CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a divisional of co-pending U.S. application Ser. No. 12/616,298, filed on Nov. 11, 2009, which claims the benefit of U.S. Provisional Application Ser. No. 61/146,676, filed Jan. 23, 2009, the content of which is herein incorporated by reference in its entirety.
FIELD OF THE INVENTION
The present invention relates to naturally-occurring and recombinant nucleic acids obtained from novel Bacillus thuringiensis genes that encode pesticidal polypeptides characterized by pesticidal activity against insect pests. Compositions and methods of the invention utilize the disclosed nucleic acids, and their encoded pesticidal polypeptides, to control plant pests.
BACKGROUND OF THE INVENTION
Insect pests are a major factor in the loss of the world's agricultural crops. For example, armyworm feeding, black cutworm damage, or European corn borer damage can be economically devastating to agricultural producers. Insect pest-related crop loss from European corn borer attacks on field and sweet corn alone has reached about one billion dollars a year in damage and control expenses.
Traditionally, the primary method for impacting insect pest populations is the application of broad-spectrum chemical insecticides. However, consumers and government regulators alike are becoming increasingly concerned with the environmental hazards associated with the production and use of synthetic chemical pesticides. Because of such concerns, regulators have banned or limited the use of some of the more hazardous pesticides. Thus, there is substantial interest in developing alternative pesticides.
Biological control of insect pests of agricultural significance using a microbial agent, such as fungi, bacteria, or another species of insect affords an environmentally friendly and commercially attractive alternative to synthetic chemical pesticides. Generally speaking, the use of biopesticides presents a lower risk of pollution and environmental hazards, and biopesticides provide greater target specificity than is characteristic of traditional broad-spectrum chemical insecticides. In addition, biopesticides often cost less to produce and thus improve economic yield for a wide variety of crops.
Certain species of microorganisms of the genus Bacillus are known to possess pesticidal activity against a broad range of insect pests including Lepidoptera, Diptera, Coleoptera, Hemiptera, and others. Bacillus thuringiensis (Bt) and Bacillus papilliae are among the most successful biocontrol agents discovered to date. Insect pathogenicity has also been attributed to strains of B. larvae, B. lentimorbus, B. sphaericus (Harwook, ed., ((1989) Bacillus (Plenum Press), 306) and B. cereus (WO 96/10083). Pesticidal activity appears to be concentrated in parasporal crystalline protein inclusions, although pesticidal proteins have also been isolated from the vegetative growth stage of Bacillus. Several genes encoding these pesticidal proteins have been isolated and characterized (see, for example, U.S. Pat. Nos. 5,366,892 and 5,840,868).
Microbial insecticides, particularly those obtained from Bacillus strains, have played an important role in agriculture as alternatives to chemical pest control. Recently, agricultural scientists have developed crop plants with enhanced insect resistance by genetically engineering crop plants to produce pesticidal proteins from Bacillus. For example, corn and cotton plants have been genetically engineered to produce pesticidal proteins isolated from strains of Bt (see, e.g., Aronson (2002) Cell Mol. Life. Sci. 59(3):417-425; Schnepf et al. (1998) Microbiol Mol Biol Rev. 62(3):775-806). These genetically engineered crops are now widely used in American agriculture and have provided the farmer with an environmentally friendly alternative to traditional insect-control methods. In addition, potatoes genetically engineered to contain pesticidal Cry toxins have been sold to the American farmer. While they have proven to be very successful commercially, these genetically engineered, insect-resistant crop plants provide resistance to only a narrow range of the economically important insect pests.
Accordingly, there remains a need for new Bt toxins with a broader range of insecticidal activity against insect pests, e.g., toxins which are active against a greater variety of insects from the order Lepidoptera. In addition, there remains a need for biopesticides having activity against a variety of insect pests and for biopesticides which have improved insecticidal activity.
SUMMARY OF THE INVENTION
Compositions and methods are provided for impacting insect pests. More specifically, the embodiments of the present invention relate to methods of impacting insects utilizing nucleotide sequences encoding insecticidal peptides to produce transformed microorganisms and plants that express a insecticidal polypeptide of the embodiments. Such pests include agriculturally significant pests, such as, for example: European corn borer (Ostrinia nubilalis) and Southwestern corn borer (Diatraea grandiosella). In some embodiments, the nucleotide sequences encode polypeptides that are pesticidal for at least one insect belonging to the order Lepidoptera.
The embodiments provide a nucleic acid and fragments and variants thereof which encode polypeptides that possess pesticidal activity against insect pests (e.g. SEQ ID NO: 1 encoding SEQ ID NO: 2). The wild-type (e.g., naturally occurring) nucleotide sequence of the embodiments, which was obtained from Bt, encodes a novel insecticidal peptide. The embodiments further provide fragments and variants of the disclosed nucleotide sequence that encode biologically active (e.g., insecticidal) polypeptides.
The embodiments further provide isolated pesticidal (e.g., insecticidal) polypeptides encoded by either a naturally occurring, or a modified (e.g., mutagenized or manipulated) nucleic acid of the embodiments. In particular examples, pesticidal proteins of the embodiments include fragments of full-length proteins and polypeptides that are produced from mutagenized nucleic acids designed to introduce particular amino acid sequences into the polypeptides of the embodiments. In particular embodiments, the polypeptides have enhanced pesticidal activity relative to the activity of the naturally occurring polypeptide from which they are derived.
The nucleic acids of the embodiments can also be used to produce transgenic (e.g., transformed) monocot or dicot plants that are characterized by genomes that comprise at least one stably incorporated nucleotide construct comprising a coding sequence of the embodiments operably linked to a promoter that drives expression of the encoded pesticidal polypeptide. Accordingly, transformed plant cells, plant tissues, plants, and seeds thereof are also provided.
In a particular embodiment, a transformed plant can be produced using a nucleic acid that has been optimized for increased expression in a host plant. For example, one of the pesticidal polypeptides of the embodiments can be back-translated to produce a nucleic acid comprising codons optimized for expression in a particular host, for example a crop plant such as a corn (Zea mays) plant. Expression of a coding sequence by such a transformed plant (e.g., dicot or monocot) will result in the production of a pesticidal polypeptide and confer increased insect resistance to the plant. Some embodiments provide transgenic plants expressing pesticidal polypeptides that find use in methods for impacting various insect pests.
The embodiments further include pesticidal or insecticidal compositions containing the insecticidal polypeptides of the embodiments, and can optionally comprise further insecticidal peptides. The embodiments encompass the application of such compositions to the environment of insect pests in order to impact the insect pests.
DETAILED DESCRIPTION OF THE INVENTION
The embodiments of the invention are drawn to compositions and methods for impacting insect pests, particularly plant pests. More specifically, the isolated nucleic acid of the embodiments, and fragments and variants thereof, comprise nucleotide sequences that encode pesticidal polypeptides (e.g., proteins). The disclosed pesticidal proteins are biologically active (e.g., pesticidal) against insect pests such as, but not limited to, insect pests of the order Lepidoptera. Insect pests of interest include, but are not limited to: Ostrinia nubilalis (European corn borer); Papaipema nebris (common stalk borer); and Diatraea grandiosella (Southwestern corn borer).
The compositions of the embodiments comprise isolated nucleic acids, and fragments and variants thereof, that encode pesticidal polypeptides, expression cassettes comprising nucleotide sequences of the embodiments, isolated pesticidal proteins, and pesticidal compositions. Some embodiments provide modified pesticidal polypeptides characterized by improved insecticidal activity against Lepidopterans relative to the pesticidal activity of the corresponding wild-type protein. The embodiments further provide plants and microorganisms transformed with these novel nucleic acids, and methods involving the use of such nucleic acids, pesticidal compositions, transformed organisms, and products thereof in impacting insect pests.
The nucleic acids and nucleotide sequences of the embodiments may be used to transform any organism to produce the encoded pesticidal proteins. Methods are provided that involve the use of such transformed organisms to impact or control plant pests. The nucleic acids and nucleotide sequences of the embodiments may also be used to transform organelles such as chloroplasts (McBride et al. (1995) Biotechnology 13: 362-365; and Kota et al. (1999) Proc. Natl. Acad. Sci. USA 96: 1840-1845).
The embodiments further relate to the identification of fragments and variants of the naturally-occurring coding sequence that encode biologically active pesticidal proteins. The nucleotide sequences of the embodiments find direct use in methods for impacting pests, particularly insect pests such as pests of the order Lepidoptera. Accordingly, the embodiments provide new approaches for impacting insect pests that do not depend on the use of traditional, synthetic chemical insecticides. The embodiments involve the discovery of naturally-occurring, biodegradable pesticides and the genes that encode them.
The embodiments further provide fragments and variants of the naturally occurring coding sequence that also encode biologically active (e.g., pesticidal) polypeptides. The nucleic acids of the embodiments encompass nucleic acid or nucleotide sequences that have been optimized for expression by the cells of a particular organism, for example nucleic acid sequences that have been back-translated (i.e., reverse translated) using plant-preferred codons based on the amino acid sequence of a polypeptide having enhanced pesticidal activity. The embodiments further provide mutations which confer improved or altered properties on the polypeptides of the embodiments. See, e.g., copending U.S. application Ser. Nos. 10/606,320, filed Jun. 25, 2003, and 10/746,914, filed Dec. 24, 2003.
In the description that follows, a number of terms are used extensively. The following definitions are provided to facilitate understanding of the embodiments.
Units, prefixes, and symbols may be denoted in their SI accepted form. Unless otherwise indicated, nucleic acids are written left to right in 5′ to 3′ orientation; amino acid sequences are written left to right in amino to carboxy orientation, respectively. Numeric ranges are inclusive of the numbers defining the range. Amino acids may be referred to herein by either their commonly known three letter symbols or by the one-letter symbols recommended by the IUPAC-IUB Biochemical Nomenclature Commission. Nucleotides, likewise, may be referred to by their commonly accepted single-letter codes. The above-defined terms are more fully defined by reference to the specification as a whole.
As used herein, “nucleic acid” includes reference to a deoxyribonucleotide or ribonucleotide polymer in either single- or double-stranded form, and unless otherwise limited, encompasses known analogues (e.g., peptide nucleic acids) having the essential nature of natural nucleotides in that they hybridize to single-stranded nucleic acids in a manner similar to that of naturally occurring nucleotides.
As used herein, the terms “encoding” or “encoded” when used in the context of a specified nucleic acid mean that the nucleic acid comprises the requisite information to direct translation of the nucleotide sequence into a specified protein. The information by which a protein is encoded is specified by the use of codons. A nucleic acid encoding a protein may comprise non-translated sequences (e.g., introns) within translated regions of the nucleic acid or may lack such intervening non-translated sequences (e.g., as in cDNA).
As used herein, “full-length sequence” in reference to a specified polynucleotide or its encoded protein means having the entire nucleic acid sequence or the entire amino acid sequence of a native (non-synthetic), endogenous sequence. A full-length polynucleotide encodes the full-length, catalytically active form of the specified protein.
As used herein, the term “antisense” used in the context of orientation of a nucleotide sequence refers to a duplex polynucleotide sequence that is operably linked to a promoter in an orientation where the antisense strand is transcribed. The antisense strand is sufficiently complementary to an endogenous transcription product such that translation of the endogenous transcription product is often inhibited. Thus, where the term “antisense” is used in the context of a particular nucleotide sequence, the term refers to the complementary strand of the reference transcription product.
The terms “polypeptide,” “peptide,” and “protein” are used interchangeably herein to refer to a polymer of amino acid residues. The terms apply to amino acid polymers in which one or more amino acid residues is an artificial chemical analogue of a corresponding naturally occurring amino acid, as well as to naturally occurring amino acid polymers.
The terms “residue” or “amino acid residue” or “amino acid” are used interchangeably herein to refer to an amino acid that is incorporated into a protein, polypeptide, or peptide (collectively “protein”). The amino acid may be a naturally occurring amino acid and, unless otherwise limited, may encompass known analogues of natural amino acids that can function in a similar manner as naturally occurring amino acids.
Polypeptides of the embodiments can be produced either from a nucleic acid disclosed herein, or by the use of standard molecular biology techniques. For example, a protein of the embodiments can be produced by expression of a recombinant nucleic acid of the embodiments in an appropriate host cell, or alternatively by a combination of ex vivo procedures.
As used herein, the terms “isolated” and “purified” are used interchangeably to refer to nucleic acids or polypeptides or biologically active portions thereof that are substantially or essentially free from components that normally accompany or interact with the nucleic acid or polypeptide as found in its naturally occurring environment. Thus, an isolated or purified nucleic acid or polypeptide is substantially free of other cellular material or culture medium when produced by recombinant techniques, or substantially free of chemical precursors or other chemicals when chemically synthesized.
An “isolated” nucleic acid is generally free of sequences (such as, for example, protein-encoding sequences) that naturally flank the nucleic acid (i.e., sequences located at the 5′ and 3′ ends of the nucleic acid) in the genomic DNA of the organism from which the nucleic acid is derived. For example, in various embodiments, the isolated nucleic acids can contain less than about 5 kb, 4 kb, 3 kb, 2 kb, 1 kb, 0.5 kb, or 0.1 kb of nucleotide sequences that naturally flank the nucleic acids in genomic DNA of the cell from which the nucleic acid is derived.
As used herein, the term “isolated” or “purified” as it is used to refer to a polypeptide of the embodiments means that the isolated protein is substantially free of cellular material and includes preparations of protein having less than about 30%, 20%, 10%, or 5% (by dry weight) of contaminating protein. When the protein of the embodiments or biologically active portion thereof is recombinantly produced, culture medium represents less than about 30%, 20%, 10%, or 5% (by dry weight) of chemical precursors or non-protein-of-interest chemicals.
Throughout the specification the word “comprising,” or variations such as “comprises” or “comprising,” will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
As used herein, the term “impacting insect pests” refers to effecting changes in insect feeding, growth, and/or behavior at any stage of development, including but not limited to: killing the insect; retarding growth; preventing reproductive capability; antifeedant activity; and the like.
As used herein, the terms “pesticidal activity” and “insecticidal activity” are used synonymously to refer to activity of an organism or a substance (such as, for example, a protein) that can be measured by, but is not limited to, pest mortality, pest weight loss, pest repellency, and other behavioral and physical changes of a pest after feeding and exposure for an appropriate length of time. Thus, an organism or substance having pesticidal activity adversely impacts at least one measurable parameter of pest fitness. For example, “pesticidal proteins” are proteins that display pesticidal activity by themselves or in combination with other proteins.
As used herein, the term “pesticidally effective amount” connotes a quantity of a substance or organism that has pesticidal activity when present in the environment of a pest. For each substance or organism, the pesticidally effective amount is determined empirically for each pest affected in a specific environment. Similarly, an “insecticidally effective amount” may be used to refer to a “pesticidally effective amount” when the pest is an insect pest.
As used herein, the term “recombinantly engineered” or “engineered” connotes the utilization of recombinant DNA technology to introduce (e.g., engineer) a change in the protein structure based on an understanding of the protein's mechanism of action and a consideration of the amino acids being introduced, deleted, or substituted.
As used herein, the term “mutant nucleotide sequence” or “mutation” or “mutagenized nucleotide sequence” connotes a nucleotide sequence that has been mutagenized or altered to contain one or more nucleotide residues (e.g., base pair) that is not present in the corresponding wild-type sequence. Such mutagenesis or alteration consists of one or more additions, deletions, or substitutions or replacements of nucleic acid residues. When mutations are made by adding, removing, or replacing an amino acid of a proteolytic site, such addition, removal, or replacement may be within or adjacent to the proteolytic site motif, so long as the object of the mutation is accomplished (i.e., so long as proteolysis at the site is changed).
A mutant nucleotide sequence can encode a mutant insecticidal toxin showing improved or decreased insecticidal activity, or an amino acid sequence which confers improved or decreased insecticidal activity on a polypeptide containing it. As used herein, the term “mutant” or “mutation” in the context of a protein a polypeptide or amino acid sequence refers to a sequence which has been mutagenized or altered to contain one or more amino acid residues that are not present in the corresponding wild-type sequence. Such mutagenesis or alteration consists of one or more additions, deletions, or substitutions or replacements of amino acid residues. A mutant polypeptide shows improved or decreased insecticidal activity, or represents an amino acid sequence which confers improved insecticidal activity on a polypeptide containing it. Thus, the term “mutant” or “mutation” refers to either or both of the mutant nucleotide sequence and the encoded amino acids. Mutants may be used alone or in any compatible combination with other mutants of the embodiments or with other mutants. A “mutant polypeptide” may conversely show a decrease in insecticidal activity. Where more than one mutation is added to a particular nucleic acid or protein, the mutations may be added at the same time or sequentially; if sequentially, mutations may be added in any suitable order.
As used herein, the term “improved insecticidal activity” or “improved pesticidal activity” refers to an insecticidal polypeptide of the embodiments that has enhanced insecticidal activity relative to the activity of its corresponding wild-type protein, and/or an insecticidal polypeptide that is effective against a broader range of insects, and/or an insecticidal polypeptide having specificity for an insect that is not susceptible to the toxicity of the wild-type protein. A finding of improved or enhanced pesticidal activity requires a demonstration of an increase of pesticidal activity of at least 10%, against the insect target, or at least 20%, 25%, 30%, 35%, 40%, 45%, 50%, 60%, 70%, 100%, 150%, 200%, or 300% or greater increase of pesticidal activity relative to the pesticidal activity of the wild-type insecticidal polypeptide determined against the same insect.
For example, an improved pesticidal or insecticidal activity is provided where a wider or narrower range of insects is impacted by the polypeptide relative to the range of insects that is affected by a wild-type Bt toxin. A wider range of impact may be desirable where versatility is desired, while a narrower range of impact may be desirable where, for example, beneficial insects might otherwise be impacted by use or presence of the toxin. While the embodiments are not bound by any particular mechanism of action, an improved pesticidal activity may also be provided by changes in one or more characteristics of a polypeptide; for example, the stability or longevity of a polypeptide in an insect gut may be increased relative to the stability or longevity of a corresponding wild-type protein.
The term “toxin” as used herein refers to a polypeptide showing pesticidal activity or insecticidal activity or improved pesticidal activity or improved insecticidal activity. “Be” or “Bacillus thuringiensis” toxin is intended to include the broader class of Cry toxins found in various strains of Bt, which includes such toxins as, for example, Cry1s, Cry2s, or Cry3s.
The terms “proteolytic site” or “cleavage site” refer to an amino acid sequence which confers sensitivity to a class of proteases or a particular protease such that a polypeptide containing the amino acid sequence is digested by the class of proteases or particular protease. A proteolytic site is said to be “sensitive” to the protease(s) that recognize that site. It is appreciated in the art that the efficiency of digestion will vary, and that a decrease in efficiency of digestion can lead to an increase in stability or longevity of the polypeptide in an insect gut. Thus, a proteolytic site may confer sensitivity to more than one protease or class of proteases, but the efficiency of digestion at that site by various proteases may vary. Proteolytic sites include, for example, trypsin sites, chymotrypsin sites, and elastase sites.
Research has shown that the insect gut proteases of Lepidopterans include trypsins, chymotrypsins, and elastases. See, e.g., Lenz et al. (1991) Arch. Insect Biochem. Physiol. 16: 201-212; and Hedegus et al. (2003) Arch. Insect Biochem. Physiol. 53: 30-47. For example, about 18 different trypsins have been found in the midgut of Helicoverpa armigera larvae (see Gatehouse et al. (1997) Insect Biochem. Mol. Biol. 27: 929-944). The preferred proteolytic substrate sites of these proteases have been investigated. See, e.g., Peterson et al. (1995) Insect Biochem. Mol. Biol. 25: 765-774.
Efforts have been made to understand the mechanism of action of Bt toxins and to engineer toxins with improved properties. It has been shown that insect gut proteases can affect the impact of Bt Cry proteins on the insect. Some proteases activate the Cry proteins by processing them from a “protoxin” form into a toxic form, or “toxin.” See, Oppert (1999) Arch. Insect Biochem. Phys. 42: 1-12; and Carroll et al. (1997) J. Invertebrate Pathology 70: 41-49. This activation of the toxin can include the removal of the N- and C-terminal peptides from the protein and can also include internal cleavage of the protein. Other proteases can degrade the Cry proteins. See Oppert, ibid.
A comparison of the amino acid sequences of Cry toxins of different specificities reveals five highly-conserved sequence blocks. Structurally, the toxins comprise three distinct domains which are, from the N- to C-terminus: a cluster of seven alpha-helices implicated in pore formation (referred to as “domain 1”), three anti-parallel beta sheets implicated in cell binding (referred to as “domain 2”), and a beta sandwich (referred to as “domain 3”). The location and properties of these domains are known to those of skill in the art. See, for example, Li et al. (1991) Nature, 305:815-821 and Morse et al. (2001) Structure, 9:409-417. When reference is made to a particular domain, such as domain 1, it is understood that the exact endpoints of the domain with regard to a particular sequence are not critical so long as the sequence or portion thereof includes sequence that provides at least some function attributed to the particular domain. Thus, for example, when referring to “domain 1,” it is intended that a particular sequence includes a cluster of seven alpha-helices, but the exact endpoints of the sequence used or referred to with regard to that cluster are not critical. One of skill in the art is familiar with the determination of such endpoints and the evaluation of such functions.
In an effort to better characterize and improve Bt toxins, strains of the bacterium Bt were studied. Crystal preparations prepared from cultures of the Bt strains were discovered to have pesticidal activity against European corn borer (see, e.g., Experimental Example 1). An effort was undertaken to identify the nucleotide sequences encoding the crystal proteins from the selected strains, and the wild-type (i.e., naturally occurring) nucleic acids of the embodiments were isolated from these bacterial strains, cloned into an expression vector, and transformed into E. Coli. Depending upon the characteristics of a given preparation, it was recognized that the demonstration of pesticidal activity sometimes required trypsin pretreatment to activate the pesticidal proteins. Thus, it is understood that some pesticidal proteins require protease digestion (e.g., by trypsin, chymotrypsin, and the like) for activation, while other proteins are biologically active (e.g., pesticidal) in the absence of activation.
Such molecules may be altered by means described, for example, in U.S. application Ser. Nos. 10/606,320, filed Jun. 25, 2003, and 10/746,914, filed Dec. 24, 2003. In addition, nucleic acid sequences may be engineered to encode polypeptides that contain additional mutations that confer improved or altered pesticidal activity relative to the pesticidal activity of the naturally occurring polypeptide. The nucleotide sequences of such engineered nucleic acids comprise mutations not found in the wild type sequences.
The mutant polypeptides of the embodiments are generally prepared by a process that involves the steps of: obtaining a nucleic acid sequence encoding a Cry family polypeptide; analyzing the structure of the polypeptide to identify particular “target” sites for mutagenesis of the underlying gene sequence based on a consideration of the proposed function of the target domain in the mode of action of the toxin; introducing one or more mutations into the nucleic acid sequence to produce a desired change in one or more amino acid residues of the encoded polypeptide sequence; and assaying the polypeptide produced for pesticidal activity.
Many of the Bt insecticidal toxins are related to various degrees by similarities in their amino acid sequences and tertiary structure and means for obtaining the crystal structures of Bt toxins are well known. Exemplary high-resolution crystal structure solution of both the Cry3A and Cry3B polypeptides are available in the literature. The solved structure of the Cry3A gene (Li et al. (1991) Nature 353:815-821) provides insight into the relationship between structure and function of the toxin. A combined consideration of the published structural analyses of Bt toxins and the reported function associated with particular structures, motifs, and the like indicates that specific regions of the toxin are correlated with particular functions and discrete steps of the mode of action of the protein. For example, many toxins isolated from Bt are generally described as comprising three domains: a seven-helix bundle that is involved in pore formation, a three-sheet domain that has been implicated in receptor binding, and a beta-sandwich motif (Li et al. (1991) Nature 305: 815-821).
As reported in U.S. Pat. No. 7,105,332, and pending U.S. application Ser. No. 10/746,914, filed Dec. 24, 2003, the toxicity of Cry proteins can be improved by targeting the region located between alpha helices 3 and 4 of domain 1 of the toxin.
This theory was premised on a body of knowledge concerning insecticidal toxins, including: 1) that alpha helices 4 and 5 of domain 1 of Cry3A toxins had been reported to insert into the lipid bilayer of cells lining the midgut of susceptible insects (Gazit et al. (1998) Proc. Natl. Acad. Sci. USA 95: 12289-12294); 2) the inventors\' knowledge of the location of trypsin and chymotrypsin cleavage sites within the amino acid sequence of the wild-type protein; 3) the observation that the wild-type protein was more active against certain insects following in vitro activation by trypsin or chymotrypsin treatment; and 4) reports that digestion of toxins from the 3′ end resulted in decreased toxicity to insects.
A series of mutations may be created and placed in a variety of background sequences to create novel polypeptides having enhanced or altered pesticidal activity. See, e.g., U.S. application Ser. Nos. 10/606,320, filed Jun. 25, 2003, now abandoned, and 10/746,914, filed Dec. 24, 2003. These mutants include, but are not limited to: the addition of at least one more protease-sensitive site (e.g., trypsin cleavage site) in the region located between helices 3 and 4 of domain 1; the replacement of an original protease-sensitive site in the wild-type sequence with a different protease-sensitive site; the addition of multiple protease-sensitive sites in a particular location; the addition of amino acid residues near protease-sensitive site(s) to alter folding of the polypeptide and thus enhance digestion of the polypeptide at the protease-sensitive site(s); and adding mutations to protect the polypeptide from degradative digestion that reduces toxicity (e.g., making a series of mutations wherein the wild-type amino acid is replaced by valine to protect the polypeptide from digestion). Mutations may be used singly or in any combination to provide polypeptides of the embodiments.
In this manner, the embodiments provide sequences comprising a variety of mutations, such as, for example, a mutation that comprises an additional, or an alternative, protease-sensitive site located between alpha-helices 3 and 4 of domain 1 of the encoded polypeptide. A mutation which is an additional or alternative protease-sensitive site may be sensitive to several classes of proteases such as serine proteases, which include trypsin and chymotrypsin, or enzymes such as elastase. Thus, a mutation which is an additional or alternative protease-sensitive site may be designed so that the site is readily recognized and/or cleaved by a category of proteases, such as mammalian proteases or insect proteases. A protease-sensitive site may also be designed to be cleaved by a particular class of enzymes or a particular enzyme known to be produced in an organism, such as, for example, a chymotrypsin produced by the corn earworm Heliothis zea (Lenz et al. (1991) Arch. Insect Biochem. Physiol. 16: 201-212). Mutations may also confer resistance to proteolytic digestion, for example, to digestion by chymotrypsin at the C-terminus of the peptide.
The presence of an additional and/or alternative protease-sensitive site in the amino acid sequence of the encoded polypeptide can improve the pesticidal activity and/or specificity of the polypeptide encoded by the nucleic acids of the embodiments. Accordingly, the nucleotide sequences of the embodiments can be recombinantly engineered or manipulated to produce polypeptides having improved or altered insecticidal activity and/or specificity compared to that of an unmodified wild-type toxin. In addition, the mutations disclosed herein may be placed in or used in conjunction with other nucleotide sequences to provide improved properties. For example, a protease-sensitive site that is readily cleaved by insect chymotrypsin, e.g., a chymotrypsin found in the bertha armyworm or the corn earworm (Hegedus et al. (2003) Arch. Insect Biochem. Physiol. 53: 30-47; and Lenz et al. (1991) Arch. Insect Biochem. Physiol. 16: 201-212), may be placed in a Cry background sequence to provide improved toxicity to that sequence. In this manner, the embodiments provide toxic polypeptides with improved properties.
For example, a mutagenized Cry nucleotide sequence can comprise additional mutants that comprise additional codons that introduce a second trypsin-sensitive amino acid sequence (in addition to the naturally occurring trypsin site) into the encoded polypeptide. An alternative addition mutant of the embodiments comprises additional codons designed to introduce at least one additional different protease-sensitive site into the polypeptide, for example, a chymotrypsin-sensitive site located immediately 5′ or 3′ of the naturally occurring trypsin site. Alternatively, substitution mutants may be created in which at least one codon of the nucleic acid that encodes the naturally occurring protease-sensitive site is destroyed and alternative codons are introduced into the nucleic acid sequence in order to provide a different (e.g., substitute) protease-sensitive site. A replacement mutant may also be added to a Cry sequence in which the naturally-occurring trypsin cleavage site present in the encoded polypeptide is destroyed and a chymotrypsin or elastase cleavage site is introduced in its place.
It is recognized that any nucleotide sequence encoding the amino acid sequences that are proteolytic sites or putative proteolytic sites (for example, sequences such as NGSR, RR, or LKM) can be used and that the exact identity of the codons used to introduce any of these cleavage sites into a variant polypeptide may vary depending on the use, i.e., expression in a particular plant species. It is also recognized that any of the disclosed mutations can be introduced into any polynucleotide sequence of the embodiments that comprises the codons for amino acid residues that provide the native trypsin cleavage site that is targeted for modification. Accordingly, variants of either full-length toxins or fragments thereof can be modified to contain additional or alternative cleavage sites, and these embodiments are intended to be encompassed by the scope of the embodiments disclosed herein.
It will be appreciated by those of skill in the art that any useful mutation may be added to the sequences of the embodiments so long as the encoded polypeptides retain pesticidal activity. Thus, sequences may also be mutated so that the encoded polypeptides are resistant to proteolytic digestion by chymotrypsin. More than one recognition site can be added in a particular location in any combination, and multiple recognition sites can be added to or removed from the toxin. Thus, additional mutations can comprise three, four, or more recognition sites. It is to be recognized that multiple mutations can be engineered in any suitable polynucleotide sequence; accordingly, either full-length sequences or fragments thereof can be modified to contain additional or alternative cleavage sites as well as to be resistant to proteolytic digestion. In this manner, the embodiments provide Cry toxins containing mutations that improve pesticidal activity as well as improved compositions and methods for impacting pests using other Bt toxins.
Mutations may protect the polypeptide from protease degradation, for example by removing putative proteolytic sites such as putative serine protease sites and elastase recognition sites from different areas. Some or all of such putative sites may be removed or altered so that proteolysis at the location of the original site is decreased. Changes in proteolysis may be assessed by comparing a mutant polypeptide with wild-type toxins or by comparing mutant toxins which differ in their amino acid sequence. Putative proteolytic sites and proteolytic sites include, but are not limited to, the following sequences: RR, a trypsin cleavage site; LKM, a chymotrypsin site; and NGSR, a trypsin site. These sites may be altered by the addition or deletion of any number and kind of amino acid residues, so long as the pesticidal activity of the polypeptide is increased. Thus, polypeptides encoded by nucleotide sequences comprising mutations will comprise at least one amino acid change or addition relative to the native or background sequence, or 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 32, 35, 38, 40, 45, 47, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, or 280 or more amino acid changes or additions. Pesticidal activity of a polypeptide may also be improved by truncation of the native or full-length sequence, as is known in the art.
Compositions of the embodiments include nucleic acids, and fragments and variants thereof, that encode pesticidal polypeptides. In particular, the embodiments provide for isolated nucleic acid molecules comprising nucleotide sequences encoding the amino acid sequence shown in SEQ ID NO: 2, or the nucleotide sequences encoding said amino acid sequence, for example the nucleotide sequence set forth in SEQ ID NO: 1, and fragments and variants thereof.
Also of interest are optimized nucleotide sequences encoding the pesticidal proteins of the embodiments. As used herein, the phrase “optimized nucleotide sequences” refers to nucleic acids that are optimized for expression in a particular organism, for example a plant. Optimized nucleotide sequences may be prepared for any organism of interest using methods known in the art. See, for example, U.S. application Ser. Nos. 10/606,320, filed Jun. 25, 2003, now abandoned, and 10/746,914, filed Dec. 24, 2003, which describe an optimized nucleotide sequence encoding a disclosed pesticidal protein. In this example, the nucleotide sequence was prepared by reverse-translating the amino acid sequence of the protein and changing the nucleotide sequence so as to comprise maize-preferred codons while still encoding the same amino acid sequence. This procedure is described in more detail by Murray et al. (1989) Nucleic Acids Res. 17:477-498. Optimized nucleotide sequences find use in increasing expression of a pesticidal protein in a plant, for example monocot plants of the Gramineae (Poaceae) family such as, for example, a maize or corn plant.
The embodiments further provide isolated pesticidal (e.g., insecticidal) polypeptides encoded by either a naturally-occurring or modified nucleic acid of the embodiments. More specifically, the embodiments provide polypeptides comprising an amino acid sequence set forth in SEQ ID NO: 2, and the polypeptides encoded by nucleic acids described herein, for example those set forth in SEQ ID NO: 1, and fragments and variants thereof.
In particular embodiments, pesticidal proteins of the embodiments provide full-length insecticidal polypeptides, fragments of full-length insecticidal polypeptides, and variant polypeptides that are produced from mutagenized nucleic acids designed to introduce particular amino acid sequences into polypeptides of the embodiments. In particular embodiments, the amino acid sequences that are introduced into the polypeptides comprise a sequence that provides a cleavage site for an enzyme such as a protease.
It is known in the art that the pesticidal activity of Bt toxins is typically activated by cleavage of the peptide in the insect gut by various proteases. Because peptides may not always be cleaved with complete efficiency in the insect gut, fragments of a full-length toxin may have enhanced pesticidal activity in comparison to the full-length toxin itself. Thus, some of the polypeptides of the embodiments include fragments of a full-length insecticidal polypeptide, and some of the polypeptide fragments, variants, and mutations will have enhanced pesticidal activity relative to the activity of the naturally occurring insecticidal polypeptide from which they are derived, particularly if the naturally occurring insecticidal polypeptide is not activated in vitro with a protease prior to screening for activity. Thus, the present application encompasses truncated versions or fragments of the sequences.
Mutations may be placed into any background sequence, including such truncated polypeptides, so long as the polypeptide retains pesticidal activity. One of skill in the art can readily compare two or more proteins with regard to pesticidal activity using assays known in the art or described elsewhere herein. It is to be understood that the polypeptides of the embodiments can be produced either by expression of a nucleic acid disclosed herein, or by the use of standard molecular biology techniques.
It is recognized that the pesticidal proteins may be oligomeric and will vary in molecular weight, number of residues, component peptides, activity against particular pests, and other characteristics. However, by the methods set forth herein, proteins active against a variety of pests may be isolated and characterized. The pesticidal proteins of the embodiments can be used in combination with other Bt toxins or other insecticidal proteins to increase insect target range. Furthermore, the use of the pesticidal proteins of the embodiments in combination with other Bt toxins or other insecticidal principles of a distinct nature has particular utility for the prevention and/or management of insect resistance. Other insecticidal agents include protease inhibitors (both serine and cysteine types), α-amylase, and peroxidase.
Fragments and variants of the nucleotide and amino acid sequences and the polypeptides encoded thereby are also encompassed by the embodiments. As used herein the term “fragment” refers to a portion of a nucleotide sequence of a polynucleotide or a portion of an amino acid sequence of a polypeptide of the embodiments. Fragments of a nucleotide sequence may encode protein fragments that retain the biological activity of the native or corresponding full-length protein and hence possess pesticidal activity. Thus, it is acknowledged that some of the polynucleotide and amino acid sequences of the embodiments can correctly be referred to as both fragments and mutants.
It is to be understood that the term “fragment,” as it is used to refer to nucleic acid sequences of the embodiments, also encompasses sequences that are useful as hybridization probes. This class of nucleotide sequences generally does not encode fragment proteins retaining biological activity. Thus, fragments of a nucleotide sequence may range from at least about 20 nucleotides, about 50 nucleotides, about 100 nucleotides, and up to the full-length nucleotide sequence encoding the proteins of the embodiments.
A fragment of a nucleotide sequence of the embodiments that encodes a biologically active portion of a pesticidal protein of the embodiments will encode at least 15, 25, 30, 50, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1,000, 1,100, or 1,200 contiguous amino acids, or up to the total number of amino acids present in a pesticidal polypeptide of the embodiments (for example, 1,058 amino acids for SEQ ID NO: 2). Thus, it is understood that the embodiments also encompass polypeptides that are fragments of the exemplary pesticidal proteins of the embodiments and having lengths of at least 15, 25, 30, 50, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1,000, 1,100, or 1,200 contiguous amino acids, or up to the total number of amino acids present in a pesticidal polypeptide of the embodiments (for example, 1,058 amino acids for SEQ ID NO: 2). Fragments of a nucleotide sequence of the embodiments that are useful as hybridization probes or PCR primers generally need not encode a biologically active portion of a pesticidal protein. Thus, a fragment of a nucleic acid of the embodiments may encode a biologically active portion of a pesticidal protein, or it may be a fragment that can be used as a hybridization probe or PCR primer using methods disclosed herein. A biologically active portion of a pesticidal protein can be prepared by isolating a portion of one of the nucleotide sequences of the embodiments, expressing the encoded portion of the pesticidal protein (e.g., by recombinant expression in vitro), and assessing the activity of the encoded portion of the pesticidal protein.
Nucleic acids that are fragments of a nucleotide sequence of the embodiments comprise at least 16, 20, 50, 75, 100, 150, 200, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1,000, 1,200, 1,400, 1,600, 1,800, or 2,000 nucleotides, or up to the number of nucleotides present in a nucleotide sequence disclosed herein (for example, 3,174 nucleotides for SEQ ID NO: 1). Particular embodiments envision fragments derived from (e.g., produced from) a first nucleic acid of the embodiments, wherein the fragment encodes a truncated toxin characterized by pesticidal activity. Truncated polypeptides encoded by the polynucleotide fragments of the embodiments are characterized by pesticidal activity that is either equivalent to, or improved, relative to the activity of the corresponding full-length polypeptide encoded by the first nucleic acid from which the fragment is derived. It is envisioned that such nucleic acid fragments of the embodiments may be truncated at the 3′ end of the native or corresponding full-length coding sequence. Nucleic acid fragments may also be truncated at both the 5′ and 3′ end of the native or corresponding full-length coding sequence.
The term “variants” is used herein to refer to substantially similar sequences. For nucleotide sequences, conservative variants include those sequences that, because of the degeneracy of the genetic code, encode the amino acid sequence of one of the pesticidal polypeptides of the embodiments. Naturally occurring allelic variants such as these can be identified with the use of well-known molecular biology techniques, such as, for example, polymerase chain reaction (PCR) and hybridization techniques as outlined herein.
Variant nucleotide sequences also include synthetically derived nucleotide sequences, such as those generated, for example, by using site-directed mutagenesis but which still encode a pesticidal protein of the embodiments, such as a mutant toxin. Generally, variants of a particular nucleotide sequence of the embodiments will have at least about 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or more sequence identity to that particular nucleotide sequence as determined by sequence alignment programs described elsewhere herein using default parameters. A variant of a nucleotide sequence of the embodiments may differ from that sequence by as few as 1-15 nucleotides, as few as 1-10, such as 6-10, as few as 5, as few as 4, 3, 2, or even 1 nucleotide.
Variants of a particular nucleotide sequence of the embodiments (i.e., an exemplary nucleotide sequence) can also be evaluated by comparison of the percent sequence identity between the polypeptide encoded by a variant nucleotide sequence and the polypeptide encoded by the reference nucleotide sequence. Thus, for example, isolated nucleic acids that encode a polypeptide with a given percent sequence identity to the polypeptide of SEQ ID NO: 2 are disclosed. Percent sequence identity between any two polypeptides can be calculated using sequence alignment programs described elsewhere herein using default parameters. Where any given pair of polynucleotides of the embodiments is evaluated by comparison of the percent sequence identity shared by the two polypeptides they encode, the percent sequence identity between the two encoded polypeptides is at least about 40%, 45%, 50%, 55%, 60%, 65%, 70%, generally at least about 75%, 80%, 85%, at least about 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, or at least about 98%, 99% or more sequence identity.
As used herein, the term “variant protein” encompasses polypeptides that are derived from a native protein by: deletion (so-called truncation) or addition of one or more amino acids to the N-terminal and/or C-terminal end of the native protein; deletion or addition of one or more amino acids at one or more sites in the native protein; or substitution of one or more amino acids at one or more sites in the native protein. Accordingly, the term “variant protein” encompasses biologically active fragments of a native protein that comprise a sufficient number of contiguous amino acid residues to retain the biological activity of the native protein, i.e., to have pesticidal activity. Such pesticidal activity may be different or improved relative to the native protein or it may be unchanged, so long as pesticidal activity is retained.
Variant proteins encompassed by the embodiments are biologically active, that is they continue to possess the desired biological activity of the native protein, that is, pesticidal activity as described herein. Such variants may result from, for example, genetic polymorphism or from human manipulation. Biologically active variants of a native pesticidal protein of the embodiments will have at least about 60%, 65%, 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or more sequence identity to the amino acid sequence for the native protein as determined by sequence alignment programs described elsewhere herein using default parameters. A biologically active variant of a protein of the embodiments may differ from that protein by as few as 1-15 amino acid residues, as few as 1-10, such as 6-10, as few as 5, as few as 4, 3, 2, or even 1 amino acid residue.
The embodiments further encompass a microorganism that is transformed with at least one nucleic acid of the embodiments, with an expression cassette comprising the nucleic acid, or with a vector comprising the expression cassette. In some embodiments, the microorganism is one that multiplies on plants. An embodiment of the invention relates to an encapsulated pesticidal protein which comprises a transformed microorganism capable of expressing at least one pesticidal protein of the embodiments.
The embodiments provide pesticidal compositions comprising a transformed microorganism of the embodiments. In such embodiments, the transformed microorganism is generally present in the pesticidal composition in a pesticidally effective amount, together with a suitable carrier. The embodiments also encompass pesticidal compositions comprising an isolated protein of the embodiments, alone or in combination with a transformed organism of the embodiments and/or an encapsulated pesticidal protein of the embodiments, in an insecticidally effective amount, together with a suitable carrier.
The embodiments further provide a method of increasing insect target range by using a pesticidal protein of the embodiments in combination with at least one other or “second” pesticidal protein. Any pesticidal protein known in the art can be employed in the methods of the embodiments. Such pesticidal proteins include, but are not limited to, Bt toxins, protease inhibitors, α-amylases, and peroxidases.
The embodiments also encompass transformed or transgenic plants comprising at least one nucleotide sequence of the embodiments. In some embodiments, the plant is stably transformed with a nucleotide construct comprising at least one nucleotide sequence of the embodiments operably linked to a promoter that drives expression in a plant cell. As used herein, the terms “transformed plant” and “transgenic plant” refer to a plant that comprises within its genome a heterologous polynucleotide. Generally, the heterologous polynucleotide is stably integrated within the genome of a transgenic or transformed plant such that the polynucleotide is passed on to successive generations. The heterologous polynucleotide may be integrated into the genome alone or as part of a recombinant expression cassette.
It is to be understood that as used herein the term “transgenic” includes any cell, cell line, callus, tissue, plant part, or plant the genotype of which has been altered by the presence of heterologous nucleic acid including those transgenics initially so altered as well as those created by sexual crosses or asexual propagation from the initial transgenic. The term “transgenic” as used herein does not encompass the alteration of the genome (chromosomal or extra-chromosomal) by conventional plant breeding methods or by naturally occurring events such as random cross-fertilization, non-recombinant viral infection, non-recombinant bacterial transformation, non-recombinant transposition, or spontaneous mutation.
As used herein, the term “plant” includes whole plants, plant organs (e.g., leaves, stems, roots, etc.), seeds, plant cells, and progeny of same. Parts of transgenic plants are within the scope of the embodiments and comprise, for example, plant cells, protoplasts, tissues, callus, embryos as well as flowers, stems, fruits, leaves, and roots originating in transgenic plants or their progeny previously transformed with a DNA molecule of the embodiments and therefore consisting at least in part of transgenic cells.
As used herein, the term plant includes plant cells, plant protoplasts, plant cell tissue cultures from which plants can be regenerated, plant calli, plant clumps, and plant cells that are intact in plants or parts of plants such as embryos, pollen, ovules, seeds, leaves, flowers, branches, fruit, kernels, ears, cobs, husks, stalks, roots, root tips, anthers, and the like. The class of plants that can be used in the methods of the embodiments is generally as broad as the class of higher plants amenable to transformation techniques, including both monocotyledonous and dicotyledonous plants. Such plants include, for example, Solanum tuberosum and Zea mays.
While the embodiments do not depend on a particular biological mechanism for increasing the resistance of a plant to a plant pest, expression of the nucleotide sequences of the embodiments in a plant can result in the production of the pesticidal proteins of the embodiments and in an increase in the resistance of the plant to a plant pest. The plants of the embodiments find use in agriculture in methods for impacting insect pests. Certain embodiments provide transformed crop plants, such as, for example, maize plants, which find use in methods for impacting insect pests of the plant, such as, for example, European corn borer.
A “subject plant or plant cell” is one in which genetic alteration, such as transformation, has been effected as to a gene of interest, or is a plant or plant cell which is descended from a plant or cell so altered and which comprises the alteration. A “control” or “control plant” or “control plant cell” provides a reference point for measuring changes in phenotype of the subject plant or plant cell.
A control plant or plant cell may comprise, for example: (a) a wild-type plant or cell, i.e., of the same genotype as the starting material for the genetic alteration which resulted in the subject plant or cell; (b) a plant or plant cell of the same genotype as the starting material but which has been transformed with a null construct (i.e., with a construct which has no known effect on the trait of interest, such as a construct comprising a marker gene); (c) a plant or plant cell which is a non-transformed sergeant among progeny of a subject plant or plant cell; (d) a plant or plant cell genetically identical to the subject plant or plant cell but which is not exposed to conditions or stimuli that would induce expression of the gene of interest; or (e) the subject plant or plant cell itself, under conditions in which the gene of interest is not expressed.
One of skill in the art will readily acknowledge that advances in the field of molecular biology such as site-specific and random mutagenesis, polymerase chain reaction methodologies, and protein engineering techniques provide an extensive collection of tools and protocols suitable for use to alter or engineer both the amino acid sequence and underlying genetic sequences of proteins of agricultural interest.
Thus, the proteins of the embodiments may be altered in various ways including amino acid substitutions, deletions, truncations, and insertions. Methods for such manipulations are generally known in the art. For example, amino acid sequence variants of the pesticidal proteins can be prepared by introducing mutations into a synthetic nucleic acid (e.g., DNA molecule). Methods for mutagenesis and nucleic acid alterations are well known in the art. For example, designed changes can be introduced using an oligonucleotide-mediated site-directed mutagenesis technique. See, for example, Kunkel (1985) Proc. Natl. Acad. Sci. USA 82:488-492; Kunkel et al. (1987) Methods in Enzymol. 154:367-382; U.S. Pat. No. 4,873,192; Walker and Gaastra, eds. (1983) Techniques in Molecular Biology (MacMillan Publishing Company, New York), and the references cited therein.
The mutagenized nucleotide sequences of the embodiments may be modified so as to change about 1, 2, 3, 4, 5, 6, 8, 10, 12 or more of the amino acids present in the primary sequence of the encoded polypeptide. Alternatively, even more changes from the native sequence may be introduced such that the encoded protein may have at least about 1% or 2%, or about 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, or even about 13%, 14%, 15%, 16%, 17%, 18%, 19%, or 20%, 21%, 22%, 23%, 24%, or 25%, 30%, 35%, or 40% or more of the codons altered, or otherwise modified compared to the corresponding wild-type protein. In the same manner, the encoded protein may have at least about 1% or 2%, or about 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, or even about 13%, 14%, 15%, 16%, 17%, 18%, 19%, or 20%, 21%, 22%, 23%, 24%, or 25%, 30%, 35%, or 40% or more additional codons compared to the corresponding wild-type protein. It should be understood that the mutagenized nucleotide sequences of the embodiments are intended to encompass biologically functional, equivalent peptides which have pesticidal activity, such as an improved pesticidal activity as determined by antifeedant properties against European corn borer larvae. Such sequences may arise as a consequence of codon redundancy and functional equivalency that are known to occur naturally within nucleic acid sequences and the proteins thus encoded.
One of skill in the art would recognize that amino acid additions and/or substitutions are generally based on the relative similarity of the amino acid side-chain substituents, for example, their hydrophobicity, charge, size, and the like. Exemplary amino acid substitution groups that take various of the foregoing characteristics into consideration are well known to those of skill in the art and include: arginine and lysine; glutamate and aspartate; serine and threonine; glutamine and asparagine; and valine, leucine, and isoleucine.
Guidance as to appropriate amino acid substitutions that do not affect biological activity of the protein of interest may be found in the model of Dayhoff et al. (1978) Atlas of Protein Sequence and Structure (Natl. Biomed. Res. Found., Washington, D.C.), herein incorporated by reference. Conservative substitutions, such as exchanging one amino acid with another having similar properties, may be made.