Identification, quantitation and purification of insecticidal proteins from Bacillus thuringiensis
This application is a continuation of application Ser. No. 08/102,491, filed 5 Aug. 1993, now U.S. Pat. No. 5,356,788, which is a continuation of U.S. Ser. No. 07/836,967, filed 19 Feb. 1992, now abandoned, which is a CIP of U.S. Ser. No. 07/493,453, filed 14 Mar. 1990, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to the identification, quantitation and purification of insecticidal proteins from Bacillus thuringiensis, in particular from multigene strains of Bacillus thuringiensis. 
     Bacillus thuringiensis (hereinafter abbreviated Bt) is a gram-positive soil bacterium characterized by its ability to produce large crystalline parasporal inclusions during sporulation. These inclusions consist of proteins exhibiting a highly specific insecticidal activity (protoxins). 
     Many Bt strains with different insect host toxicity spectra have been identified. Numerous strains are active against larvae of certain members of the Lepidoptera (Bt strains active against over 100 species of lepidopterans have been identified to date), but strains showing toxicity against dipteran or coleopteran species are also known. 
     Bt parasporal inclusions have proven to be a valuable alternative to conventional insecticides. They are highly toxic to the target insects and harmless to the environment owing to their specificity. Other insect orders, animals and plants appear to be unaffected by the toxic crystal proteins. Various formulations of Bt have been used for more than two decades as biological insecticides to control pests in agriculture and forestry and, more recently, to control insect vectors of a variety of human and animal diseases. 
     Bt produces several types of toxins, the precise biochemical nature of which may vary from strain to strain. Some of them, in particular α- and β-exotoxins, are toxic to a variety of insect orders or to many cell types. The parasporal crystal inclusion toxins (also called δ-endotoxins), which are proteins, have a more limited and specific host range. When consumed by a larva, Bt crystalline inclusions dissolve in the larval midgut and rapidly undergo proteolytical conversion into smaller toxic polypeptides (in the 23-80 kD molecular weight range) in the insect midgut. The produced toxin species interact with midgut epithelium cells of the host insect, generating pores in the cell membrane and disturbing the osmotic balance. The epithelium cells swell and lyse. The larva stops feeding and eventually dies. For several Bt toxins specific high-affinity binding sites have been demonstrated to exist on the midgut epithelium of susceptible insects, which could explain the extreme specificity of these toxins. 
     It has become clear in recent years that Bt is provided with a surprisingly large and variable family of insecticidal proteins. Data obtained using several experimental methods indicate that crystal protein genes in many subspecies of Bt that are toxic to lepidopterans are located on one or more large plasmids; in some subspecies, the gene may be located on the chromosome. The fact that the genes for the Bt protoxins are usually plasmid borne has made Bt a favourite candidate for genetic manipulations. This has resulted recently, among other things, in generating insect-resistant transgenic crop plants capable of expressing Bt crystal protein genes. 
     It has also become clear in recent years that many Bt strains contain several closely related genes coding for protoxins. For example Bt var. kurstaki NRD-12 strain is a three-gene strain. The presence of several such genes results in the production by a single Bt strain of several protoxins closely related by their amino acid sequences. Enzymatic action of proteolytic enzymes on such a mixture of protoxins, either in vitro or in the insect midgut, produces several toxins which may only differ by a few amino acid residues, which makes the obtained mixture of toxins difficult to separate. However, since these small differences frequently occur in key regions of the toxin sequence, they may result in significantly different toxicities towards a selected insect target. As the composition of the mixture of toxins produced by a multigene strain of Bt or the expression level of the individual toxins may vary with fermentation conditions, so may its relative toxicity to various insects, and, as a consequence, the host range of the multigene Bt strain. It becomes therefore important to monitor and quantitate different toxins present in endotoxin crystals, in order to optimize the production of the most wanted toxin from a multigene producer. 
     Various attempts to purify entomocidal toxins from δ-endotoxins of various strains of Bt are known from the prior art. The proposed methods typically include a digestion of crystalline inclusions of Bt with a proteolytic enzyme, such as trypsin, or insect digestive juices, followed by separation of the products of hydrolysis by various analytical procedures, such as electrophoresis, gel filtration and ion exchange chromatography. However, none of these methods demonstrated the ability to separate and purify closely related toxins obtained from a multigene strain of Bt. 
     Similarly, numerous methods of qualitative and quantitative characterization of Bt strains have been proposed. They involve, for example, the use of flagellar antibodies, probing for the genes with DNA probes or measuring the level of RNA production. These methods, although useful for characterization and classification purposes, are not satisfactory for quantitation of gene expression and monitoring the viability of a given strain or producing individual toxin standards for analyzing insecticidal activity, synergism, membrane studies or insect resistance. 
     More specifically in Fullmer, C. S. and Wasserman, R. H. Analytical Peptide Mapping by High Performance Liquid Chromatography. Application to intestinal calcium-binding proteins. J. Biol. Chem. 254, 7208-7212 (1979) a peptide mapping technique is described involving an exhaustive enzymatic digestion to provide a mixture of peptides. The analysis of the peptide mixture is carried out by reverse-phase HPLC, in association with an acid and an organic solvent. 
     In another reference, Yamamoto, T. Identification of Entomocidal Toxins of Bacillus thuringiensis by High Performance Liquid Chromatography. J. Gen. Microbiol. 129, 2595-2603 (1983), peptide mapping using the same Fullmer et al methodology is described. For example, see page 2601, line 5 under FIG. 4 where it is stated &#34;A peak representing the proteinase resistant core could not be located&#34;. It is emphasized here that the use of organic solvent, either permanently reduces, or completely destroys, the biological activity of the toxins. 
     In a further reference, Yamamoto, T., Ehmann, A., Gonzalez, J. M. Jr. and Carlton, B.C. Expression of Three Genes Coding for 135-Kilodalton Entomocidal Proteins in Bacillus thuringiensis kurstaki. Current Microbiol. 17, 5-12 (1988), the same peptide mapping methodology is also followed. In this reference, the protoxin(s) is/are pre-purified, prior to trypsin hydrolysis. Following hydrolysis, the toxin mixture is denatured in urea and re-digested by trypsin for peptide mapping using reverse-phase HPLC (the peptides were solubilized in acid and eluted in acid/acetonitrile mixtures). As mentioned, above, this destroys the biological activity of the toxin. Unfortunately, the resultant peptide mapping, using single gene standards to interpret the results from multi-gene strains, failed to recognize the presence of the cryIA(c)[6.6] gene product in the strain kurstaki HD-1. This gene and its protein toxin is an important component, shown to be present by other researchers. 
     Further, in Hernstadt, C. and Wilcox E. Cloning and Expression of Bacillus thuringiensis Toxin gene Toxic to Beetles of the Order Coleoptera. U.S. Pat. No. 4,853,331 (1989) the cloning of a single gene and producing a single Bt protein by E. Coli host cells is described. The toxin is purified by affinity chromatography. 
     To the contrary, none of the aforementioned techniques will provide for the identification, quantitation and purification of protoxins, expressed by a Bt gene, while retaining the biological activity of the toxin. In fact, these references have a different purpose in mind, namely peptide mapping, wherein destruction of the biological activity of the toxin is irrelevant. However, in our invention, the biological activity of the toxin must remain substantially intact. Accordingly we cannot use the reverse-phase HPLC technique employed in these prior art techniques. 
     SUMMARY OF THE INVENTION 
     The present invention provides a process for identifying a protoxin expressed by a Bacillus thuringiensis gene, which process comprises 
     hydrolyzing a Bacillus thuringiensis protoxin-containing material with a proteolytic enzyme in an aqueous suspension at a pH in the range of 10 to 12 to generate a solubilized daughter toxin, 
     subjecting the daughter toxin to high performance anion-exchange liquid chromatography at a substantially constant pH in the range of 10 to 12, and under aqueous conditions corresponding to the use of a first eluent containing only a buffer and the gradual introduction over a predetermined period of time of at least one other eluent containing the buffer and a suitable salt, the changes in time of the concentration of the salt in the eluent being such that the daughter toxin, in biologically active state, is separated from other products of hydrolysis, and 
     identifying, and where required quantifying, the protoxin from the chromatographic signals produced by the daughter toxin. 
     It will be appreciated that the term &#34;daughter toxin&#34; may cover a mixture of protoxins or an individual protoxin, depending on the starting material. Thus, when a multigene strain of Bt is involved, a mixture of protoxins is involved, and when a single gene strain of Bt is used, a single protoxin is involved. 
     Thus, a method has now been found according to which closely related toxins from a multigene strain of Bt can be separated, identified and purified in their biologically active state. According to the method purified protein crystals or a crude fermentation mixture from, for instance, a multigene strain of Bt is subjected directly to the action of proteolytic enzyme at pH of from about 10 to about 12, preferably 10.5 This proteolytic digestion liberates the toxins, which are then subjected to anion exchange high performance liquid chromatography (HPLC) at a pH range of 10 to 12, preferably in a range of 10.5 to 11.5, using an anion exchange column. The elution of the toxins is carried out with a buffer solution of suitable pH in an increasing gradient of a salt, preferably sodium chloride. The procedure provides a chromatogram showing clearly identifiable peaks characteristic of toxins and therefore permits the qualitative and quantitative characterization of the original mixture. The separation process can be scaled up using semipreparative or preparative columns, thus permitting the isolation of pure active toxins in large quantities. 
     Although the most used applications of the invention may be in the separation or identification of individual toxins from a mixture of toxins obtained from a multigene strain of Bt, the invention can be used to characterize a single toxin obtained from a single-gene strain of Bt. When the toxin or toxins from the single-gene or multi-gene strain of Bt, respectively, are characterized, the strain of Bt is also characterized. 
     According to the present method also the level of expression of known toxins from a known Bt strain can be determined and the viability of the strain can be monitored. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 represents gradient conditions of chromatographic separation according to one preferred embodiment of the invention, using Protein PAK DEAE 5 PW semipreparative column, 
     FIG. 2 represents a chromatogram obtained with Protein PAK DEAE 5 PW semipreparative column, under gradient conditions of FIG. 1 of a tryptic digest of parasporal crystals of the Bt kurstaki NRD-12 strain, 
     FIGS. 3, 4 and 5 represent chromatograms obtained under gradient conditions of FIG. 1 of components B, C and D of FIG. 2, respectively, reinjected after isolation, 
     FIG. 6 represents gradient conditions of chromatographic separation according to another preferred embodiment of the invention, using a MonoQ column, 
     FIG. 7 represents a chromatogram obtained with a MonoQ column, under gradient conditions of FIG. 6 of a tryptic digest of parasporal crystals of a Bt kurstaki NRD-12 strain, 
     FIG. 8 represents gradient conditions of chromatographic separation shown in FIG. 9, 
     FIG. 9 represents a chromatogram similar to that represented by FIG. 2 and obtained with a Protein PAK DEAE column under gradient conditions of FIG. 8 (toxin peaks not resolved), and 
     FIGS. 10a and 10b show amino acid sequence of 3 toxins of Bt kurstaki NRD-12 strain and the peptide fragments identified from the isolated single protein toxins. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One embodiment according to the invention comprises typically the following steps: 
     a) purification of Bt endotoxin crystals from a fermentation mixture; 
     b) direct proteolytic enzyme digestion of the crystals; and 
     c) anion exchange HPLC separation of the products of hydrolysis. 
     Although the material digested by proteolytic enzymes is normally crystalline endotoxin purified from crude fermentation mixtures, such a purification, although preferred, is not essential. Recognizable peaks characteristic of toxins in the anion exchange HPLC elution pattern can be also obtained using a crude Bt material separated from the fermentation broth prior to enzymatic digestion. 
     The purified crystals or washed crude material are then subjected directly to hydrolysis with a proteolytic enzyme, such as trypsin, chymotrypsin or elastase. Trypsin is the preferred enzyme. Also insect gut juices, such as gut juice from silk worm (Bombyx mori) are effective. Each particular enzyme gives a slightly different set of toxins, since each has a unique specificity with respect to endotoxin proteins. Commercially available preparations of enzymes may be used. The concentration range of the enzyme is from 0.1 to 2 mg/mL, preferably 1 mg/mL. 
     The temperature of digestion of the endotoxin with the proteolytic enzyme is not critical and may vary from 20° to 40° C. preferably, about 37° C. The duration of hydrolysis is also not critical but should be long enough to assure a nearly complete liberation of toxins under given pH, temperature and enzyme concentration conditions. For trypsin digestion, the time of digestion is about 10 minutes to 12 hours. 
     The hydrolysis can be carried out in an unbuffered or buffered solution having pH in the specified range. Examples of suitable solutions are 3-cyclohexylamino-1-propanesulfonic acid/NaOH buffer (CAPS) borate buffer or unbuffered solution of NaOH. Not all solutions are equally efficient. Since the toxins released during hydrolysis are apparently hydrophobic and poorly soluble in water, it is preferable that the solution contains components which assist solubilization of the toxin protein molecules. One example of such a component is 3-cyclohexylamino-1-propanesulfonic acid (CAPS). The preferred solution is 0.1M CAPS/NaOH buffer having pH 10.5. 
     After hydrolysis the solid components of the reaction mixture are removed, for example by centrifugation and filtration. Samples of the filtrate (supernatant) are used for HPLC analysis. The pellet should not contain any recognisable inclusion body by phase contrast microscope (as a sign of complete digestion). 
     The HPLC separation of the products of proteolytic hydrolysis provides a fingerprint by which the toxins can be identified and quantified. To achieve an adequate solubility of the toxins the separation must be conducted at pH not lower than about 10, preferably 10.5 to 11.5. On the other hand, pH should not be higher than about 12, to avoid the denaturation of the toxins and to assure the proper operation of the column. In the specified range of pH the molecules of toxins are negatively charged and the separation is carried out as a liquid-solid anion exchange chromatography. The use of HPLC technique allows the achieving of the separation in short periods of time. 
     It should be noted that although the separation of the toxins according to the invention can be carried out at any value of pH in the indicated broad range, increasing the pH above the preferred range of 10.5 to 11.5 unnecessarily increases the danger of denaturation of the separated proteins and irreversible damages to the column. On the other hand, decreasing the pH below 10.5 increases the danger of toxins being precipitated in the HPLC column, at least partially, which adversly affects the reliability of quantitative determinations. For these reasons the pH range of about 10.5 to 11.5 is considered to be the optimum. 
     Columns packed with anion exchangers are used for the separation. Both weak and strong anion exchangers can be used. Weak anion exchangers are preferred. Diethylaminoethyl poly(methyl methacrylate) is especially preferred. 
     The elution of the toxins from the column is carried out at room temperature employing a suitable buffer, in an increasing gradient of a salt, such as sodium chloride. The employed buffer is in first place responsible for maintaining the required pH of the eluent. 
     However, it appears that not only the pH but also the composition of the buffer is important for successful separation. In one preferred embodiment, 0.05M CAPS/NaOH buffer (pH=10.5) is employed. However, it will be appreciated that alternative buffer systems with a new set of elution conditions may be found. 
     The conditions of salt gradient are specific for the column used. These conditions are normally achieved by employing a series of buffers having increasing concentration of the salt and introduced in a predetermined order and at a predetermined time. 
     The salt used in the separation should bind rather strongly to the column, but should not bind to or otherwise adversly affect the separated proteins. Examples of suitable salts include sodium chloride, potassium chloride, ammonium chloride and sodium acetate. Bromides and iodides may also be used. Sodium chloride is preferred. Some salts, for instance calcium salts, bind to some proteins and affect the properties of the protein. These salts are therefore not suitable. Selection of a suitable salt will present no problem to a person skilled in the art. 
     According to the most preferred embodiment of the invention, the elution from the chromatographic column is effected, immediately after hydrolysis, in two stages. A first eluent is 0.05M CAPS/NaOH buffer, pH 11.5, followed by a second eluent of 0.05M CAPS/NaOH buffer and 0.5M NaOl, pH 11.5. The two eluents are simultaneously introduced into the column in such a manner that the amount of the second eluent increases linearly from 0 to 33% over a period of 25 to 40 minutes, depending upon the nature of the sample. This step was followed by a 20-50 minute isocratic period while the toxinx are eluting. 
     Parts of the eluate corresponding to candidate peaks may be collected, concentrated and reinjected to establish identity and purity of the separated toxins. When the separation is carried out using a semipreparative or preparative column, an active, native toxin may be isolated and collected. 
     The method of rapid identification, quantitation and purification of toxins resulting from proteolytic hydrolysis of protoxins of endotoxin crystals is of considerable interest for the art. It provides means of screening and testing new Bt isolates by a rapid comparison against known strains. It also allows experiments in the manipulation of fermentation conditions to optimise the production of the most wanted toxin from multigene strains. It also enables producers to characterize the Bt toxins produced by competitors&#39; strains. Since separated toxins may be collected in pure and active form, the method will permit testing of individual purified toxins and mixtures of individual toxins prepared in a specified and controlled manner. Although of particular interest for multigene strains of Bt, the method has obviously similar application for single gene strains of Bt and for analysis of inclusion bodies from cloned Bt genes, e.g. in E. coli. 
     EXPERIMENTAL 
     EXAMPLE 1 
     Separation and Isolation of Toxins from Bacillus thuringiensis var. kurstaki NRD-12 Multigene Strain 
     1. Purification of Parasporal Crystals 
     Purified crystals were prepared following literature procedures (e.g. Carey et al. Biochim Biophys. Acta, 872, 169 (1986)) 
     2. Tryptic Digestion 
     20 mg/mL NRD-12 crystal suspension in 0.1M CAPS/NaOH, pH 10.5 buffer was treated with 1 mg/mL commercial pancreatic trypsin. The reaction mixture was stirred at room temperature overnight and centrifuged at 10,000 rpm for 15 min. The decanted supernatant was filtered through a 0.22 μm cut-off membrane. 
     3. MPLC Separation Using a Weak Anion-exchanger 
     The separation was carried out using Waters 990 solvent delivery system equipped with an automatic injector and a photodiode array detector. Protein PAK DEAE 5PW anion exchange column (7.5×75 mm analytical or 21.5×150 mm semipreparative, Waters) was used; injection volume--1-20,000 μL; flow rate--4 mL/min. 
     Complex elution gradients employed 3 buffers: 
     A: 0.05M CAPS pH 10.5 
     B: 0.05M CAPS pH 10.5+0.17M NaCl 
     C: 0.05M CAPS pH 10.5+0.5M NaCl 
     
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Gradient table:                                                           
Time [min.]                                                               
           Flow [mL/min.]                                                 
                        % A      % B  % C                                 
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Initial    4.00         100.0    0.0  0.0                                 
20.00      4.00         0.0      100.0                                    
                                      0.0                                 
35.00      4.00         0.0      100.0                                    
                                      0.0                                 
50.00      4.00         0.0      90.0 10.0                                
55.00      4.00         0.0      90.0 10.0                                
60.00      4.00         0.0      0.0  100.0                               
65.00      4.00         0.0      0.0  100.0                               
70.00      4.00         100.0    0.0  0.0                                 
79.90      4.00         100.0    0.0  0.0                                 
80.00      0.50         100.0    0.0  0.0                                 
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     The employed gradient conditions are shown in FIG. 1. FIG. 2 shows a chromatogram of the tryptic digest under these gradient conditions, where the peaks of separated toxins are marked as B, C and D. Peak B corresponds to the cryIA(a)[4.5] gene toxin, peak C corresponds to the cryIA(b)[5.3] gene product and peak D represents the cryIA(c)[6.6] gene product. The condition of the column or very slight changes of pH or salt concentration can result in the shifting of the retention time of certain proteins by several minutes. As can be seen from FIG. 2 the separation takes place during approximately the first 60 minutes of elution. The following elution is for cleaning and regeneration of the column for subsequent separations. 
     In a comparative test, no separation of the toxins was observed when a linear gradient of salt concentration increasing from 0 to 0.5M during 30 minutes was employed for the Protein PAK DEAE 5PW analytical column, as illustrated by FIG. 8. The chromatogram of the tryptic digest under these gradient conditions is shown in FIG. 9. The area around peak 7 contains the toxins. 
     4. Toxin Isolation and Purification 
     Parts corresponding to B, C and D candidate peaks were collected from the HPLC eluate, concentrated using 8 kDa polyethylene glycol/3.5 kDa cut-off dialysis tubing and reinjected into the HPLC column to establish purity. The respective chromatograms are shown in FIGS. 3, 4 and 5. With repeated injection-isolation procedure 100% purity of the isolated toxins can be achieved. 
     5. Confirmation of Toxin Identification 
     Toxins isolated and purified as described above were cleaved using CNBr and the resulting fragments were subjected to SDS-polyacrylamide gel electrophoresis (SDS-PAGE). The gel was electroblotted onto polyvinylidene difluoride membrane. The peptide bands in the membrane were stained, the visualized peptide bands were excised and candidate fragments were sequenced by an Applied Biosystems 475A protein sequencing system comprising a 470A gas phase sequencer equipped with an on-line 120A PTH analyser with a 900A control/data analysis module. 
     For the three gene strain of Bt var. kurstaki NRD-12 three candidate proteins were isolated from the HPLC eluate (peaks B, C, D in FIG. 2 and in FIGS. 3, 4 and 5, respectively). For these proteins SDS-PAGE showed three different patterns of cleavage. Sequencing of individual bands within each cleavage pattern demonstrated amino acid sequences which corresponded to unique sequences for each of the three toxins. These sequences are known from experimentally determined DNA sequences of the respective genes. 
     FIG. 10 shows the sequences of the three toxins activated by trypsin and aligned according to identical sequence segments, identified respectively as cryIA(a)[4.5], cryIA(b)[5.3] and cryIA(c)[6.6]. The sequences underlined in FIG. 10 correspond to peptides which have been isolated and sequenced and most of which contain amino acid sequences unique to only one protein. 
     6. Bt Strains Examined 
     In the manner described above the toxin content of the following strains of Bt was examined: 
     
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Bt var. kurstaki HD-1  (3 proteins)                                       
Bt var. kurstaki NRD-12                                                   
                       (3 proteins)                                       
Bt var. kurstaki HD-73 (1 protein)                                        
Bt var. entomocidus    (2 proteins)                                       
Bt var. aizawai HD-133 (2 proteins)                                       
Bt var. kurstaki A20   (3 proteins)                                       
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     and three E. coli clones, each containing a single Bt gene and giving rise to a single toxin product. Each of these genes corresponds to one of the three genes in the natural HD-1 strain. The HPLC retention times for the E. coli toxins and corresponding HD-1 toxins were identical. 
     7. Alternative HPLC Separation Using a Strong Anion Exchanger 
     This separation was carried out using a MonoQ HR 5/5 (Pharmacia) anion exchange column having --CH 2  --N(CH 3 ) 3  charged groups; injection volume 1-20,000 L; flow rate--1 mL/min. 
     The elution gradient employed 2 buffers: 
     A: 0.05M CAPS pH=10.5 
     B: 0.05M CAPS pH=10.5+0.5M NaCl 
     
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Gradient table:                                                           
Time [min.]                                                               
          Flow [mL/min.]   % A    % B                                     
______________________________________                                    
Initial   1.0              100.0  0.0                                     
35.00     1.0              0.0    100.0                                   
40.00     1.0              0.0    100.0                                   
45.00     1.0              100.0  0.0                                     
49.99     1.0              100.0  0.0                                     
50.00     0.1              100.0  0.0                                     
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     The employed gradient conditions are shown in FIG. 6. FIG. 7 shows a chromatogram of the tryptic digest under these gradient conditions, where peaks 10, 11 and 12, appearing approximately between 22 and 28 minute of the elution, correspond to separated toxins being cryIA(a)[4.5], cryIA(b)[5.3] and cryIA(c)[6.6] gene products. As can be seen from FIG. 6, the separation takes place under a linear gradient of sodium chloride increasing from 0 to 0.5M during 35 minutes. 
     EXAMPLE 2 
     The following Bt strains were analysed and gene products identified, using the same methodology as in example 1, with the exception that the elution was effected using two buffers, according to the following gradient table. The weak anion exchanger was also employed 
     A: 0.05M CAPS/NaOH pH 11.5 
     B: 0.05M CAPS/NaOH+0.5M NaCl pH 11.5 
     
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Gradient table for analytical scale:                                      
Time [min.]                                                               
          Flow [mL/min.]   % A    % B                                     
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Initial   1                100    0                                       
2         1                100    0                                       
40*       1                67     33                                      
41        0.5              67     33                                      
60        0.5              67     33                                      
61        1                67     33                                      
65        1                0      100                                     
70        1                100    0                                       
71        0.1              100    0                                       
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 *The gradient time can vary between 25-40 min depending on the nature of 
 sample                                                                   
 
    
     The results were confirmed by microsequencing and bioassay: 
     
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                             Relative                                     
Strain          Expressed gene                                            
                             Ratio                                        
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Bt var kurstaki HD-1                                                      
                cryIA(a)     28                                           
                cryIA(b)     39                                           
                cryIA(c)     33                                           
Bt var kurstaki NRD-12                                                    
                cryIA(a)     41                                           
                cryIA(b)     36                                           
                cryIA(c)     23                                           
Bt var kurstaki A-20                                                      
                cryIA(a)     17                                           
                cryIA(b)     17                                           
                cryIA(c)     66                                           
Bt var entomocidus                                                        
                cryIA(a)     40                                           
                cryIA(b)     &lt;5                                           
                cryIA(c)     &lt;5                                           
                cryIB        &lt;5                                           
                cryIC        50                                           
Bt var kurstaki HD-73                                                     
                cryIA(c)     100                                          
HD-2            cryIC (+4 other                                           
                             25                                           
                proteins)                                                 
Bt var thuringiensis                                                      
                cryIB                                                     
Bt var tenebrionis                                                        
                cryIIIA      &gt;90                                          
Bt var israelensis                                                        
                cytA         30                                           
                (+4 other    (in                                          
                proteins)    cryIVC +                                     
                             D mixture)                                   
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