The biotechnology of Bacillus thuringiensis

Extracellular biomineralization of uranium and its toxicity alleviation to Bacillus thuringiensis X-27

Uranium biomineralization can slow uranium migration in the environment and thus prevent it from further contaminating the surroundings. Investigations into the uranium species, pH, inorganic phosphate (Pi) concentration, and microbial viability during biomineralization by microorganisms are crucial for understanding the mineralization mechanism. In this study, Bacillus thuringiensis X-27 was isolated from soil contaminated with uranium and was used to investigate the formation process of uranium biominerals induced by X-27. The results showed that as biomineralization proceeded, amorphous uranium-containing deposits were generated and transformed into crystalline minerals outside cells, increasing the overall concentration of uramphite. This is a cumulative rather than abrupt process. Notably, B. thuringiensis X-27 precipitated uranium outside the cell surface within 0.5 h, while the release of Pi into the extracellular environment and the change of pH to alkalescence further promoted the formation of uramphite. In addition, cell viability determination showed that the U(VI) biomineralization induced by B. thuringiensis X-27 was instrumental in alleviating the toxicity of U(VI) to cells. This work offers insight into the mechanism of U(VI) phosphate biomineralization and is a reference for bioremediation-related studies.

Identification of a delta-Endotoxin Gene Product Specifically Active against Spodoptera littoralis Bdv. among Proteolysed Fractions of the Insecticidal Crystals of Bacillus thuringiensis subsp. aizawai 7.29

At least three different insecticidal crystal protein genes were shown to be expressed in Bacillus thuringiensis subsp. aizawai 7.29, a strain that is potentially active against the cotton leafworm Spodoptera littoralis Bdv. Among crude K-60 fractions (60- to 70-kilodalton [kDa] molecules) that were products of proteolysed crystals containing the active domains of the protoxin molecules, we were able to distinguish several distinct components on the basis of their antigenic relationship and their larvicidal properties. A purified fraction designated SF2 was a 61-kDa component specifically active against Pieris brassicae L. and homologous to the B. thuringiensis subsp. berliner 1715 plasmid-encoded crystal protein. A second fraction designated SF1 was composed of 63- and 65-kDa polypeptides and was specifically active against S. littoralis. The SF1 fraction and particularly the 65-kDa component were not antigenically related to the 61-kDa component. The purified fractions were compared with the products of three different crystal protein genes we previously cloned from total DNA of B. thuringiensis subsp. aizawai, among them a new type of crystal protein gene encoding a protein that is specifically active against S. littoralis and other insects of the Noctuidae family. This approach led us to consider the 65-kDa component as a minimum active part of a delta-endotoxin that is encoded by this new gene. Products of the two other cloned genes can be correlated with the 61- and 63-kDa components, respectively. Thus, in B. thuringiensis subsp. aizawai 7.29, multiple delta-endotoxin genes of different structural types direct the synthesis of several delta-endotoxins with different host specificities which were identified as components of the insecticidal crystals.

Isolation of Bacillus thuringiensis from Stored Tobacco and Lasioderma serricorne (F.)

Bacillus thuringiensis was isolated from dried tobacco residues and dead tobacco beetles (Lasioderma serricorne (F.); Coleoptera: Anobiidae) collected in a large number of locations worldwide. Eighty-eight samples of stored tobacco were analyzed and yielded 78 B. thuringiensis strains which were characterized on the basis of parasporal crystal morphology, sodium dodecyl sulfate-polyacrylamide gel electrophoresis profiles, and the results of an immunoblot analysis of the insecticidal crystal proteins. Flagellar antigen identification was used to differentiate selected isolates. Strains that produced rhomboidal crystals associated with the Coleoptera-specific pathotype (Cry III group) were the most abundant strains (59% of the isolates). Preliminary toxicity assays were performed with L. serricorne larvae, and the results suggested that activity is not restricted to isolates related to the Coleoptera-specific group. The results of our survey indicate that B. thuringiensis is part of the natural microflora in the stored-tobacco environment and that this special habitat represents a source of B. thuringiensis isolates that may be used to control stored-product pests.

Plant toxic proteins with insecticidal properties. A review on their potentialities as bioinsecticides

To meet the demands for food of the expanding world population, there is need of new ways for protecting plant crops against predators and pathogens while avoiding the use of environmentally aggressive chemicals. A milestone in this field was the introduction into crop plants of genes expressing Bacillus thuringiensis entomotoxic proteins. In spite of the success of this new technology, however, there are difficulties for acceptance of these 'anti-natural' products by the consumers and some concerns about its biosafety in mammals. An alternative could be exploring the plant's own defense mechanisms, by manipulating the expression of their endogenous defense proteins, or introducing an insect control gene derived from another plant. This review deals with the biochemical features and mechanisms of actions of plant proteins supposedly involved in defense mechanisms against insects, including lectins, ribosome-inactivating proteins, enzymes inhibitors, arcelins, chitinases, ureases, and modified storage proteins. The potentialities of genetic engineering of plants with increased resistance to insect predation relying on the repertoire of genes found in plants are also discussed. Several different genes encoding plant entomotoxic proteins have been introduced into crop genomes and many of these insect resistant plants are now being tested in field conditions or awaiting commercialization.

Characterization study of the sporulation kinetics of Bacillus thuringiensis

A wild-type and an rDNA strain of Bacillus thuringiensis were cultured in a net-draft-tube modified 20-L airlift bioreactor. A comparison of the sporulation patterns suggests that the early sporulation strain has a lower final spore count. Results from off-gas analysis suggests that the CO(2) profile could be an alternative indication to spore counts for the examination of fermentation performance or even the mortality in bioassay of the cultivation product. The difference in mortality tests exhibited by the microorganism was attributed to different patterns of sporulation as well as different levels of gene control inside the cell itself. The sporulation kinetics of B. thuringiensis was simulated by a simple modified Hill equation, where the initial glucose concentration could affect the timing of the onset of sporulation. The equation matches well with the experimental sporulation data for B. thuringiensis in both wild-type and rDNA strains.

Expression of chitinase-encoding genes from Aeromonas hydrophila and Pseudomonas maltophilia in Bacillus thuringiensis subsp. israelensis

Fifty isolates of chitinase (Cts)-producing bacteria were collected from soil samples and tested for their ability to degrade chitin using colloidal chitin agar as the primary plating medium. The results indicated that three isolates could degrade chitin at high pH. Further studies also demonstrated that crude Cts preparations from Bacillus circulans (Bc) No. 4.1 could enhance the toxicity of Bacillus thuringiensis subsp. kurstaki (Bt-k) toward diamondback moth larvae. Thus, it might be useful to increase the toxicity of B. thuringiensis (Bt) toward target insects by introducing a Cts-encoding gene (cts) into Bt. To investigate the expression of cts in Bt, cloned cts from Aeromonas hydrophila (pHYA1) and Pseudomonas maltophilia (pHYB1, pHYB2 and pHYB3) were cloned into the shuttle vector pHY300PLK and transformed into Escherichia coli DH5 alpha using 4-methylumbelliferyl beta-D-N,N'-diacetylchitobioside (4-MUF GlcNAc) as the detecting substrate. The four plasmids were then introduced into B. thuringiensis subsp. israelensis (Bt-i) strain c4Q272 by electroporation. Various transformants harboring cloned cts were selected, and expression and stability of the plasmids in Bt were studied.

Identification of novel cry-type genes from Bacillus thuringiensis strains on the basis of restriction fragment length polymorphism of the PCR-amplified DNA

Two pairs of universal oligonucleotide primers were designed to probe the most conserved regions of all known cryI-type gene sequences so that the amplified PCR fragments of the DNA template from Bacillus thuringiensis strains may contain all possible cryI-type gene sequences. The restriction fragment length polymorphism (RFLP) patterns of the PCR-amplified fragments revealed that 14 distinct cry-type genes have been identified from 20 B. thuringiensis strains. Those cry-type genes included cryIA(a), cryIA(a), cryIA(b), cryIA(b), cryIA(c), cryIB, cryIC, cryIC, cryIC(b), cryID, cryIE, cryIF, cryIF, and cryIII (a dagger at the end of a gene designation indicates a novel cry-type gene determined by restriction mapping or DNA sequences). Among them, the sequences of cryIA(a), cryIA(b), cryIB, cryIC, cryIF, and cryIII were found to be different from the corresponding published cry gene sequences. Interestingly, five cry-type genes [cryIA(a)-, cryIB-, cryIC-, cryIC(b)-, and cryIF-type genes] and seven cry-type genes [cryIA(a)-, cryIA(b)-, cryIB-, cryIC-, cryIC(b)-, cryIF-, and cryIII-type genes] have been detected from B. thuringiensis subsp. morrisoni HD-12 and B. thuringiensis subsp. wuhanensis, respectively. Therefore, the PCR-RFLP typing system is a facile method to detect both known and novel cry genes existing in B. thuringiensis strains.

Comparative analysis of the 16S to 23S ribosomal intergenic spacer sequences of Bacillus thuringiensis strains and subspecies and of closely related species

Bacillus thuringiensis spacer regions between the 16S and 23S rRNAs were amplified with conserved primers, designated 19-mer and 23-mer primers. A spacer region of 144 bp was determined for all of 6 B. thuringiensis strains, 7 B. thuringiensis subspecies, and 11 B. thuringiensis field isolates, as well as for the closely related species Bacillus cereus and Bacillus anthracis. Computer analysis and alignment of nucleotide sequences identified three mutations and one deletion in the intergenic spacer region (ISR) of B. thuringiensis subsp. kurstaki HD-1 when compared with ISR sequences from other subspecies. The same differences were identified between the ISR of B. thuringiensis strains and the ISR of B. cereus and B. anthracis. These minor differences do not seem to be sufficient to allow the design of a species-specific oligonucleotide probe.

Structural stability of Bacillus thuringiensis delta-endotoxin homolog-scanning mutants determined by susceptibility to proteases

Forty homolog-scanning (double-reciprocal-crossover) mutant proteins of two Bacillus thuringiensis delta-endotoxin genes (cryIAa and cryIAc) were examined for potential structural alterations by a series of proteolytic assays. Three groups of mutants could be identified. Group 1, consisting of 13 mutants, showed no delta-endotoxin present during overexpression conditions in Escherichia coli (48 h at 37 degrees C, with a ptac promoter). These mutants produced full-sized delta-endotoxin detectable by polyacrylamide gel electrophoresis with Coomassie blue staining or Western immunoanalysis after 24 h of growth but not after 48 h, suggesting sensitivity to intracellular proteases. Group 2 consisted of 13 mutants that produced stable delta-endotoxins that were completely digested by 2% bovine trypsin. In contrast, native delta-endotoxin produces a 65,000-Da trypsin-resistant peptide, which is the active toxin. Group 3 mutants expressed delta-endotoxin and trypsin-stable toxins, similar to the wild type. In this study, 12 group 3 mutant toxins were compared with wild type toxins by thermolysin digestion at a range of temperatures. The two wild-type toxins exhibited significant differences in thermolysin digestion midpoints. Among the group 3 mutants, most possessed significantly different protein stabilities relative to their parental toxins. Two of the group 3 mutants were observed to have exchanged the thermolysin sensitivity properties of the parental toxins.

Multiplex polymerase chain reaction for detection and differentiation of the microbial insecticide Bacillus thuringiensis

A rapid identification of Bacillus thuringiensis strains was established by using multiplex polymerase chain reaction (PCR). Primers of high homology specific to regions within genes encoding three major classes of B. thuringiensis crystal proteins were used to generate a PCR product profile characteristic of each strain of B. thuringiensis subsp. kurstaki. Differentiation among these strains was made on the basis of the electrophoretic pattern of the PCR products. Known B. thuringiensis subsp. kurstaki strains as well as unidentified strains isolated from insect cadavers were analyzed by PCR. Small amounts of crude sample lysates were assayed in a two-step PCR containing five primers capable of distinguishing between the strains giving products of 1,500, 858, and 653 bp for the CryIA(a) CryIA(b), and CryIA(c) genes, respectively. The method can be applied to rapidly detect the strains of B. thuringiensis subsp. kurstaki in commercial formulations and in the field.

Bacillus thuringiensis crystal protein: effect of chemical modification of the cysteine and lysine residues

The 16 cysteine residues of reduced protoxin from Bacillus thuringiensis subsp. kurstaki HD-73 can be quantitatively reacted with: (a) iodoacetic acid, to give carboxymethyl protoxin; (b) iodoacetamide, giving carbaminomethyl protoxin and (c) N-(beta-iodoethyl)trifluoroacetamide to give aminoethyl protoxin. The carboxymethyl derivative was found to be significantly more soluble at neutral pH values where both the native protoxin and the carbaminomethyl derivative exhibit low solubilities. At the alkaline pH values (pH 9.5-10.5) normally used to solubilize the crystal protein, the native protein was slightly more soluble than either the carboxymethyl or the carbaminomethyl derivatives. The aminoethyl derivative had an extremely low solubility at all pH values. Succinic anhydride reacted with only 35% of the lysine residues in both the carboxymethyl and the carbaminomethyl protoxin derivatives. Nonetheless, these succinylated protoxins exhibited significantly increased solubilities at neutral pH values. All the derivatives were found to retain full insecticidal activity toward spruce budworm (Choristeneura fufimerana) larvae. It is concluded that all the cysteine residues and modified lysine residues are on the surface of the protein and that derivatization does not alter the conformation of the solubilized protoxin.

Cloning and characterization of two plasmids from Bacillus thuringiensis in Bacillus subtilis

Bacillus thuringiensis subspecies israliensis plasmids pTX14-1 and pTX14-3 were cloned and analyzed by Southern blot hybridization for their replication mechanism in Bacillus subtilis. The cloning of pTX14-1 into the replicon deficient vector pBOE335 showed the usual characteristics of single-stranded DNA plasmids, i.e., it generated circular single-stranded DNA and high molecular weight (HMW) multimers. The other plasmid, pTX14-3, behaved differently; it generated neither single-stranded DNA nor HMW multimers. Treatment with rifampicin did not result in the accumulation of single-stranded DNA. However, deletion of an EcoRI-PstI fragment resulted in the accumulation of both single-stranded DNA and HMW multimers. From various deletion derivatives, we have mapped the minus origin and the locus responsible for suppression of HMW multimer formation. Full activity of the minus origin and of the locus suppressing HMW formation was only observed on the native replicon, indicating a coupling to the plus strand synthesis.

A structured model for vegetative growth and sporulation in Bacillus thuringiensis

A mathematical model has been developed for the delta-endotoxin producing Bacillus thuringiensis. The structure of the model involves the processes taking place during vegetative growth, those leading to the initiation of sporulation under conditions of carbon and/or nitrogen limitation, and the sporulation events. The key features in the model are the pools of compounds, such as PRPP, IMP, ADP/ATP, GDP/GTP, pyrimidine nucleotides, NAD/NADH2, amino acids, nucleic acids, cell wall, and vegetative and sporulation proteins. These, along with sigma-factors that control the nature of RNA-polymerase during the different phases, effectively stimulate the vegetative growth and sporulation. The initiation of sporulation is controlled by the intracellular concentration of GTP. Results of simulation of vegetative growth, initiation of sporulation, spore protein formation, and production of delta-endotoxin under C- or N-limitation are presented.

In vivo generation of hybrids between two Bacillus thuringiensis insect-toxin-encoding genes

The parasporal crystal of Bacillus thuringiensis is composed of polypeptides highly toxic to a number of insect larvae. The structural genes (cryIA) encoding the Lepidoptera-specific toxin from different bacterial strains diverge primarily in a single hypervariable region, whereas the N-terminal and C-terminal parts of the proteins are highly conserved. In this report, we describe the generation of hybrid genes between two cryIA genes. Two truncated cryIA genes were cloned in a plasmid vector in such way as to have only the hypervariable region in common. The two truncated cryIA genes were separated by the tetracycline-resistance determinant (or part of it). In vivo recombination between the hypervariable regions of the cryIA genes reconstituted an entire hybrid cryIA gene. Direct sequence analysis of 17 recombinant plasmids identified eleven different crossover regions which did not alter the reading frame and allowed the production of eight different hybrid proteins. The recombination events were independent from the RecA function of Escherichia coli. Some of the hybrid gene products were more specific in their insecticidal action and one had acquired a new biological activity.

Folding and unfolding of the protoxin from Bacillus thuringiensis: evidence that the toxic moiety is present in an active conformation

The action of trypsin or papain on the 130-kDa crystal protein (protoxin) from Bacillus thuringiensis subsp. kurstaki HD-73 yields a 67-kDa proteinase-resistant toxic fragment (toxin) which is derived from the N-terminal half of the molecule. Sensitivity to proteolysis and fluorescence emission spectroscopy showed that the toxin unfolded to a much greater extent in 6 M guanidinium chloride (GuHCl) than in 8 M urea. Protoxin also unfolded extensively in 6 M GuHCl, whereas in 8 M urea only the C-terminal half of the molecule had unfolded extensively. Both unfolded protoxin and unfolded toxin refolded to their native and biologically active conformations. The biphasic unfolding observed for protoxin suggests that the C-terminal half of the molecule unfolded rapidly, whereas the N-terminal toxic moiety unfolded at a much slower rate, similar to that of the free 67-kDa toxin. A 67-kDa fragment, derived from the N-terminal half of the molecule, could be generated from the protoxin in the presence of either urea or GuHCl by treatment with proteinases. Compared to toxin in denaturants, this fragment was found to be more sensitive to proteolysis. However, on removal of the denaturants the fragment had the same proteinase resistance and cytolytic activity as native toxin. The increased proteinase sensitivity of the fragment generated in the presence of denaturants appears to be due to a perturbation in the conformation of the N-terminal toxic moiety. This perturbation is attributed to the unfolding of the C-terminal region of the protoxin prior to its proteolysis to yield the 67-kDa fragment.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative analysis of the individual protoxin components in P1 crystals of Bacillus thuringiensis subsp. kurstaki isolates NRD-12 and HD-1

Two commercially important strains (NRD-12 and HD-1) of the entomopathogenic bacterium Bacillus thuringiensis subsp. kurstaki each contain three genes of partially identical sequence coding for three classes of 130-135 kDa protoxins (termed the 4.5, 5.3 and 6.6 protoxins) that display toxicity towards various lepidopteran larvae. These gene products combine to form the intracellular bipyramidal P1 crystal. Each of the genes from both strains was cloned and expressed in Escherichia coli. Analysis of the cloned genes at the restriction-endonuclease level revealed no detectable differences among genes within a particular gene class. The composition of the P1 crystal from both strains was quantitatively analysed by CNBr cleavage of the purified P1 crystal, with the purified recombinant gene products as reference proteins. Independent verification of the presence of high 6.6-protoxin gene product in the P1 crystal was provided by a rapid in vitro lawn cell toxicity assay directed against a Choristoneura fumiferana (CF-1) insect cell line. The results indicate that, although all three gene products are represented within the P1 crystal of either NRD-12 or HD-1, only the contents of the 4.5 and 5.3 protoxins vary between the two crystals, whereas the 6.6 protoxin contents are similar and represent approximately one-third of the P1 crystal in either strain.

Unusual proteolysis of the protoxin and toxin from Bacillus thuringiensis. Structural implications

Trypsin is shown to generate an insecticidal toxin from the 130-kDa protoxin of Bacillus thuringiensis subsp. kurstaki HD-73 by an unusual proteolytic process. Seven specific cleavages are shown to occur in an ordered sequence starting at the C-terminus of the protoxin and proceeding toward the N-terminal region. At each step, C-terminal fragments of approximately 10 kDa are produced and rapidly proteolyzed to small peptides. The sequential proteolysis ends with a 67-kDa toxin which is resistant to further proteolysis. However, the toxin could be specifically split into two fragments by proteinases as it unfolded under denaturing conditions. Papain cleaved the toxin at glycine 327 to give a 34.5-kDa N-terminal fragment and a 32.3-kDa C-terminal fragment. Similar fragments could be generated by elastase and trypsin. The N-terminal fragment corresponds to the conserved N-terminal domain predicted from the gene-deduced sequence analysis of toxins from various subspecies of B. thuringiensis, and the C-terminal fragment is the predicted hypervariable sequence domain. A double-peaked transition was observed for the toxin by differential scanning calorimetry, consistent with two or more independent folding domains. It is concluded that the N- and C-terminal regions of the protoxin are two multidomain regions which give unique structural and biological properties to the molecule.

Characterization of the cysteine residues and disulphide linkages in the protein crystal of Bacillus thuringiensis

Bacillus thuringiensis produces a 130-140 kDa insecticidal protein in the form of a bipyramidal crystal. The protein in the crystals from the subspecies kurstaki HD-1 and entomocidus was found to contain 16-18 cysteine residues per molecule, present primarily in the disulphide form as cystine. Evidence that all the cysteine residues form symmetrical interchain disulphide linkages in the protein crystal was obtained from the following results: (i) the disulphide diagonal procedure [Brown & Hartley (1966) Biochem. J. 101, 214-228] gave only unpaired cysteic acid peptides in diagonal maps; (ii) the disulphide bridges were shown to be labile in dilute alkali and the crystal protein could be released quantitatively with 1 mM-2-mercaptoethanol; (iii) the thiol groups of the released crystal protein were shown by competitive labelling [Kaplan, Stevenson & Hartley (1971) Biochem. J. 124, 289-299] to have the same chemical properties as exposed groups on the surface of the protein; (iv) the thiol groups in the released crystal protein reacted quantitatively with iodoacetate or iodoacetamide. The finding that all the disulphide linkages in the protein crystal are interchain and symmetrical accounts for its alkali-lability and for the high degree of conservation in the primary structure of the cystine-containing regions of the protein from various subspecies.

Secondary structure of the entomocidal toxin from Bacillus thuringiensis subsp. kurstaki HD-73

The secondary structure of the toxin from Bacillus thuringiensis subsp. kurstaki (Btk) HD-73 was estimated by Raman, infrared, and circular dichroism spectroscopy, and by predictive methods. Circular dichroism and infrared spectroscopy gave an estimate of 33-40% alpha-helix, whereas Raman and predictive methods gave approximately 20%. Raman and circular dichroism spectra, as well as predictive methods, indicated that the toxin contains 32-40% beta-sheet structure, whereas infrared spectroscopy gave a slightly lower estimate. Thus, all of these approaches are in agreement that the native conformation of Btk HD-73 toxin is highly folded and contains considerable amounts of both alpha-helical and beta-sheet structures. No significant differences were detected in the secondary structure of the toxin either in solution or as a hydrated pellet.

Characterization of the parasporal inclusion of Bacillus thuringiensis subsp. kyushuensis

Electron microscopy of Bacillus thuringiensis subsp. kyushuensis revealed that the parasporal inclusions are composed of a homogeneous center surrounded by a thick, electron-dense coating. Antibodies directed against the 135- and 65-kilodalton B. thuringiensis subsp. israelensis peptides cross-reacted with the 70- and 26-kilodalton peptides, respectively, of B. thuringiensis subsp. kyushuensis.

Transformation of Bacillus cereus vegetative cells by electroporation

Transformation of untreated vegetative cells of Bacillus cereus 569 with plasmid pC194 (1.8 megadaltons) by high-voltage electroporation resulted in a maximum of 2 x 10(-5) transformants per viable cell. Transformation of a 130-megadalton plasmid occurred at a comparable frequency. The method was simple, rapid, and yielded transformant colonies in 14 to 24 h. Transformation was obtained with unpurified total plasmid DNA.

Expression of a cloned Bacillus thuringiensis delta-endotoxin gene in Bacillus subtilis

The entire coding region of the Bacillus thurigiensis HD73 crystal protein gene was subcloned from plasmid pJWK20 into the integration vector pUG2-15. This plasmid expresses chloramphenicol resistance when integrated into the Bacillus subtilis chromosome in the outH locus near the recE region. The correct molecular organization of the integrated plasmid was verified by hybridization to Southern blots of chromosomal DNA digests. Production of the toxic crystal protein was monitored at different time points during the life cycle of B. subtilis. Toxicity assays against Anagasta (Ephestia) larvae, direct electron microscopy crystal detection, and immunoblotting assays proved that the expression of the gene in B. subtilis is time regulated and restricted mainly to the sporulation stage. RNase protection experiments defined the transcription initiation start point and the transcription timing. All tests were made in a strain containing one to three copies of the integrated plasmid and in a strain subjected to an amplification regimen.

Tn916-dependent conjugal transfer of PC194 and PUB110 from Bacillus subtilis into Bacillus thuringiensis subsp. israelensis

The Staphylococcus aureus plasmids pC194 and pUB110 were introduced into Bacillus thuringiensis subsp. israelensis by using the Streptococcus faecalis transposon Tn916 as a mobilizing agent. Plasmid transfer occurred only when B. thuringiensis subsp. israelensis was mated with a B. subtilis donor that contained both pC194 and pUB110 and Tn916; plasmid transfer was not observed in the absence of the transposon. B. thuringiensis transconjugants resistant to chloramphenicol (Cmr) and tetracycline (Tetr) were detected at a frequency of 1.96 x 10(-6) per recipient cell, whereas the Tetr phenotype, but not the Cmr, was observed at a frequency of 1.09 x 10(-4). The converse, Cmr but not Tetr, was observed at a frequency of 2.94 X 10(-5). The transfer of pUB110 from B. subtilis to B. thuringiensis subsp. israelensis was observed at a frequency of 3.0 x 10(-6) per recipient cell but concomitant transfer of pUB110 and Tn916 was not observed. Mobilization of plasmid pE194 was not observed under these conditions. Transconjugants were detected in filter matings only, not in broth. The Tn916 phenotype was maintained during serial passage of B. thuringiensis without selection, whereas the pC194 phenotype was not. Unlike pC194, however, pUB110 remained stable in B. thuringiensis during several passages through nonselective medium. Southern hybridization analysis demonstrated that Tn916 had inserted into several different sites on the B. thuringiensis chromosome and that pC194 and pUB110 were maintained as an autonomous plasmid.

Introduction of the Streptococcus faecalis transposon Tn916 into Bacillus thuringiensis subsp. israelensis

The conjugative Streptococcus faecalis transposon Tn916 was introduced into Bacillus thuringiensis subsp. israelensis by filter matings with S. faecalis. B. thuringiensis transconjugants resistant to tetracycline (Tetr) were detected at a frequency of approximately 7.0 X 10(-7) per recipient cell during filter matings, whereas transfer of Tn916 was not observed in broth matings. The Tetr phenotype in subsp. israelensis was stable in the absence of antibiotic selection. Southern hybridization analysis revealed that Tn916 had inserted into several different sites on the B. thuringiensis subsp. israelensis chromosome but insertion into plasmid DNA was not observed. Movement of Tn916 was demonstrated when Tetr B. thuringiensis transconjugants were mated with isogenic recipients. Southern hybridizations, however, showed that the resulting Tetr isolates contained Tn916 junction fragments that were nearly identical to the donor, suggesting that this movement resulted from transfer of chromosomal DNA from donor to recipient or from a fusion of mating cells, rather than conjugative transposition of the Tn element.

Bacillus thuringiensis insecticidal crystal toxins: Gene structure and mode of action

Thanks to the techniques of recombinant DNA, there is now abundant sequence information on several endotoxin genes of Bacillus thuringiensis. The task of correlating this sequence information with the economically important aspects of the toxins such as insect specificity, LD(50) and speed of kill is now under worldwide investigation. Progress has also been made on understanding the mechanism of action of the toxins and on identifying the parts of the protoxin which are important in toxicity. Taken together, the mechanistic data and the sequence information allow the first attempts at rational design of mutant endotoxin genes and greatly facilitate the transfer of those genes to other organisms such as plants. More information is still needed, however, as to the nature of the binding site of the toxin and on the three-dimensional structure of the activated toxins.