Transcriptional regulation of Bacillus thuringiensis subsp. israelensis mosquito larvicidal crystal protein gene cryIVA

Contributions of 5'-UTR and 3'-UTR cis elements to Cyt1Aa synthesis in Bacillus thuringiensis subsp. israelensis

The biopesticide used most effectively to control mosquito and blackfly vectors of human diseases worldwide is Bacillus thuringiensis subsp. israelensis. The high efficacy of this bacterium is due to synergistic interactions among four protein entomotoxins assembled individually into a single parasporal body (PB) during sporulation. Cyt1Aa, the primary synergist, is the most abundant toxin, comprising approximately 55% of the PB's mass. The other proteins are Cry11Aa at ∼35%, and Cry4Aa and Cry4Ba, which together account for the remaining ∼10%. The molecular genetic basis for the comparatively large amount of Cyt1Aa synthesized is unknown. Here, in addition to the known strong BtI (σ^E) and BtII (σ^K) promoters, we demonstrate a third promoter (BtIII) that has high identity to the σ^E promoter of Bacillus subtilis, contributes to the large amount of Cyt1Aa synthesized. We also show that a cyt1Aa-BtIII construct was not functional in a σ^E-deficient strain of B. subtilis. Comparison of transcription levels and protein profiles for recombinant strains containing different combinations of BtI, BtII and BtIII, or each promoter alone, showed that BtIII is active throughout sporulation. We further demonstrate that a stable stem-loop in the 3'-untranslated region (3'-UTR, predicted ΔG=-27.6) contributes to the high level of Cyt1Aa synthesized.

Screening of cry-type promoters with strong activity and application in Cry protein encapsulation in a sigK mutant

To optimize the expression of cry genes in a Bacillus thuringiensis sigK mutant failing in crystal releasing, the transcriptional activity of the cry promoters cry1A, cry3A, cry4A, and cry8E was compared using lacZ gene fusions. A beta-galactosidase assay indicated that the cry8E promoter showed the highest transcriptional activity. A novel Escherichia coli-B. thuringiensis shuttle vector pHT315-8E21b was constructed for cry gene expression using the cry8E promoter and the multiple cloning sites from vector pET21b, based on vector pHT315. SDS-PAGE analysis showed that the expression of the cry1Ac gene directed by the cry8E promoter was increased by approximately 2.4-fold over the expression directed by the cry3A promoter. The cry1Ba gene was expressed in the sigK mutant with the constructed vector pHT315-8E21b. Normal bipyramidal crystals encapsulated in mother cell were observed by transmission electron microscopy (TEM). The encapsulated Cry1Ba protein expressed in the sigK mutant showed activity against Ostrinia furnacalis and Plutella xylostella similar to that of the released Cry1Ba protein expressed in the acrystalliferous strain HD73 and can be protected from inactivation by UV light. All these results suggest that the cry8E promoter can be an efficient transcriptional element for cry gene expression in sigK mutants and can be utilized for the construction of a genetically engineered strain.

Enhanced Cry1Da production in Bacillus thuringiensis by driving expression from the σ(E) -dependent BtI promoter

AIMS

To increase the Cry1Da production in Bacillus thuringiensis by enhancing BtI promoter activity and fusion with upstream sequence from cry1Ab.

METHODS AND RESULTS

The effects of joining the upstream sequence of cry1Ab that contains E2 subunit pyruvate dehydrogenase (PDH) recognition site to the cry1Da promoter as well as the effects of substitution mutation of conserved sequences of its BtI promoter on cry1Da expression was monitored by constructing cry1Da promoter-lacZ fusions. Changing the -35 region of the cry1Da BtI promoter to that of cry1Ab enhanced β-galactosidase activity about three fold as comparing to that of the wild-type promoter with its own upstream sequence. In contrast, the same cry1Da mutated promoter linked to the above upstream sequence of cry1Ab enhanced enzyme activity up to seven fold, but was five fold lower than that of the full-length cry1Ab promoter. The cry1Ab-cry1Da hybrid promoter with the -35 BtI mutation efficiently increased Cry1Da synthesis by 133% and resulted in a 2·3-fold increase in insect larval toxicity when comparing to the wild type.

CONCLUSIONS

The cry1Ab promoter as well as mutation of -35 region of BtI promoter together with fusion with E2 subunit PDH recognition site efficiently enhanced Cry1Da production in B. thuringiensis.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results provide useful information to construct an efficient cry1Da gene expression in B. thuringiensis.

Bacillus thuringiensis: a genomics and proteomics perspective

Bacillus thuringiensis (Bt) is a unique bacterium in that it shares a common place with a number of chemical compounds which are used commercially to control insects important to agriculture and public health. Although other bacteria, including B. popilliae and B. sphaericus, are used as microbial insecticides, their spectrum of insecticidal activity is quite limited compared to Bt. Importantly, Bt is safe for humans and is the most widely used environmentally compatible biopesticide worldwide. Furthermore, insecticidal Bt genes have been incorporated into several major crops, rendering them insect resistant, and thus providing a model for genetic engineering in agriculture.This review highlights what the authors consider the most relevant issues and topics pertaining to the genomics and proteomics of Bt. At least one of the authors (L.A.B.) has spent most of his professional life studying different aspects of this bacterium with the goal in mind of determining the mechanism(s) by which it kills insects. The other authors have a much shorter experience with Bt but their intellect and personal insight have greatly enriched our understanding of what makes Bt distinctive in the microbial world. Obviously, there is personal interest and bias reflected in this article notwithstanding oversight of a number of published studies. This review contains some material not published elsewhere although several ideas and concepts were developed from a broad base of scientific literature up to 2010.

Identification of a catabolite-responsive element necessary for regulation of the cry4A gene of Bacillus thuringiensis subsp. israelensis

Bacillus thuringiensis subsp. israelensis produces a potent mosquitocidal protein, Cry4A. We have identified a 15-bp catabolite responsive element (cre), overlapping the -35 element of the cry4A promoter. Changing a guanine to adenine at position -49 in the promoter abolished glucose catabolite repression of cry4A and enhanced promoter activity two- to threefold. This cis regulatory element is essential for controlled toxin synthesis, vital to evolutionary success of B. thuringiensis subsp. israelensis.

Transcriptional analysis of the toxin-coding plasmid pBtoxis from Bacillus thuringiensis subsp. israelensis

In Bacillus thuringiensis subsp. israelensis all of the insecticidal toxins are encoded on a single, large plasmid, pBtoxis. Sequencing of this plasmid revealed 125 potential coding sequences, many of which have predicted functions in gene regulation and physiological processes, such as germination. As a first step in understanding the possible role of pBtoxis in its host bacterium, a survey of the transcription of genes with predicted functions was carried out. Whereas many coding sequences, including those previously identified as probable pseudogenes, were not transcribed, mRNA was detected for 29 of the 40 sequences surveyed. Several of these sequences, including eight with similarities to the sequences of known transcriptional regulators, may influence wider gene regulation and thus may alter the phenotype of the host bacterium.

Identification of a promoter for the vegetative insecticidal protein-encoding gene vip3LB from Bacillus thuringiensis

The expression of vip3LB, one of the vegetative insecticidal protein-encoding genes of Bacillus thuringiensis, was studied at the transcriptional level. By primer extension analysis, we have identified, for the first time, the transcription start point of vip3-type gene. Upstream from vip3LB transcription start point, was found a nucleotide sequence partially homologous to the consensus sequence for the E sigma(E) holoenzyme of B. subtilis. Thus, it was strongly suggested that the identified vip3 promoter was under the control of sigma(35)-like enzyme, the B. thuringiensis homolog of sigma(E). The transcriptional activity from the promoter was detected at the vegetative stage of growth starting at mid-log phase as well as during the middle stage of sporulation.

Complete sequence and organization of pBtoxis, the toxin-coding plasmid of Bacillus thuringiensis subsp. israelensis

The entire 127,923-bp sequence of the toxin-encoding plasmid pBtoxis from Bacillus thuringiensis subsp. israelensis is presented and analyzed. In addition to the four known Cry and two known Cyt toxins, a third Cyt-type sequence was found with an additional C-terminal domain previously unseen in such proteins. Many plasmid-encoded genes could be involved in several functions other than toxin production. The most striking of these are several genes potentially affecting host sporulation and germination and a set of genes for the production and export of a peptide antibiotic.

Loss of catabolite repression function of HPr, the phosphocarrier protein of the bacterial phosphotransferase system, affects expression of the cry4A toxin gene in Bacillus thuringiensis subsp. israelensis

HPr, the phosphocarrier protein of the bacterial phosphotransferase system, mediates catabolite repression of a number of operons in gram-positive bacteria. In order to participate in the regulatory process, HPr is activated by phosphorylation of a conserved serine-46 residue. To study the potential role of HPr in the regulation of Cry4A protoxin synthesis in Bacillus thuringiensis subsp. israelensis, we produced a catabolite repression-negative mutant by replacing the wild-type copy of the ptsH gene with a mutated copy in which the conserved serine residue of HPr was replaced with an alanine. HPr isolated from the mutant strain was not phosphorylated at Ser-45 by HPr kinase, but phosphorylation at His-14 was found to occur normally. The enzyme I and HPr kinase activities of the mutant were not affected. Analysis of the B. thuringiensis subsp. israelensis mutant harboring ptsH-S45A in the chromosome showed that cry4A expression was derepressed from the inhibitory effect of glucose. The mutant strain produced both cry4A and sigma(35) gene transcripts 4 h ahead of the parent strain, but there was no effect on sigma(28) synthesis. In wild-type B. thuringiensis subsp. israelensis cells, cry4A mRNA was observed from 12 h onwards, while in the mutant it appeared at 8 h and was produced for a longer period. The total amount of cry4A transcripts produced by the mutant was higher than by the parent strain. There was a 60 to 70% reduction in the sporulation efficiency of the mutant B. thuringiensis subsp. israelensis strain compared to the wild-type strain.

Identification of a promoter for the crystal protein-encoding gene cry1Ia from Bacillus thuringiensis subsp. kurstaki

Expression of cry1Ia, one of the insecticidal protein genes of Bacillus thuringiensis subsp. kurstaki, was studied at the transcriptional level. By primer extension analysis, we have identified for the first time, the transcription start point of cry1Ia. Upstream from the cry1Ia transcription start point, was found a nucleotide sequence partially homologous to the consensus sequence for the E sigma(E) holoenzyme of Bacillus subtilis. Thus, it was strongly suggested that the identified cry1Ia promoter was under the control of sigma(35), the B. thuringiensis homolog of sigma(E). The transcriptional activity from the promoter was detected only for the sporulating cells at T2 and T5 stages.

Transcription of the insecticidal crystal protein genes of Bacillus thuringiensis

Production of a large amount of insecticidal crystal proteins encoded on large plasmids is largely dependent upon the mother cell, Bacillus thuringiensis (B. thuringiensis, also Bt), specific transcription systems attributable to sporulation. In the middle stages of sporulation, cry4A is most actively transcribed from the promoter cry4A-P1. The proximal transcriptional start point of cry4A, which is under the control of the promoter P1, is used in Bacillus subtilis (B. subtilis) in the middle stage of sporulation. The nucleotide sequence that determines the cry4A-P1 promoter is homologous to the consensus sequence for the promoter of sigma E-specific genes in B. subtilis, and to those promoters of the insecticidal protein genes that are efficiently transcribed in vitro with the RNA polymerase E sigma 35 isolated from B. thuringiensis. The sigma factor sigma 35 of B. thuringiensis is highly homologous and functionally equivalent to sigma E of B. subtilis. These results suggest that the cry4A transcription from P1 is under the control of sigma E in B. subtilis, and under the control of sigma 35 in B. thuringiensis.

Inorganic phosphate regulates CryIVA protoxin expression in Bacillus thuringiensis israelensis

The role of nutritional factors during CryIVA protoxin expression in Bacillus thuringiensis israelensis (Bti) has been investigated. Inorganic phosphate (Pi) was found to stimulate 135 kD protoxin synthesis by Bti cells. There was a corresponding increase in the cryIVA specific mRNA in the presence of Pi. Inorganic phosphate inhibited HPr kinase but activated HPr phosphatase, the two enzymes responsible for regulating the concentration of phosphorylated HPr in the cell. Addition of protein phosphatase inhibitors NaF and calyculin A during resuspension resulted in the inhibition of toxin synthesis by Bti cells. Calyculin A inhibited HPr phosphatase activity in the in vitro assay also. The concentration of phosphorylated HPr was upregulated when the cells were resuspended in the presence of calyculin A, while the levels of the same were lowered in the presence of Pi, as determined by Western blotting the respective cells. The efficiency of sporulation of Bti was not affected when Pi was added alone or along with the phosphatase inhibitor calyculin A.

Modulation of Cry IV A toxin protein expression by glucose in Bacillus thuringiensis israelensis

Bacillus thuringiensis subsp. israelensis (Bti) produces Cry IV A protoxin protein as part of the insecticidal crystal toxin during sporulation. This study was conducted with the objective of identifying environmental signals which regulate toxin synthesis by Bti. Glucose was found to repress Cry IV A toxin induction at the mRNA level. The repressive effect of glucose was dependent on a phosphorylation step since protein kinase inhibitor calphostin c relieved the 130-kD protoxin synthesis at both the mRNA and protein level. Phosphorylation of HPr, the phosphocarrier protein of the phosphotransferase system, occurred during glucose repression of Cry IV A toxin synthesis in Bti cells was seen by Western blotting with anti-phosphoserine antibody and rabbit anti-HPr serum. Phosphorylation of HPr in vivo as well as in the in vitro assay was inhibited by calphostin c, a specific inhibitor of serine/threonine kinase. Calphostin c had no effect on sporulation efficiency of Bti cells.

Bacillus thuringiensis and its pesticidal crystal proteins

During the past decade the pesticidal bacterium Bacillus thuringiensis has been the subject of intensive research. These efforts have yielded considerable data about the complex relationships between the structure, mechanism of action, and genetics of the organism's pesticidal crystal proteins, and a coherent picture of these relationships is beginning to emerge. Other studies have focused on the ecological role of the B. thuringiensis crystal proteins, their performance in agricultural and other natural settings, and the evolution of resistance mechanisms in target pests. Armed with this knowledge base and with the tools of modern biotechnology, researchers are now reporting promising results in engineering more-useful toxins and formulations, in creating transgenic plants that express pesticidal activity, and in constructing integrated management strategies to insure that these products are utilized with maximum efficiency and benefit.

Bacillus popilliae cry18Aa operon is transcribed by sigmaE and sigmaK forms of RNA polymerase from a single initiation site

Bacillus popilliae is an obligate pathogen for larvae of the insect family Scarabaeidae (Coleoptera). It forms parasporal crystals upon sporulation. The gene cry18Aa coding for the parasporal crystal protein and an upstream open reading frame, orf1, were previously isolated from B.popilliae. Here we report an analysis of cry18Aa transcription in Bacillus thuringiensis. The only transcriptional start site of cry18Aa was found 29 bp upstream of the open reading frame orf1, suggesting that orf1 and cry18Aa are transcribed as an operon. lacZ fusion to the cry18Aa promoter was used to follow the time-course of cry18Aa transcription in wild type B.thuringiensis and in various B.thuringiensis sporulation-deficient mutants (spo0A, sigE or sigK). In wild type B.thuringiensis, the cry18Aa promoter was activated 2 h after the end of exponential growth and the expression lasted to the late sporulation phase. The results of promoter activity in Spo+or Spo-backgrounds together with the results of primer extension experiments suggest that the transcription from this promoter can be driven by both sigmaE and sigmaK types of RNA polymerase at a single start site. The promoter region of cry18Aa operon fits the consensus sequences of both sigmaE and sigmaK dependent promoters of Bacillus.

Spo0A represses transcription of the cry toxin genes in Bacillus thuringiensis

The DNA regions upstream from the genes encoding polypeptides of Bacillus thuringiensis subsp. israelensis larvicidal crystals (cry4A, cry4B, cry11A) contain sequences with similarities to the spo0A box of Bacillus subtilis (or '0A' box) and the promoter recognized by the sigma H-associated RNA polymerase of B. subtilis. Expression of cry-lacZ transcriptional fusions was analysed in various B. thuringiensis genetic backgrounds. The early transcription of the toxin genes was not sporulation-dependent, whereas the late-stage expression at t4-6 was sigma E-dependent. Primer extension analysis confirmed that the cry4- and cry11-type toxin genes were weakly transcribed during the transition phase; expression analysis of a cry11A'-lacZ transcriptional fusion in B. subtilis sporulation mutants confirmed the involvement of the sigma H-RNA polymerase. Primer extension analysis showed that in B. thuringiensis subsp. israelensis, the cry4A and cry11A gene transcription observed at the end of the growth stage was turned off at the beginning of the sporulation phase. The DNA region located upstream from the cry11A gene promoter including the putative '0A' box was deleted. This led to a derepression of the expression of the cry11A operon. These results suggest that the cry4A, cry4B and cry11A toxin genes of B. thuringiensis subsp. Israelensis are transcribed during the transition phase by the RNA polymerase associated with the sigma H factor and are subject to Spo0A repression.

Identification of a second transcriptional start site for the insecticidal protein gene cryIVA of Bacillus thuringiensis subsp. israelensis

Expression of cryIVA, one of the insecticidal protein genes of B. thuringiensis subsp. israelensis, is regulated at the transcriptional level. The cryIVA gene is specifically transcribed during the stationary phase of this bacterium. As shown in our previous report [Yoshisue et al. (1993a)], the transcription from the -364 position of the cryIVA gene is conducted by the major promoter P1 that is functional during middle stages of the stationary phase of B. thuringiensis. In the present study, we have identified a second transcriptional start point P2 for the cryIVA gene in addition to P1, the major transcriptional start point. The transcription from P2 of the cryIVA gene occurred later than that from P1, during later stages of stationary phase of B. thuringiensis subsp. israelensis. The -10 and -35 nt sequences upstream from P2 of cryIVA are similar to those of the omega 28-specific promoters of B. thuringiensis genes and of the omega K-specific promoters of B. subtilis genes. It is most likely that the region upstream from P2 of cryIVA contains the nt sequences that determine the omega 28-specific promoter, the second one, for the cryIVA gene.

Analysis of cryIAa expression in sigE and sigK mutants of Bacillus thuringiensis

The sigE and sigK genes, encoding the sporulation-specific sigma factors sigma 35 and sigma 28 of Bacillus thuringiensis, were each disrupted by inserting a gene conferring resistance to kanamycin into their coding sequences. The B. thuringiensis SigE- and sigK- mutant strains were blocked at different sporulation stages and were unable to sporulate. The SigE-strain was blocked at stage II of sporulation, whereas the SigK- strain was blocked at stage IV. The expression of a cryIAa'-'lacZ transcriptional fusion was analysed in these genetic backgrounds and it was found that both sigma factors are involved in the in vivo transcription of this gene. However, the SigK- strain harbouring the cryIAa gene produced amounts of toxin similar to those produced by the B. thuringiensis Spo+ strain. The toxins accumulated in the mother cell compartment to form a crystal inclusion which remained encapsulated within the cell wall. Thus, transcription from the sigma E-dependent promoter alone (Bt I promoter) is sufficient to support high levels of toxin production in B. thuringiensis.

Mosquito larvicidal activity of Escherichia coli with combinations of genes from Bacillus thuringiensis subsp. israelensis

The genes cryIVA and cryIVD, encoding 134- and 72-kDa proteins, respectively, and the gene for a regulatory 20-kDa polypeptide of Bacillus thuringiensis subsp. israelensis (serovar H14) were cloned in all seven possible combinations by the Escherichia coli expression vectors pT7 and pUHE. The four combinations containing cryIVA (cryIVA alone, with cryIVD, with the 20-kDa-protein gene, and with both) displayed high levels of mosquito larvicidal activity in pUHE. The toxicity of the combination of cryIVA and cryIVD, with or without the 20-kDa-protein gene, was higher than has ever been achieved with delta-endotoxin genes in recombinant E. coli. Fifty percent lethal concentrations against third-instar Aedes aegypti larvae for these clones decreased (i.e., toxicity increased) continuously to about 3 x 10(5) cells ml-1 after 4 h of induction. Larvicidal activities, obtained after 30 min of induction, were lower for clones in pT7 and decreased for an additional 3.5 h. Induction of either cryIVD or the 20-kDa-protein gene alone resulted in no larvicidal activity in either pT7 or pUHE20. Cloned together, these genes were slightly toxic in pT7 but not in pUHE20. Five minutes of induction of this combination (cryIVD with the 20-kDa-protein gene) in pT7 yielded a maximal mortality of about 40%, which decreased rapidly and disappeared completely after 50 min. CryIVD is thus apparently degraded in E. coli and partially stabilized by the 20-kDa regulatory protein. Larvicidal activity of the combination of cryIVA and cryIVD was sevenfold higher than that of cryIVA alone, probably because of the cross-stabilization of the polypeptides or the synergism between their activities.

Transcriptional regulation of the cryIVD gene operon from Bacillus thuringiensis subsp. israelensis

The CryIVD protein is involved in the overall toxicity of the Bacillus thuringiensis subsp. israelensis parasporal inclusions and is one of the four major components of the crystals. Determination of the DNA sequence indicated that the cryIVD gene is the second gene of an operon which includes three genes. The first one encodes a 19-kDa polypeptide and has sequence homology with the orf1 gene of the Bacillus thuringiensis cryIIA and cryIIC operons. The second and third genes have already been identified and encode the CryIVD crystal protein and the P20 polypeptide, respectively. The promoter region was located by deletion analysis, and the 5' end of the mRNA was determined by primer extension mapping. Transcription of the cryIVD gene in B. thuringiensis subsp. israelensis strains is induced 9 h after the beginning of sporulation. Sequence analysis indicated two potential promoters, a strong one and a weak one, recognized respectively by the RNA polymerase associated with the sigma 35 or the sigma 28 factor of B. thuringiensis (sigma E and sigma K of Bacillus subtilis, respectively). Transcriptional lacZ fusion integrated in single copy into the chromosome of various B. subtilis sporulation mutants confirmed the sigma E dependence of cryIVD gene transcription.

Expression of the genes for insecticidal crystal proteins in Bacillus thuringiensis: cryIVA, not cryIVB, is transcribed by RNA polymerase containing sigma H and that containing sigma E

To investigate the mechanism of transcriptional regulation of cryIVA and cryIVB, encoding 130-kDa dipteran-active crystal proteins, in Bacillus thuringiensis subsp. israelensis, we introduced each gene into several sporulation mutants of Bacillus subtilis. A spoIIG mutation, the wild-type gene of which encodes sigma E precursor, completely blocked the cryIVB transcription. In contrast, low but detectable transcription of cryIVA was observed in the spoIIG mutant. In the wild-type B. subtilis, no transcription of cryIVB was detected before T2 (2 h after the onset of stationary phase), while the cryIVA transcription started at the late exponential phase at low levels. Furthermore, in a wild-type strain of B. thuringiensis subsp. israelensis, transcription of cryIVA began earlier than that of genes encoding other crystal components, cryIVB and cytA. A consensus sequence recognized by an RNA polymerase containing sigma H of B. subtilis was found upstream of the transcription start point of cryIVA, which overlapped with that recognized by sigma E.

Cloning and characterization of a Bacillus thuringiensis homolog of the spoIIID gene from Bacillus subtilis

The SpoIIID protein of Bacillus subtilis (Bs) is a small DNA-binding protein that is essential for gene expression of the mother cell compartment during sporulation. We have cloned a DNA fragment from Bacillus thuringiensis (Bt) that showed a specific hybridization with the Bs spoIIID gene. Sequence analysis found an open reading frame encoding 90 amino acids (aa), which are 89% identical to the deduced aa sequence of Bs spoIIID. Upstream from the transcription start point (tsp), a nucleotide sequence highly homologous to the consensus sequence motif for the sigma 35-recognized promoters was found. Northern blot analysis has indicated that the expression of the gene is induced only at the midsporulation stage, and that the gene constitutes an operon with a downstream gene, mreB. The Bs strain carrying the spoIIID delta erm or spoIIID83 mutation completely restored sporulation ability upon introduction of the spoIIID homologous gene from Bt. These results strongly suggest that the gene we have cloned is a Bt homolog of spoIIID.

Expression in Bacillus subtilis of the Bacillus thuringiensis cryIIIA toxin gene is not dependent on a sporulation-specific sigma factor and is increased in a spo0A mutant

Expression of the Bacillus thuringiensis cryIIIA gene encoding a Coleoptera-specific toxin is weak during vegetative growth and is activated at the onset of the stationary phase. cryIIIA'-'lacZ fusions and primer extension analysis show that the regulation of cryIIIA expression is similar in Bacillus subtilis and in B. thuringiensis. Activation of cryIIIA expression was not altered in B. subtilis mutant strains deficient for the sigma H and sigma E sporulation-specific sigma factors or for minor sigma factors such as sigma B, sigma D, or sigma L. This result and the nucleotide sequence of the -35 and -10 regions of the cryIIIA promoter suggest that cryIIIA expression might be directed by the E sigma A form of RNA polymerase. Expression of the cryIIIA'-'lacZ fusion is shut off after t2 (2 h after time zero) of sporulation in the B. subtilis wild-type strain grown on nutrient broth sporulation medium. However, no decrease in cryIIIA-directed beta-galactosidase activity occurred in sigma H, kinA, or spo0A mutant strains. Moreover, beta-galactosidase activity was higher and remained elevated after t2 in the spo0A mutant strain. beta-Galactosidase activity was weak in abrB and spo0A abrB mutant strains, suggesting that AbrB is responsible for the higher level of cryIIIA expression observed in a spo0A mutant. However, both in spo0A and spo0A abrB mutant strains, beta-galactosidase activity remained elevated after t2, suggesting that even in the absence of AbrB, cryIIIA expression is controlled through modulation of the phosphorylated form of Spo0A. When the cryIIIA gene is expressed in a B. subtilis spo0A mutant strain or in the 168 wild-type strain, large amounts of toxins are produced and accumulate to form a flat rectangular crystal characteristic of the coleopteran-specific B. thuringiensis strains.

Identification of a promoter for the crystal protein-encoding gene cryIVB from Bacillus thuringiensis subsp. israelensis

The cryIVB gene of Bacillus thuringiensis subsp. israelensis (Bti) codes for a 135-kDa insecticidal crystal protein, which is specifically toxic to dipteran larvae. We have identified a transcription start point (tsp) of cryIVB by a primer extension experiment. The promoter sequence alignment, together with the chronology of appearance of the transcript, suggested that cryIVB is transcribed by RNA polymerase containing sigma 35 (E sigma 35). This was confirmed by investigation of cryIVB transcription in several Bacillus subtilis sporulation mutants. Unlike the lepidopteran-specific crystal protein-encoding genes [cryIA(a) and cryIB], transcription of which is regulated by both sigma 35 and sigma 28, cryIVB transcription was controlled only by the sigma 35-dependent promoter at the midsporulation stage.

Transcriptional regulation of the Bacillus thuringiensis subsp. thompsoni crystal protein gene operon

The two predominant polypeptides of the Bacillus thuringiensis subsp. thompsoni crystal are encoded by the cry40 and cry34 genes. These crystal protein genes are located in an operon. Western analysis (immunoblotting) demonstrated that the operon promoter activity was located in the region upstream of the cry40 gene. The Cry34 protein was expressed only when the upstream promoter region was present. The crystal protein genes are the only cistrons in the operon, and they are expressed during sporulation, with the highest transcript levels detected early in sporulation (1.5 to 3 h after the onset of sporulation). Transcription initiates from two adjacent sites located 84 and 85 bases upstream of the cry40 translational start codon. The B. thuringiensis subsp. thompsoni crystal protein gene operon promoter aligned with other crystal protein gene promoters, which are activated from early to midsporulation and transcribed in vitro by the B. thuringiensis RNA polymerase E sigma 35.