Spo0A represses transcription of the cry toxin genes in Bacillus thuringiensis

Key amino acids residues enhance the ability of CpcR to activate cry gene expression in Bacillus thuringiensis

Typical Bacillus thuringiensis (Bt) produces one or more parasporal crystals composed of insecticidal Cry proteins during the sporulation, and the parasporal crystals and spores are produced from the same cell. Strain Bt LM1212 is different from typical Bt strains in that its crystals and spores are produced in different cells. Previous studies have found that the cell differentiation process of Bt LM1212 is related to the transcription factor CpcR which activates the cry-gene promoters. In addition, CpcR could activate the Bt LM1212 cry35-like gene promoter (P35) when introduced in the heterologous HD73- strain. It was shown that P35 was only activated in non-sporulating cells. In this study, the peptidic sequences of CpcR homologous proteins found in other strains of the Bacillus cereus group were used as references to identify two key amino acid sites for CpcR activity. The function of these amino acids was investigated by measuring P35 activation by CpcR in strain HD73-. These results will lay a foundation for the optimization of the insecticidal protein expression system in non-sporulating cells.

Sporulation, Structure Assembly, and Germination in the Soil Bacterium Bacillus thuringiensis: Survival and Success in the Environment and the Insect Host

Bacillus thuringiensis (Bt) is a rod-shaped, Gram-positive soil bacterium that belongs to the phylum Firmicutes and the genus Bacillus. It is a spore-forming bacterium. During sporulation, it produces a wide range of crystalline proteins that are toxic to different orders of insects. Sporulation, structure assembly, and germination are essential stages in the cell cycle of B. thuringiensis. The majority of studies on these issues have focused on the model organism Bacillus subtilis, followed by Bacillus cereus and Bacillus anthracis. The machinery for sporulation and germination extrapolated to B. thuringiensis. However, in the light of recent findings concerning the role of the sporulation proteins (SPoVS), the germination receptors (Gr), and the cortical enzymes in Bt, the theory strengthened that conservation in sporulation, structure assembly, and germination programs drive the survival and success of B. thuringiensis in the environment and the insect host. In the present minireview, the latter pinpointed and reviewed.

Identification and Functional Characterization of Two Homologous SpoVS Proteins Involved in Sporulation of Bacillus thuringiensis

Sporulation is an important part of the life cycle of Bacillus thuringiensis and the basis for the production of parasporal crystals. This study identifies and characterizes two homologous spoVS genes (spoVS1 and spoVS2) in B. thuringiensis, both of whose expression is dependent on the σ^H factor. The disruption of spoVS1 and spoVS2 resulted in defective B. thuringiensis sporulation. Similar to Bacillus subtilis, B. thuringiensis strain HD(ΔspoVS1) mutants showed delayed formation of the polar septa, decreased sporulation efficiency, and blocked spore release. Different from B. subtilis, B. thuringiensis HD(ΔspoVS1) mutants had disporic septa and failed to complete engulfment in some cells. Moreover, HD(ΔspoVS2) mutants had delayed spore release. The effect of spoVS1 deletion on polar septum delay and sporulation efficiency could be compensated by spoVS2. β-Galactosidase activity analysis showed that the expression of pro-sigE and spoIIE decreased to different degrees in the HD(ΔspoVS1) and HD(ΔspoVS2) mutants. The different effects of the two mutations on the expression of sporulation genes led to decreases in Cry1Ac production of different levels.

IMPORTANCE There is only one spoVS gene in B. subtilis, and its effects on sporulation have been reported. In this study, two homologous spoVS genes were found and identified in B. thuringiensis. The different effects on sporulation and parasporal crystal protein production in B. thuringiensis and their relationship were investigated. We found that these two homologous spoVS genes are highly conserved in the Bacillus cereus group, and therefore, the functional characterization of SpoVS is helpful to better understand the sporulation processes of members of the Bacillus cereus group.

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.

Bt toxin modification for enhanced efficacy

Insect-specific toxins derived from Bacillus thuringiensis (Bt) provide a valuable resource for pest suppression. Here we review the different strategies that have been employed to enhance toxicity against specific target species including those that have evolved resistance to Bt, or to modify the host range of Bt crystal (Cry) and cytolytic (Cyt) toxins. These strategies include toxin truncation, modification of protease cleavage sites, domain swapping, site-directed mutagenesis, peptide addition, and phage display screens for mutated toxins with enhanced activity. Toxin optimization provides a useful approach to extend the utility of these proteins for suppression of pests that exhibit low susceptibility to native Bt toxins, and to overcome field resistance.

Regulation of cry gene expression in Bacillus thuringiensis

Bacillus thuringiensis differs from the closely related Bacillus cereus group species by its ability to produce crystalline inclusions. The production of these crystals mainly results from the expression of the cry genes, from the stability of their transcripts and from the synthesis, accumulation and crystallization of large amounts of insecticidal Cry proteins. This process normally coincides with sporulation and is regulated by various factors operating at the transcriptional, post-transcriptional, metabolic and post-translational levels.

Early transcription of Bacillus thuringiensis cry genes in strains active on Lepidopteran species and the role of gene content on their expression

Six strains of Bacillus thuringiensis previously selected as highly toxic against Manduca sexta and Plutella xylostella were analyzed by PCR screening in order to identify the cry genes active on Lepidoptera. According to their gene content and insecticidal potency, these strains were cultured and aliquots taken at different pre- and post-sporulation times. Total RNA was extracted and used as template in RT-PCR analyses directed to identify mRNAs of the previously identified cry genes. Results showed transcription of genes cry1A, cry1E, cry1I, and cry2 even before the onset of sporulation. However, this early transcription did not lead to an appreciable parasporal protein synthesis until t5-t9, as deduced from SDS-PAGE profiles. As for cry1I gene, the corresponding protein was not detected, as expected, but cry1I mRNAs were present at least until t5. Interestingly, strains expressing four cry genes from the end of the log phase onwards exhibited kinetics characterized by a very long transition phase, whereas the strain expressing only one cry gene showed a very short transition phase. Strains expressing three genes showed an intermediate profile. These results indicate that the transcription of B. thuringiensis cry1 and cry2 genes in natural strains can start several hours before massive crystal synthesis occurs and that this translation is probably competing with transcriptional regulators required for the sporulation onset.

Spo0A differentially regulates toxin production in evolutionarily diverse strains of Clostridium difficile

Clostridium difficile is an important pathogen of humans and animals, representing a significant global healthcare problem. The last decade has seen the emergence of epidemic BI/NAP1/027 and ribotype 078 isolates, associated with the onset of more severe disease and higher rates of morbidity and mortality. However, little is known about these isolates at the molecular level, partly due to difficulties in the genetic manipulation of these strains. Here we report the development of an optimised Tn916-mediated plasmid transfer system, and the use of this system to construct and complement spo0A mutants in a number of different C. difficile strain backgrounds. Spo0A is a global regulator known to control sporulation, but may also be involved in the regulation of potential virulence factors and other phenotypes. Recent studies have failed to elucidate the role of Spo0A in toxin A and toxin B production by C. difficile, with conflicting data published to date. In this study, we aimed to clarify the role of Spo0A in production of the major toxins by C. difficile. Through the construction and complementation of spo0A mutants in two ribotype 027 isolates, we demonstrate that Spo0A acts as a negative regulator of toxin A and toxin B production in this strain background. In addition, spo0A was disrupted and subsequently complemented in strain 630Δerm and, for the first time, in a ribotype 078 isolate, JGS6133. In contrast to the ribotype 027 strains, Spo0A does not appear to regulate toxin production in strain 630Δerm. In strain JGS6133, Spo0A appears to negatively regulate toxin production during early stationary phase, but has little effect on toxin expression during late stationary phase. These data suggest that Spo0A may differentially regulate toxin production in phylogenetically distinct C. difficile strain types. In addition, Spo0A may be involved in regulating some aspects of C. difficile motility.

C. difficile 630Δerm Spo0A regulates sporulation, but does not contribute to toxin production, by direct high-affinity binding to target DNA

Clostridium difficile is a Gram positive, anaerobic bacterium that can form highly resistant endospores. The bacterium is the causative agent of C. difficile infection (CDI), for which the symptoms can range from a mild diarrhea to potentially fatal pseudomembranous colitis and toxic megacolon. Endospore formation in Firmicutes, including C. difficile, is governed by the key regulator for sporulation, Spo0A. In Bacillus subtilis, this transcription factor is also directly or indirectly involved in various other cellular processes. Here, we report that C. difficile Spo0A shows a high degree of similarity to the well characterized B. subtilis protein and recognizes a similar binding sequence. We find that the laboratory strain C. difficile 630Δerm contains an 18bp-duplication near the DNA-binding domain compared to its ancestral strain 630. In vitro binding assays using purified C-terminal DNA binding domain of the C. difficile Spo0A protein demonstrate direct binding to DNA upstream of spo0A and sigH, early sporulation genes and several other putative targets. In vitro binding assays suggest that the gene encoding the major clostridial toxin TcdB may be a direct target of Spo0A, but supernatant derived from a spo0A negative strain was no less toxic towards Vero cells than that obtained from a wild type strain, in contrast to previous reports. These results identify for the first time direct (putative) targets of the Spo0A protein in C. difficile and make a positive effect of Spo0A on production of the large clostridial toxins unlikely.

Weak transcription of the cry1Ac gene in nonsporulating Bacillus thuringiensis cells

The cry1Ac gene of Bacillus thuringiensis subsp. kurstaki HD-73 (B. thuringiensis HD-73) is a typical example of a sporulation-dependent crystal gene and is controlled by sigma E and sigma K during sporulation. To monitor the production and accumulation of Cry1Ac at the cellular level, we developed a green fluorescent protein-based reporter system. The production of Cry1Ac was monitored in spo0A, sigE, and sigK mutants, and these mutants were able to express the Cry1Ac-green fluorescent protein fusion protein. In nonsporulating B. thuringiensis HD-73 cells, low-level expression of cry1Ac was also observed. Reverse transcription-PCR and Western blotting results confirmed that the cry1Ac promoter has low activity in nonsporulating B. thuringiensis cells. A beta-galactosidase assay demonstrated that the transcription of the cry1Ac gene during exponential and transition phases is positively regulated by Spo0A. Additional bioassay results indicated that spo0A and sigE mutants containing the cry1Ac-gfp fusion exhibited insecticidal activity against Plutella xylostella larvae.

Identification of the promoter in the intergenic region between orf1 and cry8Ea1 controlled by sigma H factor

Bacillus thuringiensis Cry8Ea toxin is specifically toxic to larvae of the Asian cockchafer, Holotrichia parallela. Here we investigated the mechanism of transcriptional regulation of the cry8Ea1 gene. Reverse transcription-PCR (RT-PCR) results indicated that cry8Ea1 and an upstream gene (orf1) were cotranscribed. Transcriptional fusions with the lacZ gene demonstrated that transcription of the cry8Ea1 gene started from two promoters: P(orf1), which is located upstream of the orf1 gene, and P(cry8E), located in the intergenic region mapping between orf1 and cry8Ea1. Of the known, similar orf1-cry operons, this is the first report of the existence of a promoter in the intergenic region between the orf1 and cry genes. The transcriptional activity of P(orf1) was found during sporulation in B. thuringiensis subsp. kurstaki HD-73 and was almost abolished in the sigE mutant, while the transcriptional activity of P(cry8E) was detected after the end of the exponential phase in HD-73 and was considerably lower in the sigH mutant. The transcription start sites generated by the two cry8Ea1 promoters were determined by the 5' -SMARTer rapid amplification of cDNA ends (RACE) method. The -35 and -10 regions of P(orf1) and P(cry8E) showed high sequence similarity with the σ(E) and σ(H) promoters, respectively. These results indicated that P(orf1) is controlled by the σ(E) factor and P(cry8E) by the σ(H) factor.

Large crystal toxin formation in chromosomally engineered Bacillus thuringiensis subsp. aizawai due to σE accumulation

Seven distinct Bacillus thuringiensis subsp. aizawai integrants were constructed that carried the chitinase (chiBlA) gene from B. licheniformis under the control of the cry11Aa promoter and terminator with and without p19 and p20 genes. The toxicity of B. thuringiensis subsp. aizawai integrants against second-instar Spodoptera litura larvae was increased 1.8- to 4.6-fold compared to that of the wild-type strain (BTA1). Surprisingly, the enhanced toxicity in some strains of B. thuringiensis subsp. aizawai integrants (BtaP19CS, BtaP19CSter, and BtaCAT) correlated with an increase in toxin formation. To investigate the role of these genes in toxin production, the expression profiles of the toxin genes, cry1Aa and chiBlA, as well as their transcriptional regulators (sigK and sigE), were analyzed by quantitative real-time RT-PCR (qPCR) from BTA1, BtaP19CS, and BtaCAT. Expression levels of cry1Aa in these two integrants increased about 2- to 3-fold compared to those of BTA1. The expression of the transcription factor sigK also was prolonged in the integrants compared to that of the wild type; however, sigE expression was unchanged. Western blot analysis of σ(E) and σ(K) showed the prolonged accumulation of σ(E) in the integrants compared to that of BTA1, resulting in the increased synthesis of pro-σ(K) up to T(17) after the onset of sporulation in both BtaP19CS and BtaCAT compared to that of T(13) in BTA1. The results from qPCR indicate clearly that the cry1Aa promoter activity was influenced most strongly by σ(E), whereas cry11Aa depended mostly on σ(K). These results on large-crystal toxin formation with enhanced toxicity should provide useful information for the generation of strains with improved insecticidal activity.

Potential effect of a putative sigma(H)-driven promoter on the over expression of the Cry1Ac toxin of Bacillus thuringiensis

Sequence analysis of the upstream region of the cry1Ac gene in the HD-73 strain of B. thuringiensis showed a putative sigma(H)-like promoter. The potential regulating role of this sequence was tested by transforming an acrystaliferous mutant of the HD-73 strain with three different constructs: (1) a construct consisting of the sigma(H)-, sigma(E)- and sigma(K)-like promoters, the 0A box, and the cry1Ac coding sequence (EK0AH); (2) a derivative construct that lacked the sigma(H)-promoter (EK0A); and (3) a second derivative construct that lacked the sigma(H)-promoter and the 0A box (EK). Crystals from the recombinant and the wild-type (Bt HD-73) strains were measured by transmission electron microscopy. Statistically significant differences in crystal size were detected between all the transformed and the wild-type strains, with averages of 1.54, 1.31, 1.05, and 0.95microm for the EK0AH, EK0A, HD-73, and EK constructs, respectively. SDS-PAGE analyses of the EK0AH construct corroborated a higher expression level of the cry1Ac gene than that of the EK0A construct, as well as the lower expression of the EK construct. Interestingly, RT-PCR analyses indicated that the recombinant strain carrying the construct EK0AH started the transcription of the cry gene earlier than the Bt HD-73 strain, as observed when a kinetics study was carried out, which may explain the larger crystals and the higher expression of the construct with the putative sigma(H)-like promoter, along with the vector's high copy number.

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.

Crystal protein synthesis is dependent on early sporulation gene expression inBacillus sphaericus

Insertional mutations in the spo0A and spoIIAC genes of Bacillus sphaericus 2362 were prepared by conjugation with Escherichia coli using a suicide plasmid containing cloned portions of the target genes. The mutants resembled their Bacillus subtilis counterparts phenotypically and were devoid of crystal proteins as determined by electron microscopy, SDS-PAGE and Western blots. The mutants had greatly reduced toxicity to anopheline mosquito larvae compared to the parental strain. We conclude that crystal protein synthesis in this bacterium is dependent on expression of early sporulation genes.

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.

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.

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.