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

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.

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.

The Alternative Sigma Factor SigB Is Required for the Pathogenicity of Bacillus thuringiensis

To adapt to changing and potentially hostile environments, bacteria can activate the transcription of genes under the control of alternative sigma factors, such as SigB, a master regulator of the general stress response in several Gram-positive species. Bacillus thuringiensis is a Gram-positive spore-forming invertebrate pathogen whose life cycle includes a variety of environments, including plants and the insect hemocoel or gut. Here, we assessed the role of SigB during the infectious cycle of B. thuringiensis in a Galleria mellonella insect model. We used a fluorescent reporter coupled to flow cytometry and showed that SigB was activated in vivo We also showed that the pathogenicity of the ΔsigB mutant was severely affected when inoculated via the oral route, suggesting that SigB is critical for B. thuringiensis adaptation to the gut environment of the insect. We could not detect an effect of the sigB deletion on the survival of the bacteria or on their sporulation efficiency in the cadavers. However, the gene encoding the pleiotropic regulator Spo0A was upregulated in the ΔsigB mutant cells during the infectious process.IMPORTANCE Pathogenic bacteria often need to transition between different ecosystems, and their ability to cope with such variations is critical for their survival. Several Gram-positive species have developed an adaptive response mediated by the general stress response alternative sigma factor SigB. In order to understand the ecophysiological role of this regulator in Bacillus thuringiensis, an entomopathogenic bacterium widely used as a biopesticide, we sought to examine the fate of a ΔsigB mutant during its life cycle in the natural setting of an insect larva. This allowed us, in particular, to show that SigB was activated during infection and that it was required for the pathogenicity of B. thuringiensis via the oral route of infection.

Genetically modified entomopathogenic bacteria, recent developments, benefits and impacts: A review

Entomopathogenic bacteria (EPBs), insect pathogens that produce pest-specific toxins, are environmentally-friendly alternatives to chemical insecticides. However, the most important problem with EPBs application is their limited field stability. Moreover, environmental factors such as solar radiation, leaf temperature, and vapor pressure can affect the pathogenicity of these pathogens and their toxins. Scientists have conducted intensive research to overcome such problems. Genetic engineering has great potential for the development of new engineered entomopathogens with more resistance to adverse environmental factors. Genetically modified entomopathogenic bacteria (GM-EPBs) have many advantages over wild EPBs, such as higher pathogenicity, lower spraying requirements and longer-term persistence. Genetic manipulations have been mostly applied to members of the bacterial genera Bacillus, Lysinibacillus, Pseudomonas, Serratia, Photorhabdus and Xenorhabdus. Although many researchers have found that GM-EPBs can be used safely as plant protection bioproducts, limited attention has been paid to their potential ecological impacts. The main concerns about GM-EPBs and their products are their potential unintended effects on beneficial insects (predators, parasitoids, pollinators, etc.) and rhizospheric bacteria. This review address recent update on the significant role of GM-EPBs in biological control, examining them through different perspectives in an attempt to generate critical discussion and aid in the understanding of their potential ecological impacts.

The stationary phase regulator CpcR activates cry gene expression in non-sporulating cells of Bacillus thuringiensis

Cell differentiation within an isogenic population allows the specialisation of subpopulations and a division of labour. Bacillus thuringiensis is a spore-forming bacterium that produces insecticidal crystal proteins (Cry proteins) in sporulating cells. We recently reported that strain B. thuringiensis LM1212 presents the unique ability to differentiate into two subpopulations during the stationary phase: spore-formers and crystal-producers. Here, we characterised the transcriptional regulator CpcR responsible for this differentiation and the expression of the cry genes. cpcR is located on a plasmid that also harbours cry genes. The alignment of LM1212 cry gene promoters revealed the presence of a conserved DNA sequence upstream from the -35 region. This presumed CpcR box was also found in the promoter of cpcR and we showed that cpcR transcription is positively autoregulated. Electrophoretic mobility shift assays suggested that CpcR directly controls the transcription of its target genes by binding to the CpcR box. We showed that CpcR was able to direct the production of a crystal consisting of a heterologous insecticidal Cry protein in non-sporulating cells of a typical B. thuringiensis kurstaki strain. Moreover, the expression of cpcR induced a reduction in the sporulation of this B. thuringiensis strain, suggesting an interaction between CpcR and the sporulation regulatory networks.

Regulons and protein-protein interactions of PRD-containing Bacillus anthracis virulence regulators reveal overlapping but distinct functions

Bacillus anthracis produces three regulators, AtxA, AcpA and AcpB, which control virulence gene transcription and belong to an emerging class of regulators termed 'PCVRs' (Phosphoenolpyruvate-dependent phosphotransferase regulation Domain-Containing Virulence Regulators). AtxA, named for its control of toxin gene expression, is the master virulence regulator and archetype PCVR. AcpA and AcpB are less well studied. Reports of PCVR activity suggest overlapping function. AcpA and AcpB independently positively control transcription of the capsule biosynthetic operon capBCADE, and culture conditions that enhance AtxA level or activity result in capBCADE transcription in strains lacking acpA and acpB. We used RNA-Seq to assess the regulons of the paralogous regulators in strains constructed to express individual PCVRs at native levels. Plasmid and chromosome-borne genes were PCVR controlled, with AtxA, AcpA and AcpB having a ≥ 4-fold effect on transcript levels of 145, 130 and 49 genes respectively. Several genes were coregulated by two or three PCVRs. We determined that AcpA and AcpB form homomultimers, as shown previously for AtxA, and we detected AtxA-AcpA heteromultimers. In co-expression experiments, AcpA activity was reduced by increased levels of AtxA. Our data show that the PCVRs have specific and overlapping activity and that PCVR stoichiometry and potential heteromultimerization can influence target gene expression.

Expression of the genes encoding the CasK/R two-component system and the DesA desaturase during Bacillus cereus cold adaptation

Two-component systems (TCS) allow a cell to elaborate a variety of adaptive responses to environment changes. The recently discovered CasK/R TCS plays a role in the optimal unsaturation of fatty acids necessary for cold adaptation of the foodborne-pathogen Bacillus cereus Here, we showed that the promoter activity of the operon encoding this TCS was repressed during growth at low temperature in the stationary phase in the parental strain when compared to the casK/R mutant, suggesting that CasR negatively regulates the activity of its own promoter in these conditions. The promoter activity of the desA gene encoding the Δ5 fatty acid desaturase, providing unsaturated fatty acids (UFAs) required for low temperature adaptation, was repressed in the casK/R mutant grown at 12°C versus 37°C. This result suggests that CasK/R activates desA expression during B. cereus growth at low temperature, allowing an optimal unsaturation of the fatty acids. In contrast, desA expression was repressed during the lag phase at low temperature in presence of UFAs, in a CasK/R-independent manner. Our findings confirm that the involvement of this major TCS in B. cereus cold adaptation is linked to the upregulation of a fatty acid desaturase.

Division of labour and terminal differentiation in a novel Bacillus thuringiensis strain

A major challenge in bacterial developmental biology has been to understand the mechanisms underlying cell fate decisions. Some differentiated cell types display cooperative behaviour. Cooperation is one of the greatest mysteries of evolutionary biology and microbes have been considered as an excellent system for experimentally testing evolution theories. Bacillus thuringiensis (Bt) is a spore-forming bacterium, which is genetically closely related to B. anthracis, the agent of anthrax, and to B. cereus, an opportunistic human pathogen. The defining feature that distinguishes Bt from its relatives is its ability to produce crystal inclusions in the sporulating cells. These toxins are solubilized after ingestion and are cooperative public goods in insect hosts. In this study, we describe a Bt strain LM1212 that presents the unique ability to terminally differentiate into crystal producers and spore formers. Transcriptional analysis based on lacZ and gfp reporter genes suggested that this phenotype is the consequence of a new type of cell differentiation associated with a novel regulation mode of cry gene expression. The differentiating crystal-producer phenotype has higher spore productivity than a typical Bt strain and is better able to compete with Cry toxin null 'cheaters'. Potentially, this division of labour provides additional fitness benefits in terms of spore viability or durability of Cry toxin.

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.

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.

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.

High-throughput identification of promoters and screening of highly active promoter-5'-UTR DNA region with different characteristics from Bacillus thuringiensis

In bacteria, both promoters and 5'-untranslated regions (5'-UTRs) of mRNAs play vital regulatory roles in gene expression. In this study, we identified 1203 active promoter candidates in Bacillus thuringiensis through analysis of the genome-wide TSSs based on the transcriptome data. There were 11 types of σ-factor and 34 types of transcription factor binding sites found in 723 and 1097 active promoter candidates, respectively. Moreover, within the 1203 transcriptional units (TUs), most (52%) of the 5'-UTRs were 10-50 nucleotides in length, 12.8% of the TUs had a long 5'-UTR greater than 100 nucleotides in length, and 16.3% of the TUs were leaderless. We then selected 20 active promoter candidates combined with the corresponding 5'-UTR DNA regions to screen the highly active promoter-5'-UTR DNA region complexes with different characteristics. Our results demonstrate that among the 20 selected complexes, six were able to exert their functions throughout the life cycle, six were specifically induced during the early-stationary phase, and four were specifically activated during the mid-stationary phase. We found a direct corresponding relationship between σ-factor-recognized consensus sequences and complex activity features: the great majority of complexes acting throughout the life cycle possess σ(A)-like consensus sequences; the maximum activities of the σ(F)-, σ(E)-, σ(G)-, and σ(K)-dependent complexes appeared at 10, 14, 16, and 22 h under our experimental conditions, respectively. In particular, complex Phj3 exhibited the strongest activity. Several lines of evidence showed that complex Phj3 possessed three independent promoter regions located at -251∼-98, -113∼-31, and -54∼+14, and that the 5'-UTR +1∼+118 DNA region might be particularly beneficial to both the stability and translation of its downstream mRNA. Moreover, Phj3 successfully overexpressed the active β-galactosidase and turbo-RFP, indicating that Phj3 could be a proper regulatory element for overexpression of proteins in B. thuringiensis. Therefore, our efforts contribute to molecular biology research and the biotechnological application of B. thuringiensis.

Glucose 6P binds and activates HlyIIR to repress Bacillus cereus haemolysin hlyII gene expression

Bacillus cereus is a Gram-positive spore-forming bacterium causing food poisoning and serious opportunistic infections. These infections are characterized by bacterial accumulation despite the recruitment of phagocytic cells. We have previously shown that B. cereus Haemolysin II (HlyII) induces macrophage cell death by apoptosis. In this work, we investigated the regulation of the hlyII gene. We show that HlyIIR, the negative regulator of hlyII expression in B. cereus, is especially active during the early bacterial growth phase. We demonstrate that glucose 6P directly binds to HlyIIR and enhances its activity at a post-transcriptional level. Glucose 6P activates HlyIIR, increasing its capacity to bind to its DNA-box located upstream of the hlyII gene, inhibiting its expression. Thus, hlyII expression is modulated by the availability of glucose. As HlyII induces haemocyte and macrophage death, two cell types that play a role in the sequestration of nutrients upon infection, HlyII may induce host cell death to allow the bacteria to gain access to carbon sources that are essential components for bacterial growth.

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.

Cereulide produced by Bacillus cereus increases the fitness of the producer organism in low-potassium environments

Cereulide, produced by certain Bacillus cereus strains, is a lipophilic cyclic peptide of 1152 Da that binds K(+) ions with high specificity and affinity. It is toxic to humans, but its role for the producer organism is not known. We report here that cereulide operates for B. cereus to scavenge potassium when the environment is growth limiting for this ion. Cereulide-producing B. cereus showed higher maximal growth rates (µ(max)) than cereulide non-producing B. cereus in K(+)-deficient medium (K(+) concentration ~1 mM). The cereulide-producing strains grew faster in K(+)-deficient than in K(+)-rich medium with or without added cereulide. Cereulide non-producing B. cereus neither increased µ(max) in K(+)-deficient medium compared with K(+)-rich medium, nor benefited from added cereulide. Cereulide-producing strains outcompeted GFP-labelled Bacillus thuringiensis in potassium-deficient (K(+) concentration ~1 mM) but not in potassium-rich (K(+) concentration ~30 mM) medium. Exposure to 2 µM cereulide in potassium-free medium lacking an energy source caused, within seconds, a major efflux of cellular K(+) from B. cereus not producing cereulide as well as from Bacillus subtilis. Cereulide depleted the cereulide non-producing B. cereus and B. subtilis cells of a major part of their K(+) stores, but did not affect cereulide-producing B. cereus strains. Externally added 6-10 µM cereulide triggered the generation of biofilms and pellicles by B. cereus. The results indicate that both endogenous and externally accessible cereulide supports the fitness of cereulide-producing B. cereus in environments where the potassium concentration is low.

Development of a stationary phase-specific autoinducible expression system in Bacillus subtilis

Bacillus thuringiensis produces crystal proteins (Cry) that account for up to 25% of the dry cell weight during the stationary phase. The high-level expression and stationary phase-specific autoinduction of the cry gene led to development of a cry promoter-based Bacillus expression system. Among the various cry promoters, cry3Aa promoter was selected by comparing the lacZ expression levels in Bacillus subtilis. An extracellular enzyme cellulase was highly upregulated during the stationary phase while under control of the cry3Aa promoter. Improvement of the cry3Aa promoter was obtained by modification of the promoter sequence. Specifically, a 5-fold increase in lacZ expression was obtained by changing both the -35 and -10 boxes of the cry3Aa promoter to the consensus sequence of the sigma(A)-dependent promoter of B. subtilis. The modified cry3Aa promoter produced a significantly higher yield of AprE, which suggests that the promoter may be useful for high-level protein expression in B. subtilis.

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.

The InhA metalloproteases of Bacillus cereus contribute concomitantly to virulence

The virulence of Bacillus cereus requires that bacteria have the capacity to colonize their host, degrade specific tissues, and circumvent the host immune system. To study this aspect of pathogenesis, we focused on three metalloproteases, InhA1, InhA2, and InhA3, which share more than 66% identity. The expression of these metalloprotease genes was assessed by transcriptional fusions with a lacZ reporter gene. The expression profiles suggest a complementary time course of InhA production. Indeed, the genes are simultaneously expressed but are oppositely controlled during stationary phase. We constructed single and multiple inhA mutants and assessed the bacterial locations of the proteins as well as their individual or additive roles in macrophage escape and toxicity, antibacterial-peptide cleavage, and virulence. InhA1, a major component of the spore exosporium, is the only InhA metalloprotease involved in bacterial escape from macrophages. A mutant lacking inhA1, inhA2, and inhA3 shows a strong decrease in the level of virulence for insects. Taken together, these results show that the InhA metalloproteases of B. cereus are important virulence factors that may allow the bacteria to counteract the host immune system.

Involvement of motility and flagella in Bacillus cereus biofilm formation

Bacillus cereus is a food-borne pathogen and a frequent contaminant of food production plants. The persistence of this pathogen in various environments results from the formation of spores and of biofilms. To investigate the role of the B. cereus flagellar apparatus in biofilm formation, we constructed a non-flagellated mutant and a flagellated but non-motile mutant. Unexpectedly, we found that the presence of flagella decreased the adhesion of the bacterium to glass surfaces. We hypothesize that this decrease is a consequence of the flagella hindering a direct interaction between the bacterial cell wall and the surface. In contrast, in specific conditions, motility promotes biofilm formation. Our results suggest that motility could influence biofilm formation by three mechanisms. Motility is necessary for the bacteria to reach surfaces suitable for biofilm formation. In static conditions, reaching the air-liquid interface, where the biofilm forms, is a strong requirement, whereas in flow cells bacteria can have access to the bottom glass slide by sedimentation. Therefore, motility is important for biofilm formation in glass tubes and in microtitre plates, but not in flow cells. Motility also promotes recruitment of planktonic cells within the biofilm by allowing motile bacteria to invade the whole biofilm. Finally, motility is involved in the spreading of the biofilm on glass surfaces.

Transfer of Bacillus cereus spores from packaging paper into food

Food packaging papers are not sterile, as the manufacturing is an open process, and the raw materials contain bacteria. We modeled the potential transfer of the Bacillus cereus spores from packaging paper to food by using a green fluorescent protein-expressing construct of Bacillus thuringiensis Bt 407Cry(-) [pHT315Omega(papha3-gfp)], abbreviated BT-1. Paper (260 g m(-2)) containing BT-1 was manufactured with equipment that allowed fiber formation similar to that of full-scale manufactured paper. BT-1 adhered to pulp during papermaking and survived similar to an authentic B. cereus. Rice and chocolate were exposed to the BT-1-containing paper for 10 or 30 days at 40 or 20 degrees C at relative air humidity of 10 to 60%. The majority of the spores remained immobilized inside the fiber web; only 0.001 to 0.03% transferred to the foods. This amount is low compared with the process hygiene criteria and densities commonly found in food, and it does not endanger food safety. To measure this, we introduced BT-1 spores into the paper in densities of 100 to 1,000 times higher than the amounts of the B. cereus group bacteria found in commercial paper. Of BT-1 spores, 0.03 to 0.1% transferred from the paper to fresh agar surface within 5 min of contact, which is more than to food during 10 to 30 days of exposure. The findings indicate that transfer from paper to dry food is restricted to those microbes that are exposed on the paper surface and readily detectable with a contact agar method.

Intrinsic curvature associated with the coordinately regulated anthrax toxin gene promoters

The current model for virulence gene regulation in Bacillus anthracis involves several trans-acting factors, the most important of which appears to be the anthrax toxin activator encoded by the atxA gene. AtxA is a positive regulator of the toxin genes pagA, cya and lef, and of a number of other plasmid- and chromosome-encoded genes. The AtxA protein (56 kDa) possesses a predicted winged-helix DNA-binding domain and phosphotransferase system-regulated domains, but the mechanism for positive regulation of AtxA target genes is not known. Sequence similarities in the promoter regions of AtxA-regulated genes are not apparent, and recombinant AtxA binds DNA with a high affinity in a non-specific manner. We hypothesized that the toxin genes possess common structural features or cis-acting elements that are required for positive regulation. We employed deletion analyses to determine the minimal sequences required for atxA-mediated toxin gene expression. In silico modelling and in vitro experiments using double-stranded DNA corresponding to the toxin gene promoter regions indicated significant curvature associated with these regions. These findings suggest that the structural topology of the DNA plays an important role in the control of anthrax toxin gene expression.

The PlcR virulence regulon of Bacillus cereus

PlcR is a Bacillus cereus transcriptional regulator, which activates gene expression by binding to a nucleotidic sequence called the ‘PlcR box’. To build a list of all genes included in the PlcR regulon, a consensus sequence was identified by directed mutagenesis. The reference strain ATCC14579 sequenced genome was searched for occurrences of this consensus sequence to produce a virtual regulon. PlcR control of these genes was confirmed by comparing gene expression in the reference strain and its isogenic Δ-plcR strain using DNA microarrays, lacZ fusions and proteomics methods. The resulting list included 45 genes controlled by 28 PlcR boxes. Forty of the PlcR controlled proteins were exported, of which 22 were secreted in the extracellular medium and 18 were bound or attached to cell wall structures (membrane or peptidoglycan layer). The functions of these proteins were related to food supply (phospholipases, proteases, toxins), cell protection (bacteriocins, toxins, transporters, cell wall biogenesis) and environment-sensing (two-component sensors, chemotaxis proteins, GGDEF family regulators). Four genes coded for cytoplasmic regulators. The PlcR regulon appears to integrate a large range of environmental signals, including food deprivation and self cell-density, and regulate the transcription of genes designed to overcome obstacles that hinder B. cereus growth within the host: food supply, host barriers, host immune defenses, and competition with other bacterial species. PlcR appears to be a key component in the efficient adaptation of B. cereus to its host environment.

Comparative study on effect of different promoters on expression of cry1Ac in Bacillus thuringiensis chromosome

AIMS

The aim of this work was to investigate the effect of cry3A promoter on the expression of cry1Ac in Bacillus thuringiensis chromosome and stably enhance the production of different cry genes under the control of cry3A promoter.

METHODS AND RESULTS

The cry1Ac gene, which is specific to Lepidopteran larvae, was integrated into the chromosome of a B. thuringiensis plasmid-free and acrystalliferous strain BMB171, under the control of cry3A promoter and cry1Ac promoter, respectively. The expression of cry1Ac genes in the chromosome of host strain was investigated. The results from sodium dodecyl sulfate-polyacrymide gel electrophoresis, crystal observation and bioassay showed that either integrated with cry3A promoter (cry3Apro-cry1Ac) or with its native promoter (cry1Acpro-cry1Ac), cry1Ac gene could efficiently and stably express in the chromosome. The production of cry3Apro-cry1Ac gene was higher than that of cry1Acpro-cry1Ac gene.

CONCLUSIONS

The cry3A promoter enhanced the expression of cry1Ac gene efficiently either on the chromosome or on the plasmid in B. thuringiensis strain.

SIGNIFICANCE AND IMPACT OF THE STUDY

So far, the comparative studies on cry3A promoter and other cry promoters were carried on B. thuringiensis plasmids. This system offers an additional method for potentially improving the efficacy of B. thuringiensis insecticidal proteins efficiently, stably and safely.

Characterization of a small PlcR-regulated gene co-expressed with cereolysin O

Background

In the human pathogen Bacillus cereus, the expression of most extracellular virulence factors is controlled by the transcriptional activator PlcR. Among these virulence factors, cereolysin O (Clo) is an haemolysin belonging to the cholesterol-dependant cytolysins, a protein family extensively studied in Gram-positive bacteria.

Results

In the genomes of bacteria belonging to the B. cereus group, including Bacillus anthracis and Bacillus thuringiensis, a small gene encoding a 26-amino acid peptide was present in multicopy. One copy was always found upstream from the gene encoding Clo. In B. cereus ATCC 14579, the small gene and the clo gene are co-transcribed. Transcriptional fusions showed that the three paralogues identified in this strain were expressed in a PlcR-dependent manner. We propose to name these peptides Spp for small PlcR-regulated peptides. We show that a synthetic peptide corresponding to the deduced product of the spp genes displayed antibacterial activity.

Conclusion

The co-expression of spp, a small PlcR-regulated multicopy gene with clo suggests a yet unidentified relationship between Spp and the cholesterol-dependent cytolysin in bacteria belonging to the B.cereus group.

Restraining Erwinia virulence by expression of N-acyl homoserine lactonase gene pro3A-aiiA in Bacillus thuringiensis subsp leesis

To widen the biological control function of a genetically modified Bacillus thuringiensis subsp leesis strain BMB-005, an acyl homoserine lactonase (AHL lactonase) gene aiiA transcribed by the promoter of insecticidal crystal protein coding gene cry3A, was transformed into strain BMB-005. The amount of AHL lactonase protein produced by transformant BMB821A was 2.4-fold more than that produced by BMB-005. AHL-degradation assay showed that transformant BMB821A could degrade more AHLs molecules than the original strain BMB-005. The result of Erwinia carotovora pathogenicity test showed that the parental strain BMB-005 had no restraint of Erwinia infection, but the transformants exhibited strong restraint of E. carotovora infection on potato slices and cactus stems. Insecticidal bioassay against lepidopteran Spodoptera exigua showed that both strain BMB-005 and transformant BMB821A were toxic to S. exigua. The toxicity of transformant BMB821A (LC(50) was 3.8) was a little attenuated comparing with the toxicity of the original strain BMB-005 (LC(50) was 2.9). The B. thuringiensis strain BMB-005 has high toxicity against Helicoverpa armigera, Plutella xylostella, and S. exigua. This work provided new strategy for developing genetically engineered multi-functional B. thuringiensis strain that possesses insecticidal activity together with restraint of bacterial pathogenicity.

Vegetative insecticidal protein enhancing the toxicity of Bacillus thuringiensis subsp kurstaki against Spodoptera exigua

AIMS

The objective of this work was to enhance the insecticidal activity or widen the pesticidal spectrum of a commercial Bacillus thuringiensis strain YBT1520.

METHODS AND RESULTS

A vegetative insecticidal protein gene vip3Aa7, under the control of its native promoter and cry3A promoter, was subcloned into B. thuringiensis acrystalliferous BMB171 to generate BMB8901 and BMBvip respectively. It was found that the amount of Vip3Aa7 protein produced by BMBvip was 3.2-fold more than that produced by BMB8901. Therefore, the vip3Aa7 gene under the control of cry3A promoter was transformed into strain YBT1520. The toxicity of the resulting strain BMB218V against Spodoptera exigua was 10-fold more than that of YBT1520, and that the toxicity of BMB218V against Helicoverpa armigera retained the same level as that of strain YBT1520.

CONCLUSIONS

Strain YBT1520 obtained high toxicity against S. exigua after it was transformed and expressed the foreign vip3Aa7 gene.

SIGNIFICANCE AND IMPACT OF THE STUDY

Commercial B. thuringiensis strain YBT1520 has high toxicity against H. armigera and Plutella xylostella, but almost no activity against S. exigua, which is a major crop pest in China. This work provides a new strategy for widening the activity spectrum of B. thuringiensis against agriculture pests.

A cryptic Bacillus isolate exhibited narrow 16S rRNA gene sequence divergence with Bacillus thuringiensis and showed low maintenance requirements in hyper-osmotic complex substrate cultivations

A cryptic Bacillus (K90) isolate obtained from soil samples from the Kuwait desert exhibited lower maintenance requirements in complex substrate cultivations than Bacillus thuringiensis. A mathematical model was used to estimate apparent maintenance coefficients (m(c)) and these were found to be 0.336 and 0.041/h for B. thuringiensis and K90, respectively. The results also showed that the values of apparent maintenance coefficients were inversely related to the specific growth rates. Furthermore, 16S rRNA gene sequencing showed that K90 exhibited 99.81% sequence similarity to that of B. mojavensis and 92.9% with B. thuringiensis. It is evident from the dendrogram that the evolution of B. mojavensis (K90) (B. subtilis group), which may have originated after B. licheniformis could have been influenced by prolonged hyper-osmotic conditions, while B. thuringiensis that evolved before B. oleronius exhibited greater sensitivity as implied by the higher maintenance coefficient obtained for the hyper-osmotic cultures. As K90 exhibited low maintenance requirements in hyperosmotic cultures, close phylogenetic relationship with B. thuringiensis, along with the reported property of encapsulation of insecticidal crystal proteins (Cry) in Bacillus strains and endophytic nature of B. mojavensis, strongly suggest that K90 could be a promising surrogate host for the transgenic delivery of "Cry" proteins.

Efficient expression of vip184DeltaP gene under the control of promoters plus Shine-Dalgarno (SD) sequences of cry genes from Bacillus thuringiensis

AIMS

To compare vip184DeltaP gene expression time course and Vip184 protein yield under the control of promoters and Shine-Dalgarno (SD) sequences of vip184, cry3A and cry1A gene from Bacillus thuringiensis respectively.

METHODS AND RESULTS

Derived from the shuttle vector pHT3101, recombinant plasmids pHPT3, pHTP3A(Delta)P and pHTP1A(Delta)P were constructed with the native vip184 gene and the vip184(Delta)P gene, either under the control of promoters and SD sequences of cry3A or cry1A genes. When the above plasmids were transformed into an acrystalliferous B. thuringiensis strain Cry(-)B, their expression time course were consistent with those of vip184, cry3A and cry1A gene respectively. The maximum yields of Vip184 protein were increased when under the control of promoters plus SD sequences of cry3A and cry1A gene.

CONCLUSIONS

The results showed that both cry3A and cry1A promoter/SD sequence combinations were able to enhance synthesis of Vip184 and change its expression time course.

SIGNIFICANCE AND IMPACT OF THE STUDY

Both cry3A and cry1A promoter/SD systems offer a method for improving the expression efficacy of the vip184 gene in B. thuringiensis and it is possible to co-express the vip184 gene and cry genes and accumulate Vip184 in the form of inclusion bodies by these systems in order to construct novel useful B. thuringiensis engineered strains.

Growth, Sporulation, δ-Endotoxins Synthesis, and Toxicity During Culture of Bacillus thuringiensis H14

Growth, sporulation, synthesis of delta-endotoxins, and toxicity against the larvae of Aedes aegypti and Culex pipiens were studied during fermentation of Bacillus thuringiensis H14 in a 20-L fermentor. Measurements of optical density and dielectric permittivity for biomass determination suggest a highly promising technique for on-line evaluation of sporulation. The synthesis of 65-, 25- and 130-kDa proteins started at 16, 18, and 23 h, respectively. These proteins were enriched in different ways until the end of culture (48 h). Toxicity in the course of sporulation was significantly different for the larvae of both mosquito species. Maximal activity against Ae. aegypti was obtained at the end of culture, whereas for Cx. pipiens, the sample at 38 h was the most active.

Broadening the insecticidal spectrum of Lepidoptera-specificBacillus thuringiensis strains by chromosomal integration ofcry3A

A TnpI-TnpIA-mediated and thermosensitive recombination system was developed to construct genetically modified Bacillus thuringiensis strains encoding a crystal protein particularly active against Coleopteran species. Based on B. thuringiensis transposon Tn4430, an integrative vector, pBMB-R14E, was constructed, by which the cry3A delta-endotoxin gene highly toxic to Lepidoptera was delivered into a wildtype B. thuringiensis subsp. kurstaki strain YBT1520. The cry3A gene was integrated into the chromosome of the host strain. Then the integrative vector was eliminated by moving recombinant cultures to 46 degrees C. Two recombinant B. thuringiensis strains, BMB1520-S and BMB1520-T, were obtained. In recombinant strains, the cry3A gene was stably expressed in measurable amounts and did not reduce the expression of endogenous crystal protein genes. Bioassay results showed that BMB1520-S and BMB1520-T, in addition to the activity against lepidopteran Plutella xylostella third-instar larvae present in the parental strains, exhibited a high level of activity against coleopteran Rhyllodecta vulgatissima third-instar larvae, absent from the parental strains.

Characterization of two Bacillus thuringiensis genes identified by in vivo screening of virulence factors

Bacillus thuringiensis vegetative cells are known to be highly pathogenic when injected into the hemocoel of susceptible insect larvae. This pathogenicity is due to the capacity of B. thuringiensis to cause septicemia in the host. We screened a B. thuringiensis mini-Tn10 insertion library for loss of virulence against Bombyx mori larvae on injection into the hemocoel. Three clones with attenuated virulence were isolated, corresponding to two different mini-Tn10 insertions mapping to the yqgB/yqfZ locus. Single disruptions of the yqgB and yqfZ genes did not affect virulence against B. mori. In contrast, the inactivation of both genes simultaneously reproduced the effect of the mini-Tn10 insertion and resulted in a significant delay to infection. The double DeltayqgB DeltayqfZ mutant was also nonmotile, and its growth was affected at 25 degrees C. We analyzed lacZ transcriptional fusions and detected promoter activity upstream from yqgB at 25 and 37 degrees C. Overall, our findings suggest that the yqgB and yqfZ genes encode adaptive factors that may act in synergy, enabling the bacteria to cope with the physical environment in vivo, facilitating colonization of the host.

Comparison of cytotoxin cytK promoters from Bacillus cereus strain ATCC 14579 and from a B. cereus food-poisoning strain

The cytotoxin CytK produced by Bacillus cereus is believed to be involved in food-borne diseases. The transcriptional activity of the cytK promoter region in a food-poisoning strain was studied using a reporter gene and compared with that in the reference B. cereus strain ATCC 14579. In the food-poisoning strain, cytK is more strongly transcribed, possibly explaining the pathogenicity. The global regulator PlcR in B. cereus controls several putative virulence factors. It was found that PlcR regulates cytK in this clinical strain despite a mismatch in the PlcR recognition site, as currently defined. This suggests that the PlcR box consensus should be reconsidered and that the PlcR regulon might be larger than suspected. It is also shown that the high level of cytK transcription is not caused by a modification in the PlcR recognition site.

An extracytoplasmic-function sigma factor is involved in a pathway controlling beta-exotoxin I production in Bacillus thuringiensis subsp. thuringiensis strain 407-1

Beta-exotoxin I is an insecticidal nucleotide analogue secreted by various Bacillus thuringiensis strains. In this report, we describe the characterization and transcriptional analysis of a gene cluster, designated sigW-ecfX-ecfY, that is essential for beta-exotoxin I production in B. thuringiensis subsp. thuringiensis strain 407-1. In this strain, the disruption of the sigW cluster resulted in nontoxic culture supernatants. sigW encodes a protein of 177 residues that is 97 and 94% identical to two putative RNA polymerase extracytoplasmic-function-type sigma factors from Bacillus anthracis strain Ames and Bacillus cereus strain ATCC 14579, respectively. It is also 50, 30, and 26% identical to SigW from Clostridium perfringens and SigW and SigX from Bacillus subtilis, respectively. EcfX, encoded by the gene following sigW, significantly repressed the expression of sigW when both genes were overtranscribed, suggesting that it could be the anti-sigma factor of SigW. Following the loss of its curable cry plasmid, strain 407 became unable to synthesize crystal toxins, in contrast to the mutant strain 407-1(Cry-)(Pig+), which overproduced this molecule in the absence of this plasmid. Transcriptional analysis of sigW indicated that this gene was expressed during the stationary phase and only in the 407-1(Cry-)(Pig+) mutant. This suggests that in the wild type-407(Cry+) strain, beta-exotoxin I was produced from determinants located on a cry gene-bearing plasmid and that sigW is able to induce beta-exotoxin I production in B. thuringiensis in the absence of cry gene-bearing plasmids. Although the signal responsible for this activation is unknown, these results indicate that beta-exotoxin I production in B. thuringiensis can be restored or induced via an alternative pathway that requires sigW expression.

The Bacillus thuringiensis PlcR-regulated gene inhA2 is necessary, but not sufficient, for virulence

We previously reported that Bacillus thuringiensis strain 407 Cry 32(-) secretes a zinc-requiring metalloprotease, InhA2, that is essential for virulence in orally infected insects. Analysis of the inhA2-lacZ transcriptional fusion showed that inhA2 expression is repressed in a PlcR(-) background. Using DNase I footprinting experiments, we demonstrated that PlcR activates inhA2 transcription directly by binding to a DNA sequence showing a one-residue mismatch with the previously reported PlcR box. It was previously reported that PlcR is essential for B. thuringiensis virulence in oral infection by contributing to the synergistic properties of the spores on the insecticidal activity of the Cry1C protein. We used complementation experiments to investigate whether the PlcR(-) phenotype was due to the absence of InhA2. The results indicated that overexpression of inhA2 in the (Delta)plcR strain did not restore the wild-type phenotype. However, virulence was fully restored in the (Delta)inhA2 complemented mutant. Thus, inhA2 is the first example of a PlcR-regulated gene found to be directly involved in virulence. However, it is not sufficient for pathogenicity when the other members of the PlcR regulon are lacking. This suggests that InhA2 may act in concert with other PlcR-regulated gene products to provide virulence.

Distinct clpP genes control specific adaptive responses in Bacillus thuringiensis

ClpP and ClpC are subunits of the Clp ATP-dependent protease, which is ubiquitous among prokaryotic and eukaryotic organisms. The role of these proteins in stress tolerance, stationary-phase adaptive responses, and virulence in many bacterial species has been demonstrated. Based on the amino acid sequences of the Bacillus subtilis clpC and clpP genes, we identified one clpC gene and two clpP genes (designated clpP1 and clpP2) in Bacillus thuringiensis. Predicted proteins ClpP1 and ClpP2 have approximately 88 and 67% amino acid sequence identity with ClpP of B. subtilis, respectively. Inactivation of clpC in B. thuringiensis impaired sporulation efficiency. The clpP1 and clpP2 mutants were both slightly susceptible to salt stress, whereas disruption of clpP2 negatively affected sporulation and abolished motility. Virulence of the clp mutants was assessed by injecting bacteria into the hemocoel of Bombyx mori larvae. The clpP1 mutant displayed attenuated virulence, which appeared to be related to its inability to grow at low temperature (25 degrees C), suggesting an essential role for ClpP1 in tolerance of low temperature. Microscopic examination of clpP1 mutant cells grown at 25 degrees C showed altered bacterial division, with cells remaining attached after septum formation. Analysis of lacZ transcriptional fusions showed that clpP1 was expressed at 25 and 37 degrees C during the entire growth cycle. In contrast, clpP2 was expressed at 37 degrees C but not at 25 degrees C, suggesting that ClpP2 cannot compensate for the absence of ClpP1 in the clpP1 mutant cells at low temperature. Our study demonstrates that ClpP1 and ClpP2 control distinct cellular regulatory pathways in B. thuringiensis.

The InhA2 metalloprotease of Bacillus thuringiensis strain 407 is required for pathogenicity in insects infected via the oral route

The entomopathogenic bacterium Bacillus thuringiensis is known to secrete a zinc metalloprotease (InhA) that specifically cleaves antibacterial peptides produced by insect hosts. We identified a second copy of the inhA gene, named inhA2, in B. thuringiensis strain 407 Cry(-). The inhA2 gene encodes a putative polypeptide showing 66.2% overall identity with the InhA protein and harboring the zinc-binding domain (HEXXH), which is characteristic of the zinc-requiring metalloproteases. We used a transcriptional inhA2'-lacZ fusion to show that inhA2 expression is induced at the onset of the stationary phase and is overexpressed in a Spo0A minus background. The presence of a reverse Spo0A box in the promoter region of inhA2 suggests that Spo0A directly regulates the transcription of inhA2. To determine the role of the InhA and InhA2 metalloproteases in pathogenesis, we used allelic exchange to isolate single and double mutant strains for the two genes. Spores and vegetative cells of the mutant strains were as virulent as those of the parental strain in immunized Bombyx mori larvae infected by the intrahemocoelic route. Exponential phase cells of all the strains displayed the same in vitro potential for colonizing the vaccinated hemocoel. We investigated the synergistic effect of the mutant strain spores on the toxicity of Cry1C proteins against Galleria mellonella larvae infected via the oral pathway. The spores of DeltainhA2 mutant strain were ineffective in providing synergism whereas those of the DeltainhA mutant strain were not. These results indicate that the B. thuringiensis InhA2 zinc metalloprotease has a vital role in virulence when the host is infected via the oral route.

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.

Regulation of toxin and virulence gene transcription in Bacillus thuringiensis

Bacillus thuringiensis is a spore-forming bacterium well known for its insecticidal properties and its ability to produce a crystal inclusion during sporulation. The specific activity of B. thuringiensis against insect larvae is due to the crystal proteins (Cry proteins). Two different transcriptional mechanisms (dependent and independent of sporulation) are responsible for cry gene transcription during the stationary phase. In addition to these specific insecticidal toxins, B. thuringiensis produces potential virulence factors including haemolysins, degradative enzymes and enterotoxins. A pleiotropic regulator (PlcR) that activates the transcription of various genes encoding such extracellular proteins has been identified. Its expression at the onset of the stationary phase is dependent on the growth medium and is controlled by the transition state regulator, SpoOA.

Development and field performance of a broad-spectrum nonviable asporogenic recombinant strain of Bacillus thuringiensis with greater potency and UV resistance

The main problems with Bacillus thuringiensis products for pest control are their often narrow activity spectrum, high sensitivity to UV degradation, and low cost effectiveness (high potency required). We constructed a sporulation-deficient SigK(-) B. thuringiensis strain that expressed a chimeric cry1C/Ab gene, the product of which had high activity against various lepidopteran pests, including Spodoptera littoralis (Egyptian cotton leaf worm) and Spodoptera exigua (lesser [beet] armyworm), which are not readily controlled by other Cry delta-endotoxins. The SigK(-) host strain carried the cry1Ac gene, the product of which is highly active against the larvae of the major pests Ostrinia nubilalis (European corn borer) and Heliothis virescens (tobacco budworm). This new strain had greater potency and a broader activity spectrum than the parent strain. The crystals produced by the asporogenic strain remained encapsulated within the cells, which protected them from UV degradation. The cry1C/Ab gene was introduced into the B. thuringiensis host via a site-specific recombination vector so that unwanted DNA was eliminated. Therefore, the final construct contained no sequences of non-B. thuringiensis origin. As the recombinant strain is a mutant blocked at late sporulation, it does not produce viable spores and therefore cannot compete with wild-type B. thuringiensis strains in the environment. It is thus a very safe biopesticide. In field trials, this new recombinant strain protected cabbage and broccoli against a pest complex under natural infestation conditions.

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 thuringiensis insecticidal proteins: molecular mode of action

Growing interest in biorational pesticides has placed the Bacillus thuringiensis insecticidal crystal proteins at the forefront of pesticides for plant genetic engineering. The development of improvement pesticides, both in enhanced activity and broader host range, depends on an understanding of its mechanism of action. This review presents a complete overview of the bacterium and the group of insecticidal proteins known as Cry proteins or delta-endotoxins. The molecular mode of action is described in detail, including the mapping of receptor binding sites by site-directed mutagenesis, the known receptors, and the ion-channel activity of the toxins.

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.

Engineering Bacillus thuringiensis bioinsecticides with an indigenous site-specific recombination system

The cry genes of Bacillus thuringiensis encode a diverse group of crystal-forming proteins that exhibit insecticidal activity, particularly against the larvae of lepidopteran, coleopteran, and dipteran insects. The efficacy of B. thuringiensis-based biopesticides may be improved through the genetic manipulation of these genes. A gene transfer system has been developed for the introduction and maintenance of cloned insecticidal cry genes on small plasmids in B. thuringiensis. This vector system combines a B. thuringiensis plasmid replicon and an indigenous site-specific recombination system that allows for the selective removal of ancillary or foreign DNA from the recombinant bacterium after introduction of the Cry-encoding plasmid. The site-specific recombination system is useful for engineering strains with unique combinations of cry genes, resulting in new active ingredients with improved insecticidal properties.

Construction of new insecticidal Bacillus thuringiensis recombinant strains by using the sporulation non-dependent expression system of cryIIIA and a site specific recombination vector

Bacillus thuringiensis (Bt) delta-endotoxins are safe biological insecticidal proteins whose usefulness has long been recognized. The first commercialized Bt insecticidal formulations were composed of spore-crystal preparations derived from wild-type strains. These products generally have a limited insecticidal host range and several genetically modified strains have, therefore, been constructed using transformation procedures. However, addition of a new delta-endotoxin gene to strains already harboring other delta-endotoxin genes often resulted in broader-spectrum but less potent products because they produced significantly less of each of the crystal proteins. We report expression of the coding sequence of the sporulation specific cryIC gene from the non-sporulation-dependent cryIIIA promoter. Large amounts of CryIC accumulated in various Bt strains with different genetic backgrounds. Sporulation deficient Spo0A mutants, acrystalliferous derivatives and wild-type Bt strains expressing the engineered cryIII-cryIC gene were obtained. Introduction of the cryIII-cryIC gene whose product is highly active against Spodoptera littoralis into the Kto strain harboring the cryIA(c) gene active against Ostrinia nubilalis resulted in the construction of a new strain with increased potency and broader activity spectrum than the parent strain. Large amounts of each toxin were produced and the expression of the two genes seemed to be summed, presumably because the expression systems of the two genes are different. The plasmid shuttle vector used to introduce the cryIII-cryIC gene into the different Bt hosts utilizes the specific resolution site of transposon Tn4430 to enable construction of recombinant Bt strains that are free of foreign non-Bt DNA. This should facilitate the approval and acceptance for environmental release of the insecticidal recombinant products.