Mechanism of inhibition of DNA-dependent RNA polymerase by exotoxin of Bacillus thuringiensis

Thuringiensin: a thermostable secondary metabolite from Bacillus thuringiensis with insecticidal activity against a wide range of insects

Thuringiensin (Thu), also known as β-exotoxin, is a thermostable secondary metabolite secreted by Bacillus thuringiensis. It has insecticidal activity against a wide range of insects, including species belonging to the orders Diptera, Coleoptera, Lepidoptera, Hymenoptera, Orthoptera, and Isoptera, and several nematode species. The chemical formula of Thu is C22H32O19N5P, and it is composed of adenosine, glucose, phosphoric acid, and gluconic diacid. In contrast to the more frequently studied insecticidal crystal protein, Thu is not a protein but a small molecule oligosaccharide. In this review, a detailed and updated description of the characteristics, structure, insecticidal mechanism, separation and purification technology, and genetic determinants of Thu is provided.

The role of ATP-binding cassette transporters in bacterial pathogenicity

The ATP-binding cassette transporter superfamily is present in all three domains of life. This ubiquitous class of integral membrane proteins have diverse biological functions, but their fundamental role involves the unidirectional translocation of compounds across cellular membranes in an ATP coupled process. The importance of this class of proteins in eukaryotic systems is well established as typified by their association with genetic diseases and roles in the multi-drug resistance of cancer. In stark contrast, the ABC transporters of prokaryotes have not been exhaustively investigated due to the sheer number of different roles and organisms in which they function. In this review, we examine the breadth of functions associated with microbial ABC transporters in the context of their contribution to bacterial pathogenicity and virulence.

Screening and identification of a Bacillus thuringiensis strain S1/4 with large and efficient insecticidal activities

The bacterium Bacillus thuringiensis was recognized for its entomopathogenic activities related to Cry and Cyt proteins forming the δ-endotoxins and some extracellular activities like the vegetative insecticidal proteins (VIP) and Cry1I. These activities may act specifically against diverse organisms and some of them typically characterize each strain. Here, we screened a set of 212 B. thuringiensis strains to search the higher insecticidal activities. These strains had bipyramidal and cubic crystal morphologies and 30% of them showed PCR amplification of vip3 internal region, from which five isolates (S1/4, S17, S122, S123, and S144) showed plasmid profile variability. These five strains contained the cry1I, cry1Aa and/or cry1Ac, cry1Ab and cry2 genes, and S1/4 harbored in addition the cry1C, vip1, and vip2 genes. They produced from 25 to 46 µg δ-endotoxin per 10(7) spores. Their δ-endotoxins displayed distinct lethal concentrations 50% against either Spodoptera littoralis or Ephestia kuehniella larvae with the lowest one for S1/4, which was also active against Tuta absoluta. Fortunately, the analysis of the culture supernatants revealed that S1/4 had the higher toxicity towards these lepidopteron but it did not show any toxicity against the Tribolium castaneum coleopteran larvae; additionally, S1/4 displayed an antibacterial activity. S1/4 is a good candidate for agricultural pest control, as it is more efficient than the reference strain HD1.

Genome-wide screening reveals the genetic determinants of an antibiotic insecticide in Bacillus thuringiensis

Thuringiensin is a thermostable secondary metabolite in Bacillus thuringiensis and has insecticidal activity against a wide range of insects. Until now, the regulatory mechanisms and genetic determinants involved in thuringiensin production have remained unclear. Here, we successfully used heterologous expression-guided screening in an Escherichia coli-Bacillus thuringiensis shuttle bacterial artificial chromosome library, to clone the intact thuringiensin synthesis (thu) cluster. Then the thu cluster was located on a 110-kb endogenous plasmid bearing insecticide crystal protein gene cry1Ba in strain CT-43. Furthermore, the plasmid, named pBMB0558, was indirectly cloned and sequenced. The gene functions on pBMB0558 were annotated by BLAST based on the GenBank(TM) and KEGG databases. The genes on pBMB0558 could be classified into three functional modules: a thuringiensin synthesis cluster, a type IV secretion system-like module, and mobile genetic elements. By HPLC coupling mass spectrometer, atmospheric pressure ionization with ion trap, and TOF technologies, biosynthetic intermediates of thuringiensin were detected. The thuE gene is proved to be responsible for the phosphorylation of thuringiensin at the last step by vivo and vitro activity assays. The thuringiensin biosynthesis pathway was deduced and clarified. We propose that thuringiensin is an adenine nucleoside oligosaccharide rather than an adenine nucleotide analog, as is traditionally believed, based on the predicted functions of the key enzymes, glycosyltransferase (ThuF) and exopolysaccharide polymerization protein (Thu1).

Screening and identification of vip genes in Bacillus thuringiensis strains

AIMS

To identify known vip genes and to detect potentially novel vip genes in a collection of 507 strains of Bacillus thuringiensis.

METHODS AND RESULTS

Following a polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) strategy, four restriction patterns were found within the vip1 family: vip1Aa1, vip1Ba1/vip1Ba2 and vip1Ca. In the screening of vip2 genes, patterns similar to those of vip2Aa1, vip2Ba1/vip2Ba2 and vip2Ac1 genes were observed. Patterns for vip3Aa1, vip3Ae2 and vip3Af1 were found among vip3 genes. Two new patterns revealed novel vip1 and vip3A genes. The observed frequency of genes belonging to vip1 and vip2 families was around 10%, whereas 48.9% of the strains showed amplification of vip3 genes. A tendency of vip and cry genes to occur together has been observed in this collection of B. thuringiensis strains.

CONCLUSIONS

Ten different patterns of vip genes belonging to the three vip families and two novel vip genes have been identified in this study.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first time that vip1 and vip2 genes have been identified by PCR-RFLP. Furthermore, the results show that the strategy used in this study can lead to the classification of known vip genes as well as the identification of novel vip genes.

Vip3A is responsible for the potency of Bacillus thuringiensis 9816C culture supernatant against Helicoverpa armigera and Spodoptera exigua

Culture supernatant of Bacillus thuringiensis 9816C had high toxicity against Helicoverpa armigera and Spodoptera exigua. However, it lost insecticidal activities after being bathed in boiling water for 5 min. Acrystalliferous mutants of Bt9816C (Bt9816C-NP1 and Bt9816C-NP2) cured of its endogenous plasmids no longer possessed vip3A gene and toxicity. The 89 kD protein which existed in Bt9816C supernatant disappeared in the two mutants' supernatant; nevertheless, the two mutants still exhibited hemolytic and phospholipase C activity as Bt9816C did. The vip3A gene of Bt9816C, vip3Aa18, was cloned and expressed in Escherichia coli BL21. Bioassay demonstrated that the recombinant E. coli had high toxicity against S. exigua. Taken together, it suggested that Vip3A protein was responsible for the toxicity of Bt9816C culture supernatants.

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.

Biopesticides

The term 'biopesticide' encompasses a broad array of microbial pesticides, biochemicals derived from micro-organisms and other natural sources, and processes involving the genetic incorporation of DNA into agricultural commodities that confer protection against pest damage (plant-incorporated protectants). Some microbial pesticides, such as Bacillus thuringiensis, have a long history of safe and effective use as a biological insecticide. More recent developments in microbial pest control include the utilisation of other bacterial and fungal species that may competitively inhibit the growth of pathogenic and toxigenic micro-organisms on important agricultural commodities. The use of microbes and their gene products introduces additional considerations to the toxicological dose-response relationship, including a need to determine the plausibility of infectious and immunological effects in association with human exposure to these biopesticides in food or the environment. Studies of substantial equivalence suggest that foods currently derived from plant-incorporated protectants are not likely to differ from conventional foods. However, there is general consensus that the scientific methods to assess risks from genetically modified foods and micro-organisms will continue to evolve in the future.

An ABC transporter from Bacillus thuringiensis is essential for beta-exotoxin I production

beta-Exotoxin I is a nonspecific insecticidal metabolite secreted by some Bacillus thuringiensis strains. Several studies of B. thuringiensis strains that have lost the capacity to produce beta-exotoxin I have suggested that there is a strong correlation between high levels of beta-exotoxin I production and the ability to synthesize crystal proteins. In this study, we showed that a mutant strain, B. thuringiensis 407-1(Cry(-))(Pig(+)), with no crystal gene, produced considerable amounts of beta-exotoxin I together with a soluble brown melanin pigment. Therefore, beta-exotoxin I production can take place after a strain has lost the plasmids bearing the cry genes, which suggests that these curable plasmids probably contain determinants involved in the regulation of beta-exotoxin I production. Using a mini-Tn10 transposon, we constructed a library of strain 407-1(Cry(-))(Pig(+)) mutants. We screened for nonpigmented mutants with impaired beta-exotoxin I production and identified a genetic locus harboring two genes (berA and berB) essential for beta-exotoxin I production. The deduced amino acid sequence of the berA gene displayed significant similarity to the ATP-binding domains of the DRI (drug resistance and immunity) family of ATP-binding cassette (ABC) proteins involved in drug resistance and immunity to bacteriocins and lantibiotics. The berB gene encodes a protein with six putative transmembrane helices, which probably constitutes the integral membrane component of the transporter. The demonstration that berAB is required for beta-exotoxin I production and/or resistance in B. thuringiensis adds an adenine nucleotide analog to the wide range of substrates of the superfamily of ABC proteins. We suggest that berAB confers beta-exotoxin I immunity in B. thuringiensis, through active efflux of the molecule.

Correspondence of high levels of beta-exotoxin I and the presence of cry1B in Bacillus thuringiensis

Examination of 640 natural isolates of Bacillus thuringiensis showed that the 58 strains (9%) whose supernatants were toxic to Anthonomus grandis (Coleoptera: Curculionidae) produced between 10 and 175 micro g of beta-exotoxin I per ml. We also found that 55 (46%) of a sample of 118 strains whose culture supernatants were not toxic to A. grandis nevertheless produced between 2 and 5 micro g/ml. However, these amounts of beta-exotoxin I were below the threshold for detectable toxicity against this insect species. Secretion of large amounts of beta-exotoxin I was strongly associated with the presence of cry1B and vip2 genes in the 640 natural B. thuringiensis isolates studied. We concluded that strains carrying cry1B and vip2 genes also possess, on the same plasmid, genetic determinants necessary to promote high levels of production of beta-exotoxin I.

Sample preparation for beta-exotoxin determination in Bacillus thuringiensis cultures by reversed-phase high-performance liquid chromatography

Beta-exotoxin is a nucleotide analogue produced by the entomopathogenic bacterium Bacillus thuringiensis. We have defined two new HPLC procedures for quantification of this exotoxin in culture supernatants of B. thuringiensis grown in poor or rich medium. The sample is prepared either by precipitation in solvent or by solid-phase extraction. Solvent precipitation is achieved treating the sample with acetone and acetonitrile. Solid-phase extraction is performed with a C18 and an anion-exchange cartridge. Reversed-phase HPLC with gradient elution of the prepared samples gives a limit of quantitation of 2 microg/ml for samples prepared by solvent precipitation and of 0.3 microg/ml for samples prepared by solid-phase extraction.

Update on the detection of beta-exotoxin in Bacillus thuringiensis strains by HPLC analysis

AIMS

The current work aimed to study the presence of beta-exotoxin by high-performance liquid chromatography (HPLC) in supernatant fluids from final whole cultures of the 69 type strains and 13 subtypes of Bacillus thuringiensis strains, as well as from some insecticidal strains.

METHODS AND RESULTS

Results from HPLC and bioassays with Ephestia kuhniella (Lepidoptera Pyralidae) were compared. Type I beta-exotoxin was only detected in type strains representing serotypes H1, H9 and H10a,10b. Discrepancies between HPLC and bioassays were found in H8a,8b and some insecticidal strains, which suggests the occurrence of another soluble toxin different from type I beta-exotoxin, possibly type II beta-exotoxin.

CONCLUSION

This study shows the need to use bioassays to determine the presence of beta-exotoxin activity. However, HPLC is a fast and sensitive technique if only type I beta-exotoxin is to be determined. The occurrence of beta-exotoxin in a type strain does not imply production of this metabolite by other strains belonging to the same serovar.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results complete the characterization of type strains belonging to the International Entomopathogenic Bacillus Collection (Institut Pasteur, Paris, France).

Characterization of an avirulent pleiotropic mutant of the insect pathogen Bacillus thuringiensis: reduced expression of flagellin and phospholipases

An avirulent pleiotropic mutant of the insect pathogen Bacillus thuringiensis subsp. gelechiae, isolated by Heierson et al. (A. Heierson, I. Sidén, A. Kivaisi, and H. G. Boman, J. Bacteriol. 167:18-24, 1986) as a spontaneous phage-resistant mutant, was further characterized and found to lack the expression of phosphatidylcholine- and phosphatidylinositol-degrading phospholipase C, beta-lactamase, and flagellin because of the absence of corresponding mRNAs. The avirulent mutant was also found to be less efficient in killing insect cells in vitro than the wild type and to have altered behavior in vivo; wild-type B. thuringiensis does not circulate in the insect hemolymph after injection, whereas the avirulent mutant and nonpathogenic control bacteria remain in circulation. Flagella and motility may be important for virulence in the early stages of an infection; mutants with decreased motility appear less virulent when fed to Trichoplusia ni but not when injected. The 50% lethal doses of wild-type strain Bt13 and avirulent mutant strain Bt1302 were estimated to be 0.52 +/- 0.25 and 2,600 +/- 1,300 CFU per injected larva, respectively.

Identification of beta-exotoxin production, plasmids encoding beta-exotoxin, and a new exotoxin in Bacillus thuringiensis by using high-performance liquid chromatography

An improved high-performance liquid chromatography separation was developed to detect and quantify beta-exotoxin production in Bacillus thuringiensis culture supernatants. Exotoxin production was assigned to a plasmid in five strains, from three subspecies (B. thuringiensis subsp. thuringiensis serotype 1, B. thuringiensis subsp. tolworthi serotype 9, and B. thuringiensis subsp. darmstadiensis serotype 10). A new exotoxin, called type II beta-exotoxin in this report, was discovered in B. thuringiensis subsp. morrisoni serotype 8ab, purified, and partially characterized. This material is more specific than type I beta-exotoxin and is very active against the Colorado potato beetle, Leptinotarsa decemlineata.

Bacteriophage-resistant mutants of Bacillus thuringiensis with decreased virulence in pupae of Hyalophora cecropia

Starting from a crystal-negative parental strain of Bacillus thuringiensis, we isolated certain bacteriophage-resistant mutants which showed decreased virulence in pupae of the cecropia moth (Hyalophora cecropia). These strains (class I mutants) were highly pleiotropic and showed resistance to seven or eight different phages, sensitivity to methicillin, and loss of flagella. They were also more sensitive to cecropia immune hemolymph in vitro. In addition, the export of at least three proteins was reduced. Revertants (class II mutants) were sensitive to phages, virulent, and resistant to penicillin derivatives. One class II mutant was a complete revertant in all properties examined. The other class II mutant was an incomplete revertant still susceptible to immune hemolymph and with repressed export of proteins. Virulence was not coupled to phage resistance as such or to lack of flagella because other mutants affected in these properties were virulent. Other factors which could be excluded as causes of virulence were production of extracellular protease and hemolysin.

Evidence for two immune inhibitors from Bacillus thuringiensis interfering with the humoral defense system of saturniid pupae

Mutants of Bacillus thuringiensis lacking either beta-exotoxin or gamma-endotoxin were compared for their virulence using pupae of a giant silk moth. Known doses of viable log-phase bacteria were injected, and the response was followed as the number of viable bacteria in the hemolymph. The results obtained imply that, in the system used, neither the beta-exotoxin nor the gamma-endotoxin and the sporeforming ability are of importance for virulence. Results with sterile culture filtrate from B. thuringiensis have given evidence for the production of two inhibitors, A and B, which interfere with the humoral defense system in pupae of Hyalophora cecropia. Inhibitor A, which blocked the lysis of Escherichia coli,was precipitated by trichloroacetic acid and sensitive to heating. Inhibitor B, which blocked the killing of Bacillus cereus, was soluble in trichloroacetic acid and resistant to 90 degrees C for 5 min. Both inhibitors are believed to contribute to the insecticidal nature of B. thuringiensis.

Inhibition of hepatic deoxyribonucleic acid-dependent ribonucleic acid polymerases by the exotoxin of Bacillus thuringiensis in comparison with the effects of -amanitin and cordycepin

The action of Bacillus thuringiensis exotoxin, a structural analogue of ATP, on mouse liver DNA-dependent RNA polymerases was studied and its effects were compared with those of alpha-amanitin and cordycepin. (1) Administration of exotoxin in vivo caused a marked decrease in RNA polymerase activity of isolated nuclei at various concentrations of Mg(2+), Mn(2+) and (NH(4))(2)SO(4). A similar action was recorded after addition of exotoxin to isolated nuclei from control or exotoxin-treated mice. (2) Chromatographic separation of nuclear RNA polymerases from mice treated in vivo with exotoxin showed a drastic decrease of the peak of nucleoplasmic RNA polymerase, whereas the peak of nucleolar RNA polymerase remained unaltered. The same effect was observed after administration of alpha-amanitin in vivo, but cordycepin did not alter the relative amounts of the two main RNA polymerase peaks. (3) Administration of exotoxin in vivo did not alter the template activity of isolated DNA or chromatin tested with different fractions of RNA polymerase from control or exotoxin-treated mice. (4) Addition of exotoxin to isolated liver RNA polymerases inhibited both enzyme fractions. However, the alpha-amanitin-sensitive RNA polymerase was also 50-100-fold more sensitive to exotoxin inhibition than was the alpha-amanitin-insensitive RNA polymerase. Kinetic analysis indicated the exotoxin produces a competitive inhibition with ATP on the nucleolar enzyme, but a mixed type of inhibition with nucleoplasmic enzyme. The results obtained indicate that the B. thuringiensis exotoxin inhibits liver RNA synthesis by affecting nuclear RNA polymerases, showing a preferential inhibition of the nucleoplasmic alpha-amanitin-sensitive RNA polymerase.