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β-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-ecfXecfY, that is essential for β-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, β-exotoxin I was produced from determinants located on a cry gene-bearing plasmid and that sigW is able to induce β-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 β-exotoxin I production in B. thuringiensis can be restored or induced via an alternative pathway that requires sigW expression. [ABSTRACT FROM AUTHOR]

Little is known about the occurrence and linkage between secreted insecticidal virulence factors in natural populations ofBacillus thuringiensis(Bt). We carried out a survey of 392Btstrains isolated from various samples originating from 31 countries. The toxicity profile of the culture supernatants of these strains was determined individually againstAnthonomus grandis(Coleoptera) andSpodoptera littoralis(Lepidoptera). We analyzed ß-exotoxin I production and searched for the genes encoding Vip1-2, Vip3, and Cry1I toxins in 125 of these strains. Our results showed that these insecticidal toxins were widespread inBtbut that their distribution was nonrandom, with significant linkage observed betweenvip3andcry1Iand betweenvip1-2and ß-exotoxin I. Strains producing significant amounts of ß-exotoxin I were more frequently isolated from invertebrate samples than from dust, water, soil, or plant samples. [ABSTRACT FROM AUTHOR]

Analyzes the Bacillus thuringiensis strain which overproduces β-exotoxin. Involvement of β-exotoxin I production; Toxicity of the culture supernatants of strain; Transposon mutagenesis of strain.

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 p,g of β-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 μg/ml. However, these amounts of β-exotoxin I were below the threshold for detectable toxicity against this insect species. Secretion of large amounts of β-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 β-exotoxin I. [ABSTRACT FROM AUTHOR]