An Extracytoplasmic-Function Sigma Factor Is Involved in a Pathway Controlling β-Exotoxin I Production in Bacillus thuringiensis subsp. thuringiensis Strain 407-1

Abstract

ABSTRACT 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 - ecfX - ecfY , 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.