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Abstract
Pairs of the ECF sigma factor and its anti-sigma factor, SigW and RsiW, of Bacillus-related species that inhabit extreme environments were heterologously expressed in B. subtilis. All the RsiWs, membrane proteins, failed to fill their function of repressing cognate SigW activity, despite their close structural similarities. Particularly, uncontrolled expression of Oceanobacillus iheyensis OISigW due to abortive OIRsiW was harmful to B. subtilis. Analysis of revertants of this growth defect and site-directed mutagenesis indicated that the insertion of six and a minimum of three hydrophobic amino acid residues occurring in the transmembrane region allowed OIRsiW to function as anti-OISigW. Subcellular localization of OIRsiW was detected by immunoblot analysis, suggesting that both the wild-type and the mutant form of OIRsiW were localized to the membrane. An appropriate length of a transmembrane region required for proper integration into the membrane after translocation might vary among these Bacillus-related species.