We demonstrate that the Bacillus subtilis fosB(yndN) gene encodes a fosfomycin resistance protein. Expression of fosB requires ς ^W , and both fosB and sigW mutants are fosfomycin sensitive. FosB is a metallothiol transferase related to the FosA class of Mn ²⁺ -dependent glutathione transferases but with a preference for Mg ²⁺ and l -cysteine as cofactors.

Bacillus subtilis contains seven extracytoplasmic-function σ factors that activate partially overlapping regulons. We here identify four additional members of the σ ^X regulon, pbpX (penicillin-binding protein), ywnJ , the dlt operon ( d -alanylation of teichoic acids), and the pss ybfM psd operon (phosphatidylethanolamine biosynthesis). Modification of teichoic acids by esterification with d -alanine and incorporation of phosphatidylethanolamine into the cell membrane have a common consequence: in both cases positively charged amino groups are introduced into the cell envelope. The resulting reduction in the net negative charge of the cell envelope has been previously implicated as a resistance mechanism specific for cationic antimicrobial peptides. Consistent with this notion, we find that both sigX and dltA mutants are more sensitive to nisin than wild-type cells. We conclude that activation of the σ ^X regulon serves to alter cell surface properties to provide protection against antimicrobial peptides.

The Bacillus subtilis sigW gene encodes an extracytoplasmic function (ECF) σ factor that is expressed in early stationary phase from a sigW‐dependent autoregulatory promoter, P_W. Using a consensus‐based search procedure, we have identified 15 operons preceded by promoters similar in sequence to P_W. At least 14 of these promoters are dependent on σ^W both in vivo and in vitro as judged by lacZ reporter fusions, run‐off transcription assays and nucleotide resolution start site mapping. We conclude that σ^W controls a regulon of more than 30 genes, many of which encode membrane proteins of unknown function. The σ^W regulon includes a penicillin binding protein (PBP4*) and a co‐transcribed amino acid racemase (RacX), homologues of signal peptide peptidase (YteI), flotillin (YuaG), ABC transporters (YknXYZ), non‐haem bromoperoxidase (YdjP), epoxide hydrolase (YfhM) and three small peptides with structural similarities to bacteriocin precursor polypeptides. We suggest that σ^W activates a large stationary‐phase regulon that functions in detoxification, production of antimicrobial compounds or both.

Bacitracin resistance is normally conferred by either of two major mechanisms, the BcrABC transporter, which pumps out bacitracin, or BacA, an undecaprenol kinase that provides C ₅₅ -isoprenyl phosphate by de novo synthesis. We demonstrate that the Bacillus subtilis bcrC ( ywoA ) gene, encoding a putative bacitracin transport permease, is an important bacitracin resistance determinant. A bcrC mutant strain had an eightfold-higher sensitivity to bacitracin. Expression of bcrC initiated from a single promoter site that could be recognized by either of two extracytoplasmic function (ECF) σ factors, σ ^X or σ ^M . Bacitracin induced expression of bcrC , and this induction was dependent on σ ^M but not on σ ^X . Under inducing conditions, expression was primarily dependent on σ ^M . As a consequence, a sigM mutant was fourfold more sensitive to bacitracin, while the sigX mutant was only slightly sensitive. A sigX sigM double mutant was similar to a bcrC mutant in sensitivity. These results support the suggestion that one function of B. subtilis ECF σ factors is to coordinate antibiotic stress responses.