Bacillus subtilis σV Confers Lysozyme Resistance by Activation of Two Cell Wall Modification Pathways, Peptidoglycan O-Acetylation and D-Alanylation of Teichoic Acids

The seven extracytoplasmic function (ECF) sigma (sigma) factors of Bacillus subtilis are broadly implicated in resistance to antibiotics and other cell envelope stressors mediated, in part, by regulation of cell envelope synthesis and modification enzymes. We here define the regulon of sigma(V) as including at least 20 operons, many of which are also regulated by sigma(M), sigma(X), or sigma(W). The sigma(V) regulon is strongly and specifically induced by lysozyme, and this induction is key to the intrinsic resistance of B. subtilis to lysozyme. Strains with null mutations in either sigV or all seven ECF sigma factor genes (Delta 7ECF) have essentially equal increases in sensitivity to lysozyme. Induction of sigma(V) in the Delta 7ECF background restores lysozyme resistance, whereas induction of sigma(M), sigma(X), or sigma(W) does not. Lysozyme resistance results from the ability of sigma(V) to activate the transcription of two operons: the autoregulated sigV-rsiV-oatA-yrhK operon and dltABCDE. Genetic analyses reveal that oatA and dlt are largely redundant with respect to lysozyme sensitivity: single mutants are not affected in lysozyme sensitivity, whereas an oatA dltA double mutant is as sensitive as a sigV null strain. Moreover, the sigV oatA dltA triple mutant is no more sensitive than the oatA dltA double mutant, indicating that there are no other sigma(V)-dependent genes necessary for lysozyme resistance. Thus, we suggest that sigma(V) confers lysozyme resistance by the activation of two cell wall modification pathways: O-acetylation of peptidoglycan catalyzed by OatA and D-alanylation of teichoic acids by DltABCDE.