The Bacillus subtilis sigmaW anti-sigma factor RsiW is degraded by intramembrane proteolysis through YluC

The Bacillus subtilis sigma(W) regulon is induced by different stresses such as alkaline shock, salt shock, phage infection and certain antibiotics that affect cell wall biosynthesis. The activity of the alternative, extracytoplasmic function (ECF) sigma factor sigma(W) is modulated by a specific anti-sigma factor (RsiW or YbbM) encoded by the rsiW (ybbM) gene located immediately downstream of sigW. The RsiW membrane topology was determined, and a specific reporter system for RsiW function was constructed. Experiments using the yeast two-hybrid system suggested a direct interaction of sigma(W) with the cytoplasmic part of RsiW. Analysis of truncated forms of the RsiW protein revealed that sigma(W) induction by alkaline shock is dependent on both the transmembrane and the extracytoplasmic domain of RsiW. Western blot and pulse-chase experiments demonstrated degradation of RsiW after an alkaline shock. A B. subtilis mutant strain deleted for the Escherichia coli yaeL orthologue yluC, encoding a transmembrane protease, was defective in inducing a sigma(W)-controlled promoter after alkaline shock and accumulated a membrane-bound truncated form of RsiW, suggesting that the activity of sigma(W) is controlled by the proteolysis of RsiW by at least two different proteolytic steps.