Activation of the extracytoplasmic function σ factor σV by lysozyme

sigma(V) is an extracytoplasmic function (ECF) sigma factor that is found exclusively in Firmicutes including Bacillus subtilis and the opportunistic pathogens Clostridioides difficile and Enterococcus faecalis. sigma(V) is activated by lysozyme and is required for lysozyme resistance. The activity of sigma(V) is normally inhibited by the anti-sigma factor RsiV, a transmembrane protein. RsiV acts as a receptor for lysozyme. The binding of lysozyme to RsiV triggers a signal transduction cascade which results in degradation of RsiV and activation of sigma(V). Like the anti-sigma factors for several other ECF sigma factors, RsiV is degraded by a multistep proteolytic cascade that is regulated at the step of site-1 cleavage. Unlike other anti-sigma factors, site-1 cleavage of RsiV is not dependent upon a site-1 protease whose activity is regulated. Instead constitutively active signal peptidase cleaves RsiV at site-1 in a lysozyme-dependent manner. The activation of sigma(V) leads to the transcription of genes, which encode proteins required for lysozyme resistance.