σ V is an extracytoplasmic function (ECF) σ factor that is found exclusively in Firmicutes including Bacillus subtilis and the opportunistic pathogens Clostridioides difficile and Enterococcus faecalis. σ V is activated by lysozyme and is required for lysozyme resistance. The activity of σ V is normally inhibited by the anti-σ 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 σ V . Like the anti-σ factors for several other ECF σ factors, RsiV is degraded by a multistep proteolytic cascade that is regulated at the step of site-1 cleavage. Unlike other anti-σ 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 σ V leads to the transcription of genes, which encode proteins required for lysozyme resistance.
Extracytoplasmic function (ECF) σ factor backgroundECF σ factors belong to the σ 70 family of σ factors and contain only the σ 2 and σ 4.2 domains. These are homologous to the σ 70 σ 2 and σ 4.2 domains and are required for binding to the -35 and -10 regions of target promoters (Helmann, 2002;2016;Staroń et al., 2009). In addition, most ECF σ factors are required for their own expression. The activity of most ECF σ factors is inhibited by a cognate anti-σ factor which is often encoded within the σ factor operon. Many but not all anti-σ factors are membrane proteins (Staroń et al., 2009). The σ factor activity is induced by inhibiting the function of the anti-σ factor. Activation of the σ factor can occur by one of several mechanisms; a conformational change in the anti-σ factor releases the σ factor, an anti-anti-σ factor binds to the anti-σ factor inducing σ factor release, or destruction of the anti-σ factor via regulated intramembrane proteolysis (Ho and Ellermeier, 2012;Helmann, 2016). Here, we will focus on activation of ECF σ factors by degradation of the anti-σ factor. In the presence of an inducing signal, a site-1 protease cleaves the extracellular domain of an anti-σ factor. Following site-1 protease cleavage, the truncated anti-σ factor is cleaved by a site-2 protease within the transmembrane region of the anti-σ factor. The remainder of the anti-σ factor is then degraded by cytosolic proteases. This frees the σ factor to interact with RNA polymerase and transcribe target genes (Brown et al., 2000;Ho and Ellermeier, 2012;Helmann, 2016).The ECF σ factor σ V is found exclusively in Firmicutes or low GC Gram-positive bacteria and belongs to the ECF30 family of ECF σ factors (Staroń et al., 2009). The σ V system is encoded by the model organism Bacillus subtilis, as well as the opportunistic pathogens Enterococcus faecalis and Clostridioides difficile (Fig. 1). σ V is not present in all Firmicutes or even closely related species. While present in B. subtilis, σ V is not present in B. anthrac...