Evidence of a Bacterial Receptor for Lysozyme: Binding of Lysozyme to the Anti- sigma Factor RsiV Controls Activation of the ECF sigma Factor sigma V: e1004643

sigma factors endow RNA polymerase with promoter specificity in bacteria. Extra-Cytoplasmic Function (ECF) sigma factors represent the largest and most diverse family of sigma factors. Most ECF sigma factors must be activated in response to an external signal. One mechanism of activation is the stepwise proteolytic destruction of an anti- sigma factor via Regulated Intramembrane Proteolysis (RIP). In most cases, the site-1 protease required to initiate the RIP process directly senses the signal. Here we report a new mechanism in which the anti- sigma factor rather than the site-1 protease is the sensor. We provide evidence suggesting that the anti- sigma factor RsiV is the bacterial receptor for the innate immune defense enzyme, lysozyme. The site-1 cleavage site is similar to the recognition site of signal peptidase and cleavage at this site is required for sigma V activation in Bacillus subtilis. We reconstitute site-1 cleavage in vitro and demonstrate that it requires both signal peptidase and lysozyme. We demonstrate that the anti- sigma factor RsiV directly binds to lysozyme and muramidase activity is not required for sigma V activation. We propose a model in which the binding of lysozyme to RsiV activates RsiV for signal peptidase cleavage at site-1, initiating proteolytic destruction of RsiV and activation of sigma V. This suggests a novel mechanism in which conformational change in a substrate controls the cleavage susceptibility for signal peptidase. Thus, unlike other ECF sigma factors which require regulated intramembrane proteolysis for activation, the sensor for sigma V activation is not the site-1 protease but the anti- sigma factor.