Identification of Bacillus subtilis SipW as a bifunctional signal peptidase that controls surface-adhered biofilm formation

Biofilms of microbial cells encased in an exopolymeric matrix can form on solid surfaces, but how bacteria sense a solid surface and upregulate biofilm genes is largely unknown. We investigated the role of the Bacillus subtilis signal peptidase, SipW, which has a unique role in forming biofilms on a solid surface and is not required at an air-liquid interface. Surprisingly, we found that the signal peptidase activity of SipW was not required for solid-surface biofilms. Furthermore, a SipW mutant protein was constructed that lacks the ability to form a solid-surface biofilm but still retains signal peptidase activity. Through genetic and gene expression tests, the non-signal peptidase role of SipW was found to activate biofilm matrix genes specifically when cells were on a solid surface. These data provide the first evidence that a signal peptidase is bifunctional and that SipW has a regulatory role in addition to its role as a signal peptidase.