| A biofilm can be defined as a structured community of microbial cells encased in a self-produced polymeric matrix and that is adherent to an air-liquid interface (i.e. pellicles) or a surface. In Bacillus subtilis, the type I signal peptidase, SipW, is required for the formation of both surface-adhered and pellicle biofilms. In this study, SipW was shown to be a bifunctional signal peptidase, having both a catalytic and a non-catalytic function. The signal peptidase catalytic activity of SipW is required to process two proteins that are gene encoded in the same operon with SipW, the major biofilm matrix protein, TasA, and a protein required for efficient TasA export, YqxM. The second, non-catalytic SipW function was found to be specifically required for surface-adhered biofilms, and not for pellicles. This surface-specific function appeared to be required for upregulation of the genes encoding the major constituents of the biofilm matrix, the epsA-O operon, required for production of the matrix exopolysaccharide, and the yqxM-sipW-tasA operon; expression of the epsA-O and yqxM-sipW-tasA operons was decreased in a strain deleted for sipW, as compared to a wild type strain. Furthermore, the SipW mutants defective for catalytic activity were able to support surface-adhered biofilm formation, indicating that the surface-specific function of SipW was a non-catalytic function. Truncation of the short carboxyterminal cytoplasmic region of SipW resulted in mutant that was defective in surface-adhered biofilm formation, but retained catalytic activity. Therefore, the C-terminal cytoplasmic tail of SipW is required for the non-catalytic function of SipW. This study represents the first characterization of a bifunctional signal peptidase, SipW, which has separable catalytic and non-catalytic functions in biofilm formation by B. subtilis. |
