Bacillus subtilis contains four closely related type I signal peptidases with overlapping substrate specificities. Constitutive and temporally controlled expression of different sip genes.

Most biological membranes contain one or two type I signal peptidases for the removal of signal peptides from secretory precursor proteins. In this respect, the Gram-positive bacterium Bacillus subtilis seems to be exceptional, because it contains at least four chromosomally-encoded type I signal peptidases, denoted SipS, SipT, SipU, and SipV. Here, we report the identification of the sipT and sipV genes, and the functional characterization of SipT, SipU, and SipV. The four signal peptidases have similar substrate specificities, as they can all process the same beta-lactamase precursor. Nevertheless, they seem to prefer different pre-proteins, as indicated by studies on the processing of the pre-alpha-amylase of Bacillus amyloliquefaciens in strains lacking SipS, SipT, SipU, or SipV. The sipU and sipV genes are constitutively transcribed at a low level, suggesting that they are required for processing of (pre-)proteins secreted during all growth phases. In contrast, the transcription of sipS and sipT is temporally controlled, in concert with the expression of the genes for most secretory proteins, which suggests that SipS and SipT serve to increase the secretory capacity of B. subtilis. Taken together, our findings suggest that SipS, SipT, SipU, and SipV serve different functions during the exponential and post-exponential growth phase of B. subtilis.