Jan Maarten van Dijl

THE ENDOGENOUS BACILLUS-SUBTILIS (NATTO) PLASMIDS PTA1015 AND PTA1040 CONTAIN SIGNAL PEPTIDASE-ENCODING GENES - IDENTIFICATION OF A NEW STRUCTURAL MODULE ON CRYPTIC PLASMIDS

Various strains of Bacillus subtilis (natto) contain small cryptic plasmids that replicate via the rolling‐circle mechanism. Like plasmids from other Gram‐positive bacteria, these plasmids are composed of several distinct structural modules. A new structural module was identified on the B. subtilis plasmids pTA1015 and pTA1040. It is composed of two genes: one specifies an unidentified protein with a putative signal peptide; and the other (sipP) specifies a functional type I signal peptidase (SPase). The homologous, but non‐identical, sipP genes of the two plasmids are the first identified plasmid‐specific SPase‐encoding genes. With respect to structure and activity, the corresponding enzymes (denoted SipP) are highly similar to the chromosomally encoded SPase, SipS, of B. subtilis and several newly identified SPases of other bacilli. Our findings suggest that plasmid‐encoded SPases have evolved because, under certain conditions, SPase can be a limiting factor for protein secretion in B. subtilis.

Do the Type I Signal Peptidases of Bacillus subtilis Compete for Binding and Cleavage of Secretory Precursor Proteins

Bacillus subtilis contains four closely related, chromosomally-encoded type I signal peptidases (SipS, SipT, SipU and SipV), which remove signal peptides from secretory precursor proteins. In the present studies, the role of SipS in protein secretion in B. subtilis was analysed. Interestingly, the absence of SipS had opposite effects on the secretion of different mature proteins into the growth medium. For example, the neutral protease NprE was secreted at reduced levels, whereas levansucrase was secreted at increased levels. Similarly, the processing of certain secretory precursor proteins was reduced, whereas processing of other precursors was improved. The latter observation indicates that the presence of SipS can interfere with efficient processing of certain precursor proteins, which raises the question whether the type I signal peptidases of B. subtilis compete for binding and cleavage of secretory precursor proteins.