THE ENDOGENOUS BACILLUS-SUBTILIS (NATTO) PLASMIDS PTA1015 AND PTA1040CONTAIN SIGNAL PEPTIDASE-ENCODING GENES - IDENTIFICATION OF A NEW STRUCTURAL MODULE ON CRYPTIC PLASMIDS

Various strains of Bacillus subtilis (natto) contain small cryptic pla smids that replicate via the rolling-circle mechanism. Like plasmids f rom other Gram-positive bacteria, these plasmids are composed of sever al distinct structural modules. A new structural module was identified on the B. subtilis plasmids pTA1015 and pTA1040. It is composed of tw o genes: one specifies an unidentified protein with a putative signal peptide; and the other (sipP) specifies a functional type I signal pep tidase (SPase). The homologous, but non-identical, sipP genes of the t wo plasmids are the first identified plasmid-specific SPase-encoding g enes, With respect to structure and activity, the corresponding enzyme s (denoted SipP) are highly similar to the chromosomally encoded SPase , SipS, of B. subtilis and several newly identified SPases of other ba cilli. Our findings suggest that plasmid-encoded SPases have evolved b ecause, under certain conditions, SPase can be a limiting factor for p rotein secretion in B. subtilis.