Rolling-circle plasmids fromBacillus subtilis: complete nucleotide sequences and analyses of genes of pTA1015, pTA1040, pTA1050 and pTA1060, and comparisons with related plasmids from Gram-positive bacteria

Most small plasmids of Gram-positive bacteria use the rolling-circle mechanism of replication and several of these have been studied in considerable detail at the DNA level and for the function of their genes. Although most of the common laboratory Bacillus subtilis 168 strains do not contain plasmids, several industrial strains and natural soil isolates do contain rolling-circle replicating (RCR) plasmids. So far, knowledge about these plasmids was mainly limited to: (i) a classification into seven groups, based on size and restriction patterns; and (ii) DNA sequences of the replication region of a limited number of them. To increase the knowledge, also with respect to other functions specified by these plasmids, we have determined the complete DNA sequence of four plasmids, representing different groups, and performed computer-assisted and experimental analyses on the possible function of their genes. The plasmids analyzed are pTA1015 (5.8 kbp), pTA1040 (7.8 kbp), pTA1050 (8.4 kbp), and pTA1060 (8.7 kbp). These plasmids have a structural organization similar to most other known RCR plasmids. They contain highly related replication functions, both for leading and lagging strand synthesis. pTA1015 and pTA1060 contain a mobilization gene enabling their conjugative transfer. Strikingly, in addition to the conserved replication modules, these plasmids contain unique module(s) with genes which are not present on known RCR plasmids of other Gram-positive bacteria. Examples are genes encoding a type I signal peptidase and genes encoding proteins belonging to the family of response regulator aspartate phosphatases. The latter are likely to be involved in the regulation of post-exponential phase processes. The presence of these modules on plasmids may reflect an adaptation to the special conditions to which the host cells were exposed.