Effects of replication termination mutants on chromosome partitioning in Bacillus subtilis.

Many circular genomes have replication termination systems, yet disruption of these systems does not cause an obvious defect in growth or viability. We have found that the replication termination system of Bacillus subtilis contributes to accurate chromosome partitioning. Partitioning of the terminus region requires that chromosome dimers, that have formed as a result of RecA-mediated homologous recombination, be resolved to monomers by the site-specific recombinase encoded by ripX. In addition, the chromosome must be cleared from the region of formation of the division septum. This process is facilitated by the spoIIIE gene product which is required for movement of a chromosome out of the way of the division septum during sporulation. We found that deletion of rtp, which encodes the replication termination protein, in combination with mutations in ripX or spoIIIE, led to an increase in production of anucleate cells. This increase in production of anucleate cells depended on recA, indicating that there is probably an increase in chromosome dimer formation in the absence of the replication termination system. Our results also indicate that SpoIIIE probably enhances the function of the RipX recombinase system. We also determined the subcellular location of the replication termination protein and found that it is a good marker for the position of the chromosome terminus.