Coupling of asymmetric division to polar placement of replication origin regions in Bacillus subtilis.

Entry into sporulation in Bacillus subtilis is characterized by the formation of a polar septum, which asymmetrically divides the developing cell into forespore (the smaller cell) and mother cell compartments, and by migration of replication origin regions to extreme opposite poles of the cell. Here we show that polar septation is closely correlated with movement of replication origins to the extreme poles of the cell. Replication origin regions were visualized by the use of a cassette of tandem copies of lacO that had been inserted in the chromosome near the origin of replication and decorated with green fluorescent protein-LacI. The results showed that extreme polar placement of replication origin regions is not under sporulation control and occurred in stationary phase under conditions under which entry into sporulation was prevented. On the other hand, the formation of a polar septum, which is under sporulation control, was almost invariably associated with the presence of a replication origin region in the forespore. Moreover, cells in which the polar placement of origin regions was perturbed by deletion of the gene (smc) for the structural maintenance of chromosomes (SMC) protein were impaired in polar division. A small proportion ( approximately 1%) of the mutant cells were able to undergo asymmetric division, but the forespore compartment of these exceptional cells was generally observed to contain a replication origin region. Immunofluorescence microscopy experiments indicated that the block in polar division caused by the absence of SMC occurred at or prior to the step of bipolar Z-ring formation by the cell division protein FtsZ. A model is discussed in which polar division is under the dual control of sporulation and an event associated with the placement of a replication origin at the cell pole.