A fixed distance for separation of newly replicated copies of oriC in Bacillus subtilis:: implications for co-ordination of chromosome segregation and cell division

The SpoOJ protein of Bacillus subtilis is required for normal chromosome segregation and forms discrete subcellular assemblies closely associated with the oriC region of the chromosome. Here we show that duplication of SpoOJ foci occurs early in the DNA replication cycle and that this requires the initiation of DNA replication at oriC but not elongation beyond the nearby STer sites. Soon after duplication, sister oriC/SpoOJ foci move rapidly apart to achieve a fixed separation of about 0.7 mu m, reminiscent of the segregation of eukaryotic chromosomes on the mitotic spindle. The magnitude of the fixed separation distance may explain how chromosome segregation is kept in close register with cell growth and the initiation mass for DNA replication. It could also explain how segregation can proceed accurately in the absence of cell division. The kinetics of focal separation suggest that one role of SpoOJ protein may be to facilitate formation of separate sister oriC complexes that can be segregated.