Cell division protein DivIB influences the Spo0J/Soj system of chromosome segregation in Bacillus subtilis

The initiation of the developmental process of sporulation in the rod-shaped bacterium Bacillus subtilis involves the activation of the Spo0A response regulator. Spo0A then drives the switch in the site of division septum formation from midcell to a polar position. Activated Spo0A is required for the transcription of key sporulation loci such as spoIIG, which are negatively regulated by the Soj protein. The transcriptional repressing activity of Soj is antagonized by Spo0J, and both proteins belong to the well-conserved Par family of partitioning proteins. Soj has been shown to jump from nucleoid to nucleoid via the cell pole. The dynamic behaviour of Soj is somehow controlled by Spo0J, which prevents the static association of Soj with the nucleoid, and presumably its transcriptional repression activity. Soj in turn is required for the proper condensation of Spo0J foci around the oriC region. The asymmetric partitioning of the sporangial cell requires DivIB and other proteins involved in vegetative (medial) division. We describe an allele of the cell division gene divIB (divIB80) that reduces the cellular levels of DivIB, and affects nucleoid structure and segregation in growing cells, yet has no major impact on cell division. In divIB80 cells Spo0J foci are not correctly condensed and Soj associates statically with the nucleoid. The divIB80 allele prevents transcription of spoIIG, and arrests sporulation prior to the formation of the asymmetric division septum. The defect in Spo0A-dependent gene expression, and the Spo(-) phenotype can be suppressed by expression of divIB in trans or by deletion of the soj-spo0J locus. However, deletion of the spo0J-soj region does not restore the normal cellular levels of DivIB. Therefore, the reduced levels of DivIB in the divIB80 mutant are sufficient for efficient cell division, but not to sustain a second, earlier function of DivIB related to the activity of the Spo0J/Soj system of chromosome segregation.