Efficient septum formation is essential for chromosome segregation in Bacillus subtilis when SMC function is impaired.

Abstract

Structural maintenance of chromosomes (SMC) complexes play conserved roles in chromosome organization, segregation, and repair in all domains of life. In Bacillus subtilis, SMC is required for the segregation of newly replicated origins. To investigate whether other proteins function with SMC in this process, we performed a synthetic lethal screen with an smc hypomorphic allele (smc*) that is mildly defective in chromosome segregation. In addition to recovering previously reported genetic interactions of smc with parB and spoIIIE, our screen identified minJ and divIVA as essential in the smc* background. We show that the synthetic lethality between smc* and ∆minJ or ∆divIVA arises from defects in segregating the replication terminus. Importantly, deletion of minD, which suppresses the cell division defects of ∆minJ and ∆divIVA, restored terminus segregation and viability in the smc* background. These findings support a model in which proper septum formation promotes chromosome terminus resolution and segregation by enabling SpoIIIE-mediated DNA clearance from the division septum during cytokinesis. These findings highlight the interdependence between chromosome segregation and cell division.IMPORTANCEThe SMC complex plays a central role in chromosome organization and segregation in Bacillus subtilis, but the cellular functions that become important when SMC activity is reduced are not well understood. Using a hypomorphic smc allele, we discovered that mutations affecting cell division become essential when chromosome organization and segregation are impaired. Our findings support a model in which efficient septum formation enables proper localization of the SpoIIIE DNA translocase, which in turn resolves and segregates the chromosome terminus region. These results highlight the critical role of cell division in supporting chromosome segregation.