On the birth and fate of bacterial division sites

Thanks to genetics, to the study of protein-protein interactions and to direct viewing of subcellular structures by the use of immunofluorescence and green fluorescent protein (GFP) fusions, the organization of the constriction apparatus of walled bacteria is gradually coming to light. The tubulin-like protein FtsZ assembles as a ring around the site of constriction and operates as an organizer and activator of septum-shaping proteins. Much less is known about the factors specifying the location of FtsZ rings. Circumstantial evidence favours the presence at future ring positions of fixed elements, the potential division sites (PDS), before FtsZ assembles. FtsZ polymerization is initiated from a point on a PDS, the nucleation site, still to be identified, and proceeds bidirectionally around the cell. We hypothesize that new PDS are specified in a manner that depends on the functioning of an active chromosome partition apparatus. This view is supported by the fact that formation of mid-cell PDS requires initiation of DNA replication, and by recent studies supporting the existence of a specialized partition apparatus in a variety of microorganisms. Although PDS may be specified directly by the partition apparatus, indirect localization linked to compartmentalized gene expression during chromosome segregation is also possible. Once created, PDS are used in a regulated manner, and several mechanisms normally operate to direct constriction to selected PDS at the correct time. One, dedicated to the permanent suppression of polar PDS, rests on the minicell suppression system and involves a protein that is able to discriminate between polar and non-polar sites. Another is involved in asymmetric site selection at the early stages of sporulation in Bacillus subtilis. Finally, a mechanism observed only in certain multi-nucleated cells appears to favour division at non-polar PDS related to the most ancient replication/DNA segregation events.