ON THE BIRTH AND FATE OF BACTERIAL DIVISION SITES

Thanks to genetics, to the study of protein-protein interactions and t o direct viewing of subcellular structures by the use of immunofluores cence 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 si te of constriction and operates as an organizer and activator of septu m-shaping proteins, Much less is known about the factors specifying th e location of FtsZ rings, Circumstantial evidence favours the presence at future ring positions of fixed elements, the potential division si tes (PDS), before FtsZ assembles. FtsZ polymerization is initiated fro m a point on a PDS, the nucleation site, still to be identified, and p roceeds bidirectionally around the cell, We hypothesize that new PDS a re specified in a manner that depends on the functioning of an active chromosome partition apparatus, This view is supported by the fact tha t formation of mid-cell PDS requires initiation of DNA replication, an d by recent studies supporting the existence of a specialized partitio n apparatus in a variety of microorganisms, Although PDS may be specif ied 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 s everal mechanisms normally operate to direct constriction to selected PDS at the correct time, One, dedicated to the permanent suppression o f polar PDS, rests on the minicell suppression system and involves a p rotein 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 onl y in certain multinucleated cells appears to favour division at nonpol ar PDS related to the most ancient replication/DNA segregation events.