Bacterial cell division: regulating Z-ring formation

The earliest stage of cell division in bacteria is the formation of a Z rin g, composed of a polymer of the FtsZ protein, at the division site. Z rings appear to be synthesized in a bi-directional manner from a nucleation site (NS) located on the inside of the cytoplasmic membrane. It is the utilizat ion of a NS specifically at the site of septum formation that determines wh ere and when division will occur. However, a Z ring can be made to form at positions other than at the division site. How does a cell regulate utiliza tion of a NS at the correct location and at the right time? In rod-shaped b acteria such as Escherichia coli and Bacillus subtilis, two factors involve d in this regulation are the Min system and nucleoid occlusion, It is sugge sted that in B. subtilis, the main role of the Min proteins is to inhibit d ivision at the nucleoid-free cell poles. In E. coli it is currently not cle ar whether the Min system can direct a Z ring to the division site at mid-c ell or whether its main role is to ensure that division inhibition occurs a way from mid-cell, a role analogous to that in a. subtilis, While the nucle oid negatively influences Z-ring formation in its vicinity in these rod-sha ped organisms, the exact relationship between nucleoid occlusion and the ab ility to form a mid-cell Z ring is unresolved. Recent evidence suggests tha t in B. subtilis and Caulobacter crescentus, utilization of the NS at the d ivision site is intimately linked to the progress of a round of chromosome replication and this may form the basis of achieving co-ordination between chromosome replication and cell division.