Cell cycle and positional constraints on FtsZ localization and the initiation of cell division in Caulobacter crescentus

Swarmer cells of Caulobacter crescentus are devoid of the cell division ini tiation protein FtsZ and do not replicate DNA, FtsZ is synthesized during t he differentiation of swarmer cells into replicating stalked cells. We show that FtsZ first localizes at the incipient stalked pole in differentiating swarmer cells. FtsZ subsequently localizes at the mid-cell early in the ce ll cycle. In an effort to understand whether Z-ring formation and cell cons triction are driven solely by the cell cycle-regulated increase in FtsZ con centration, FtsZ was artificially expressed in swarmer cells at a level equ ivalent to that found in predivisional cells. Immunofluorescence microscopy showed that, in these swarmer cells, simply increasing FtsZ concentration was not sufficient for Z-ring formation; Z-ring formation took place only i n stalked cells. Expression of FtsZ in swarmer cells did not alter the timi ng of cell constriction initiation during the cell cycle but, instead, caus ed additional constrictions and a delay in cell separation. These additiona l constrictions were confined to sites close to the original mid-cell const riction. These results suggest that the timing and placement of Z-rings is tightly coupled to an early cell cycle event and that cell constriction is not solely dependent on a threshold level of FtsZ.