DNA replication initiation is required for mid‐cell positioning of FtsZ rings in Caulobacter crescentus

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Abstract
Summary: Polymerization of the GTPase FtsZ to form a structure called the Z‐ring is the earliest known step in bac‐terial cell division. Mid‐cell Z‐ring assembly coincides with the beginning of the replication cycle in the differentiating bacterium Caulobacter crescentus. Z‐ring disassembly occurs at the end of the division cycle, resulting in the complete degradation of FtsZ from both stalked and swarmer progeny cells. New Z‐rings can only form in the replicative stalked cell. Conditional mutants in DNA replication were used to determine what role DNA replication events play in the process of Z‐ring assembly at different stages in the cell cycle. Z‐ring assembly occurred even when early stages of DNA replication were blocked; however, the Z‐rings were localized at a subpolar region of the cell. Z‐rings only assembled at the proper mid‐cell location if DNA replication had initiated. Z‐ring assembly coincided with areas containing little or no DNA, and Z‐rings could not form over an unreplicated chromosome. Overexpressed FtsZ in the absence of DNA replication did not stimulate productive mid‐cell Z‐ring assembly but, instead, caused the ends of cells to constrict over an extended area away from the nucleoid. These results indicate that the state of chromosome replication is a major determinant of Z‐ring localization in Caulobacter.