Cell cycle-dependent transcriptional and proteolytic regulation of FtsZ in Caulobacter

In the differentiating bacterium Caulobacter crescentus, the cell division initiation protein FtsZ is present in only one of the two cell types. Stalked cells initiate a new round of DNA replication immediately after cell division and contain FtsZ, whereas the progeny swarmer cells are unable to initiate DNA replication and do not contain FtsZ. We show that FtsZ expression is controlled by cell cycle-dependent transcription and proteolysis. Transcription of ftsZ is repressed in swarmer cells and is activated concurrently with the initiation of DNA replication. At the end of the DNA replication period, transcription of ftsZ decreases substantially. We show that the global cell cycle regulator CtrA is involved in the cell cycle control of ftsZ transcription. CtrA binds to a site that overlaps the ftsZ transcription start site. Removal of the CtrA-binding site results in transcription of the ftsZ promoter in swarmer cells. Decreasing the cellular concentration of CtrA increases ftsZ transcription and conversely, increasing the concentration of CtrA decreases ftsZ transcription. Because CtrA is present in swarmer cells, is degraded at the same time as ftsZ transcription begins, and reappears when ftsZ transcription decreases at the end of the cell cycle, we propose that CtrA is a repressor of ftsZ transcription. We show that proteolysis is an important determinant of cell type-specific distribution and cell cycle variation of FtsZ. FtsZ is stable when it is synthesized and assembles into the cytokinetic ring at the beginning of the cell cycle. After the initiation of cell division, the rate of FtsZ degradation increases as both the constriction site and the FtsZ ring decrease in diameter. When ftsZ is expressed constitutively from inducible promoters, the abundance of FtsZ still varies during the cell cycle. The coupling of transcription and proteolysis to cell division ensures that FtsZ is inherited only by the progeny cell that will begin DNA replication immediately after cell division.