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

Abstract

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.[Key Words: Caulobacter; FtsZ; cell division; proteolysis; cell cycle; differentiation] Received December 22, 1997; revised version accepted January 23, 1998.The mechanism by which cells coordinate DNA replication, cell growth, and cell division are not well understood (Donachie 1993;Vicente and Errington 1996). Research on cell division in Escherichia coli has pointed to the FtsZ protein as an essential determinant of the timing and the localization of cell division (Erickson 1995; Rothfield and Justice 1997). FtsZ is a tubulin-like GTPase that polymerizes and forms a cytokinetic ring associated with the cytoplasmic membrane at the site of cell division in bacteria (Bi and Lutkenhaus 1991) and archaea (Baumann and Jackson 1996;Margolin et al. 1996;Wang and Lutkenhaus 1996). Localization of FtsZ is likely to be the key event in assembly of the cell division apparatus. FtsZ recruits other cell division proteins to the site of division Ma et al. 1996) and may constrict, providing mechanical force for division. In E. coli, the concentr...

Proteomic analysis of the bacterial cell cycle

“…Several of the differentially expressed proteins identified in this study were involved in DNA metabolism, cell division, or development (Table 1). Despite the fact that a number of replication or cell division genes have been shown to be under cell cycle control (16,27,34,35), only few of the corresponding proteins were identified in this study (SSB, cluster 21; CcrM, cluster 13; FtsZ, cluster 20). Plausible explanations for this result are that most replication and cytokinesis proteins are present at very low concentrations in the cell (36,37) and that the membraneintegral or membrane-associated cell division components were lost during the extraction for the preparative gels.…”

“…A function was assigned to an identified protein if the similarity had an expectancy value e Ͻ 10 Ϫ8 and if the function was supported by experimental data for at least one of the homologs. Antibodies against PleD (25), CcrM (26), flagellins, FtsZ (27), FliF (28), and McpA (29) were used for immunoblot analysis (25). Expression profiles, identities, and 2-D gel coordinates of all protein spots that were differentially expressed during the cell cycle, repressed, or induced by the synchrony procedure, or exhibited a change in intensity during chase are shown in Table 2 and Fig.…”

A phospho-signaling pathway controls the localization and activity of a protease complex critical for bacterial cell cycle progression

“…In Caulobacter, the essential master regulator CtrA controls the transcription of genes involved in cell division, DNA methylation, flagellar biogenesis, and pili biogenesis (6,(19)(20)(21) and also silences the initiation of DNA replication by binding to five sites in the origin of replication (7). Because of its multiple functions, the presence and activity of CtrA is tightly regulated, temporally and spatially, by two redundant mechanisms, synthesis-proteolysis and phosphorylation-dephosphorylation (3-5, 8-12, 22-27).…”

FtsZ and the division of prokaryotic cells and organelles

“…In E. coli and B. subtilis, FtsZ levels do not change significantly during the cell cycle 70 , which indicates that the activity of FtsZ is cellcycle controlled. By contrast, FtsZ levels in C. crescentus vary dramatically because of proteolysis and resynthesis 71 ; nevertheless, the artificial increase of FtsZ levels at the wrong time in the cell cycle does not trigger Z-ring assembly or change the timing of Z-ring constriction 72 . Finding the source of this cell-cycle control of FtsZ activity remains an important challenge for the future.…”