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...

SummaryThe cell division protein FtsZ is composed of three regions based on sequence similarity: a highly conserved N-terminal region of Ϸ320 amino acids; a variable spacer region; and a conserved C-terminal region of eight amino acids. We show that FtsZ mutants missing different C-terminal fragments have dominant lethal effects because they block cell division in Caulobacter crescentus by two different mechanisms. Removal of the C-terminal conserved region, the linker, and 40 amino acids from the end of the Nterminal conserved region (FtsZ⌬C281) prevents the localization or the polymerization of FtsZ. Because two-hybrid analysis indicates that FtsZ⌬C281 does not interact with FtsZ, we hypothesize that FtsZ⌬C281 blocks cell division by competing with a factor required for FtsZ localization or that it titrates a factor required for the stability of the FtsZ ring. The removal of 24 amino acids from the C-terminus of FtsZ (FtsZ⌬C485) causes a punctate pattern of FtsZ localization and affects its interaction with FtsA. This suggests that the conserved C-terminal region of FtsZ is required for proper polymerization of FtsZ in Caulobacter and for its interaction with FtsA.

Coordination of DNA replication and cell division is essential in order to ensure that progeny cells inherit a full copy of the genome. Caulobacter crescentus divides asymmetrically to produce a non-replicating swarmer cell and a replicating stalked cell. The global response regulator CtrA coordinates DNA replication and cell division by repressing replication initiation and transcription of the early cell division gene ftsZ in swarmer cells. We show that CtrA also mediates a DNA replication checkpoint of cell division by regulating the late cell division genes ftsQ and ftsA. CtrA activates transcription of the P QA promoter that cotranscribes ftsQA, thus regulating the ordered expression of early and late cell division proteins. Cells inhibited for DNA replication are unable to complete cell division. We show that CtrA is not synthesized in pre-divisional cells in which replication has been inhibited, preventing the transcription of P QA and cell division. Replication inhibition prevents the activation of the ctrA P2 promoter, which normally depends on CtrA phosphorylation. This suggests the possibility that CtrA phosphorylation may be affected by replication inhibition.

SummaryThe mechanisms by which bacterial cell division and DNA replication are co-ordinated are still unknown. We have used the easily synchronizable bacterium Caulobacter crescentus to determine when the cell division genes ftsQ and ftsA are transcribed during the DNA replication cycle and to compare their transcription with that of ftsZ. Unlike the situation in Escherichia coli, transcription of ftsQ and ftsA does not extend into ftsZ in Caulobacter. ftsQ and ftsA are co-transcribed by a strong promoter, P QA , present within the end of the ddl gene upstream of ftsQ. Transcription of P QA is turned on at the end of the DNA replication period, coincident with the end of the ftsZ transcription period. ftsA is also transcribed by another promoter, P A , present between ftsQ and ftsA. P A transcription is Ϸ10 times weaker than P QA and occurs during the DNA replication period. Transcription of ftsA by P A is sufficient for cell viability, but is not sufficient for normal cell division. When the transcription of ftsA is increased constitutively, cell division is inhibited and stalks are synthesized at aberrant positions. Thus, transcription of ftsA and ftsZ mimics their order of action in Caulobacter and proper transcription of ftsA has to be maintained for normal cell division and differentiation.

We have measured the photoresponse of two purple nonsulfur bacteria, Rhodobacter sphaeroides and Rhodospirillum centenum, under defined conditions in a light beam propagating at 90°to the optical axis of the microscope. This beam presented cells with a steep gradient of intensity perpendicular to the direction of propagation and a shallow gradient in the direction of light propagation. R. centenum, a species that reverses to change direction, accumulated in the light beam, as expected for a "scotophobic" response, while R. sphaeroides, which stops rather than reverses, accumulated outside the light beam. We also compared the behavior of liquid-grown R. centenum, which swims by using a single polar flagellum, to that of surface-grown R. centenum, which swarms over agar by using many lateral flagella and has been shown to move as colonies toward specific wavelengths of light. When suspended in liquid medium, both liquid-and surface-grown R. centenum showed similar responses to the light gradient. In all cases, free-swimming cells responded to the steep gradient of intensity but not to the shallow gradient, indicating they cannot sense the direction of light propagation but only its intensity. In a control experiment, the known phototactic alga Chlamydamonas reinhardtii was shown to swim in the direction of light propagation.