Spatial complexity of mechanisms controlling a bacterial cell cycleIntroductionChromosome structure and replicationRegulation of DNA replication initiationTemporal and spatial control of CtrA proteolysis by a two-component signal transduction pathwaySubcellular localization of signal transduction proteinsConclusionsUpdateAcknowledgementsReferences and recommended readingSpatial complexity of mechanisms controlling abacterial cell cyclePatrick H Viollier1and Lucy Shapiro2Cell cycle progression in Caulobacter is governed by amultilayered regulatory network linking chromosomereplication with polar morphogenesis and cell division.Temporal and spatial regulation have emerged as thecentral themes, with the abundance, activity and subcellularlocation of key structural and regulatory proteins changingover the course of the cell cycle. An additional layer ofcomplexity was recently uncovered, showing that eachsegment of the chromosome is located at a specific cellularposition both during and after the completion of DNAreplication, raising the possibility that this positioningcontributes to temporal and spatial control of geneexpression.Addresses1Department of Molecular Biology and Microbiology, Case WesternReserve University School of Medicine, 10900 Euclid Avenue, Cleveland,Ohio 44106, USA2Department of Developmental Biology, Stanford University School ofMedicine, Beckman Center, B300, Stanford, CA 94305, USAe-mail: [email protected] Opinion in Microbiology 2004, 7:572–578This review comes from a themed issue onGrowth and developmentEdited by Mike Tyers and Mark ButtnerAvailable online 27th October 20041369-5274/$ – see front matter# 2004 Elsevier Ltd. All rights reserved.DOI 10.1016/j.mib.2004.10.005AbbreviationsBDS bipartite degradation signalIntroductionThe crescent-shaped a-proteobacterium Caulobacter cres-centus has proven to be a valuable model system forstudying the bacterial cell cycle. The technical advantageprovided by Caulobacter’s distinctive physiology is thatsynchronized cultures can be obtained by means of asimple density gradient centrifugation that yields a homo-geneous population of swarmer cells [1–3]. Swarmer cellshave a single flagellum and several pili, with both struc-tures positioned at the same cell pole (Figure 1). Theswarmer cell is the bacterial functional equivalent of theeukaryotic G1 cell in that chromosome replication issilenced. After a defined period, the motile swarmer celldifferentiates into a sessile stalked cell. During thistransition, the late swarmer pole is remodeled: the fla-gellum is shed, pili are lost, the receptors for the chemo-taxis sensory apparatus are degraded, and growth of theenvelope is redirected to give rise to a cylindrical polarstructure (the stalk) that is capped with an adhesivepolysaccharide (the holdfast). Coincident with these mor-phological changes, the stalked cell acquires the ability toinitiate chromosome replication. As the stalked cell growsinto a pre-divisional cell, a new polar flagellum, a che-mosensory apparatus and the pili secretion machinery areassembled at the pole opposite the stalk. Shortly after thecompletion of DNA replication, but before cell separa-tion, the cytoplasm of the pre-divisional cell partitions,with each half inheriting a sister chromosome [4].Because the cytoplasm is now discontinuous, the twocompartments of the pre-divisional cell diverge and accu-mulate different cell fate determinants [4]. Finally,asymmetric cell division yields the distinct swarmer celland stalked cell progeny.Here, we review mechanisms that control the cell cycleand are spatially confined in a bacterial cell.Chromosome structure and replicationThe circular Caulobacter chromosome — with a contourlength around 1000-fold longer than that of the long axisof the cell — exists in a highly compact and reproduciblyorganized state in the non-replicative swarmer cell, withthe origin of replication (ori) positioned near the flagel-lated pole (Figure 2a,c) [5,6]. The terminus of replica-tion (ter) is situated at maximal distance from ori, near theopposite pole. Intervening chromosomal loci are locatedat positions inside the cell that correlate linearly with theirrelative genetic distance from ori on the chromosome,suggesting that compaction of the chromosome extendsuniformly across the long axis of the cell (Figure 2b,c)[6].In the swarmer cell, the components of the DNA replica-tion machinery (replisome), such as the HolB and HolCclamp loader proteins, the DnaB helicase and the ParCsubunit of the decatenating enzyme topoisomerase IV aredispersed in the cytoplasm [7,8]. The formation of thereplisome, visualized as a tight focus by epifluorescencemicroscopy, occurs at or near ori in the stalked cell, andmarks the onset of DNA replication (Figure 2a). As thecell cycle progresses, the replisome sequentially copieseach locus and disassembles upon termination of chro-mosome replication [8].The newly synthesized segments of sister DNA thatcontinuously emerge from the replisome are transportedCurrent Opinion in Microbiology 2004, 7:572–578 www.sciencedirect.cominto the incipient daughter cell [6,8], so that replicationand segregation occur concurrently in Caulobacter. Therate of transport of these segments exceeds the rate of cellelongation, indicating an active mechanism for segmentmovement. Moreover, several different segments thatwere analyzed, including ori, moved at similar rates,suggesting that the same apparatus is capable of partition-ing different regions of the chromosome [6]. The activ-ities of the structural maintenance of chromosomes(SMC) protein [5], the Caulobacter actin homolog MreBthat determines cell shape [9,10], homologs of theParAB partitioning system [11,12] and the topoisomeraseIV enzyme [7] are required — to varying degrees — forthe proper positioning of ori at the cell pole and may bepart of the still elusive motor driving chromosome seg-regation. It is unknown if the chromosomal loci are in acompacted state during the segregation process, but theywere observed to be condensed once the movement hadceased [6]. Because the deposition of the replicated lociinto the incipient daughter cell occurs sequentially, it isplausible that the orderly compaction of the segregatedloci by SMC-type condensins [13] or ‘histone-like’proteins such as HU [14] imposes physical constraintsthat retain the loci at their respective sites in the
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