A moving DNA replication factory in Caulobacter crescentus

The in vivo intracellular location of components of the Caulobacter replication apparatus was visualized during the cell cycle. Replisome assembly occurs at the chromosomal origin located at the stalked cell pole, coincident with the initiation of DNA replication. The replisome gradually moves to midcell as DNA replication proceeds and disassembles upon completion of DNA replication. Although the newly replicated origin regions of the chromosome are rapidly moved to opposite cell poles by an active process, the replisome appears to be an untethered replication factory that is passively displaced towards the center of the cell by the newly replicated DNA. These results are consistent with a model in which unreplicated DNA is pulled into the replication factory and newly replicated DNA is bidirectionally extruded from the complex, perhaps contributing to chromosome segregation.     

Dynamics of Escherichia coli chromosome segregation during multifork replication

Slowly growing Escherichia coli cells have a simple cell cycle, with replication and progressive segregation of the chromosome completed before cell division. In rapidly growing cells, initiation of replication occurs before the previous replication rounds are complete. At cell division, the chromosomes contain multiple replication forks and must be segregated while this complex pattern of replication is still ongoing. Here, we show that replication and segregation continue in step, starting at the origin and progressing to the replication terminus. Thus, early-replicated markers on the multiple-branched chromosomes continue to separate soon after replication to form separate protonucleoids, even though they are not segregated into different daughter cells until later generations. The segregation pattern follows the pattern of chromosome replication and does not follow the cell division cycle. No extensive cohesion of sister DNA regions was seen at any growth rate. We conclude that segregation is driven by the progression of the replication forks.     

Unpicking neurodegeneration in a dish with human pluripotent stem cells

Eukaryotic cell division is an orderly and timely process involving the error-free segregation of chromosomes and cytoplasmic components to give rise to two separate daughter cells. Defects in genome maintenance mechanisms such as cell cycle checkpoints and DNA repair can impact the segregation of the genome during mitosis leading to multiple chromosomal imbalances. In mammals, the DNA damage checkpoint effector Checkpoint Kinase 1 (Chk1) is essential for responses to DNA replication errors, external DNA damage, and chromatin breaks. We reported recently that Chk1 also was essential for chromosome segregation and completion of cytokinesis to prevent genomic instability. Our studies demonstrated that Chk1 deficiency in mitotic cells causes chromosome mis-alignment, lagging chromosomes, chromosome mis- segregation, cytokinetic regression, and binucleation. In addition, abrogation of Chk1 resulted in aberrant localization of mitotic Aurora B kinase at the metaphase plate, anaphase spindle midzone, and cytokinetic midbody as studied both in various cell lines and in a mouse model. Therefore, inappropriate regulation of Chk1 levels during cell cycle progression will result in failed cell division and enhanced genomic instability.     

Compaction and transport properties of newly replicated Caulobacter crescentus DNA

Upon initiating replication of the Caulobacter chromosome, one copy of the parS centromere remains at the stalked pole; the other moves to the distal pole. We identified the segregation dynamics and compaction characteristics of newly replicated Caulobacter DNA during transport (highly variable from cell to cell) using time-lapse fluorescence microscopy. The parS centromere and a length (also highly variable) of parS proximal DNA on each arm of the chromosome are segregated with the same relatively slow transport pattern as the parS locus. Newly replicated DNA further than about 100 kb from parS segregates with a different and faster pattern, while loci at 48 kb from parS segregate with the slow pattern in some cells and the fast pattern in others. The observed parS-proximal DNA compaction characteristics have scaling properties that suggest the DNA is branched. HU2-deletion strains exhibited a reduced compaction phenotype except near the parS site where only the ΔHU1ΔHU2 double mutant had a compaction phenotype. The chromosome shows speed-dependent extension during translocation suggesting the DNA polymer is under tension. While DNA segregation is highly reliable and succeeds in virtually all wild-type cells, the high degree of cell to cell variation in the segregation process is noteworthy.     

细胞周期检控点

细胞周期是高度有组织的时序调控过程,受到DNA损伤检控点、DNA复制检控点和纺锤体检控点等细胞周期检控点的精确调控。细胞周期检控点的作用主要是调节细胞周期的时序转换,以确保DNA复制、染色体分离等细胞重要生命活动的高度精确性,并对DNA损伤、DNA复制受阻、纺锤体组装和染色体分离异常等细胞损伤及时做出反应,以防止突变和遗传不稳定的发生。细胞周期检控点的功能缺陷,将导致细胞基因组的不稳定,与细胞癌变密切相关。因此细胞周期检控点对于维持细胞遗传信息的稳定性和完整性以及防止细胞癌变和遗传疾病的发生起着至关重要的作用。     

An SMC ATPase mutant disrupts chromosome segregation in Caulobacter

Accurate replication and segregation of the bacterial genome are essential for cell cycle progression. We have identified a single amino acid substitution in the Caulobacter structural maintenance of chromosomes (SMC) protein that disrupts chromosome segregation and cell division. The E1076Q point mutation in the SMC ATPase domain caused a dominant-negative phenotype in which DNA replication was able to proceed, but duplicated parS centromeres, normally found at opposite cell poles, remained at one pole. The cellular positions of other chromosomal loci were in the wild-type order relative to the parS centromere, but chromosomes remained unsegregated and appeared to be stacked upon one another. Purified SMC-E1076Q was deficient in ATP hydrolysis and exhibited abnormally stable binding to DNA. We propose that SMC spuriously links the duplicated chromosome immediately after passage of the replication fork. In wild-type cells, ATP hydrolysis opens the SMC dimer, freeing one chromosome to segregate to the opposite pole. The loss of ATP hydrolysis causes the SMC-E1076Q dimer to remain bound to both chromosomes, inhibiting segregation.     

Does the parallel evolution pattern between the replication-segregation proteins and HU have a biological significance?

The bacterial chromosome is a highly compacted nucleoproteic structure. Its apparent disordered morphology is difficult to conciliate with newly discovered mechanisms governing the propagation of genetic information between mother and daughter cells. Recent experiments in bacterial genetics, biochemistry and cytology from a number of laboratories are beginning to unravel how at each cell division, DNA replication and segregation proteins interact spatially with specific DNA motifs to orchestrate replication and movement of replication forks and chromosomes. We propose here a method to confirm and perhaps extend these experiments by in silico protein sequence comparisons and phylogeny. This analysis showed a parallel evolution between the histone-like protein HU and key protein factors involved in DNA replication and chromosome segregation.     

The chromosome cycle of prokaryotes

In both eukaryotes and prokaryotes, chromosomal DNA undergoes replication, condensation–decondensation and segregation, sequentially, in some fixed order. Other conditions, like sister‐chromatid cohesion (SCC), may span several chromosomal events. One set of these chromosomal transactions within a single cell cycle constitutes the ‘chromosome cycle’. For many years it was generally assumed that the prokaryotic chromosome cycle follows major phases of the eukaryotic one: –replication–condensation–segregation–(cell division)–decondensation–, with SCC of unspecified length. Eventually it became evident that, in contrast to the strictly consecutive chromosome cycle of eukaryotes, all stages of the prokaryotic chromosome cycle run concurrently. Thus, prokaryotes practice ‘progressive’ chromosome segregation separated from replication by a brief SCC, and all three transactions move along the chromosome at the same fast rate. In other words, in addition to replication forks, there are ‘segregation forks’ in prokaryotic chromosomes. Moreover, the bulk of prokaryotic DNA outside the replication–segregation transition stays compacted. I consider possible origins of this concurrent replication–segregation and outline the ‘nucleoid administration’ system that organizes the dynamic part of the prokaryotic chromosome cycle.     

Delayed nucleoid segregation in Escherichia coli

To study the role of cell division in the process of nucleoid segregation, we measured the DNA content of individual nucleoids in isogenic Escherichia coli cell division mutants by image cytometry. In pbpB(Ts) and ftsZ strains growing as filaments at 42 degrees C, nucleoids contained, on average, more than two chromosome equivalents compared with 1.6 in wild-type cells. Because similar results were obtained with a pbpB recA strain, the increased DNA content cannot be ascribed to the occurrence of chromosome dimers. From the determination of the amount of DNA per cell and per individual nucleoid after rifampicin inhibition, we estimated the C and D periods (duration of a round of replication and time between termination and cell division respectively), as well as the D' period (time between termination and nucleoid separation). Compared with the parent strain and in contrast to ftsQ, ftsA and ftsZ mutants, pbpB(Ts) cells growing at the permissive temperature (28 degrees C) showed a long D' period (42 min versus 18 min in the parent) indicative of an extended segregation time. The results indicate that a defective cell division protein such as PbpB not only affects the division process but also plays a role in the last stage of DNA segregation. We propose that PbpB is involved in the assembly of the divisome and that this structure enhances nucleoid segregation.     

Chromosome segregation in<Emphasis Type="Italic">Bacillus subtilis</Emphasis>

Bacillus subtilis, a Gram-positive bacterium commonly found in soil, is an excellent model organism for the study of basic cell processes, such as cell division and cell differentiation, called sporulation. In B. subtilis the essential genetic information is carried on a single circular chromosome, the correct segregation of which is crucial for both vegetative growth and sporulation. The proper completion of life cycle requires each daughter cell to obtain identical genetic information. The consequences of inaccurate chromosome segregation can lead to formation of anucleate cells, cells with two chromosomes, or cells with incomplete chromosomes. Although bacteria miss the classical eukaryotic mitotic apparatus, the chromosome segregation is undeniably an active process tightly connected to other cell processes as DNA replication and compaction. To fully understand the chromosome segregation, it is necessary to study this process in a wider context and to examine the role of different proteins at various cell life cycle stages. The life cycle of B. subtilis is characteristic by its specific cell differentiation process where, two slightly different segregation mechanisms exist, specialized in vegetative growth and in sporulation.     

Two separate DNA sequences within oriC participate in accurate chromosome segregation in Bacillus subtilis

Current views of bacterial chromosome segregation vary in respect of the likely presence or absence of an active segregation mechanism involving a mitotic-like apparatus. Furthermore, little is known about cis-acting elements for chromosome segregation in bacteria. In this report, we show that two separate DNA regions, a 3' coding region of dnaA and the AT-rich sequence between dnaA and dnaN (the initial opening site of duplex DNA during replication), are necessary for efficient segregation of the chromosome in Bacillus subtilis. When a plasmid replicon was integrated into argG, far from oriC, on the chromosome and then the oriC function was disrupted, the oriC-deleted mutant formed anucleate cells at 5% possibly because of defects in chromosome segregation. However, when the two DNA sequences were added near oriN, frequency of anucleate cells decreased to 1%. In these cells, the origin (argG) regions were localized near cell poles, whereas they were randomly distributed in cells without the two DNA sequences. These results suggest that the two DNA sequences in and downstream of the dnaA gene participate in correct positioning of the replication origin region within the cell and that this function is associated with accurate chromosome segregation in B. subtilis.     

The role of shugoshin in meiotic chromosome segregation

During meiosis, DNA replication is followed by 2 successive chromosome segregation events, resulting in the production of gametes with a haploid number of chromosomes from a diploid precursor cell. Faithful chromosome segregation in meiosis requires that sister chromatid cohesion is lost from chromosome arms during meiosis I, but retained at centromeric regions until meiosis II. Recent studies have begun to uncover the mechanisms underlying this stepwise loss of cohesion in meiosis and the role of a conserved protein, shugoshin, in regulating this process.     

A fixed distance for separation of newly replicated copies of oriC in Bacillus subtilis: implications for co-ordination of chromosome segregation and cell division

The Spo0J protein of Bacillus subtilis is required for normal chromosome segregation and forms discrete subcellular assemblies closely associated with the oriC region of the chromosome. Here we show that duplication of Spo0J foci occurs early in the DNA replication cycle and that this requires the initiation of DNA replication at oriC but not elongation beyond the nearby STer sites. Soon after duplication, sister oriC /Spo0J foci move rapidly apart to achieve a fixed separation of about 0.7 μm, reminiscent of the segregation of eukaryotic chromosomes on the mitotic spindle. The magnitude of the fixed separation distance may explain how chromosome segregation is kept in close register with cell growth and the initiation mass for DNA replication. It could also explain how segregation can proceed accurately in the absence of cell division. The kinetics of focal separation suggest that one role of Spo0J protein may be to facilitate formation of separate sister oriC complexes that can be segregated.     

Chromosome segregation

The ability to visualise specific genes and proteins within bacterial cells is revolutionising knowledge of chromosome segregation. The essential elements appear to be the driving force behind DNA replication, which occurs at fixed cellular positions, the condensation of newly replicated DNA by a chromosome condensation machine located at the cell 1/4 and 3/4 positions, and molecular machines that act at midcell to allow chromosome separation after replication and movement of the sister chromosomes away from the division septum prior to cell division. This review attempts to provide a perspective on current views of the bacterial chromosome segregation mechanism and how it relates to other cellular processes.     

The MaoP/maoS Site-Specific System Organizes the Ori Region of the E. coli Chromosome into a Macrodomain

The Ori region of bacterial genomes is segregated early in the replication cycle of bacterial chromosomes. Consequently, Ori region positioning plays a pivotal role in chromosome dynamics. The Ori region of the E. coli chromosome is organized as a macrodomain with specific properties concerning DNA mobility, segregation of loci and long distance DNA interactions. Here, by using strains with chromosome rearrangements and DNA mobility as a read-out, we have identified the MaoP/maoS system responsible for constraining DNA mobility in the Ori region and limiting long distance DNA interactions with other regions of the chromosome. MaoP belongs to a group of proteins conserved in the Enterobacteria that coevolved with Dam methylase including SeqA, MukBEF and MatP that are all involved in the control of chromosome conformation and segregation. Analysis of DNA rings excised from the chromosome demonstrated that the single maoS site is required in cis on the chromosome to exert its effect while MaoP can act both in cis and in trans. The position of markers in the Ori region was affected by inactivating maoP. However, the MaoP/maoS system was not sufficient for positioning the Ori region at the ¼–¾ regions of the cell. We also demonstrate that the replication and the resulting expansion of bulk DNA are localized centrally in the cell. Implications of these results for chromosome positioning and segregation in E. coli are discussed.     

The segregation of the Escherichia coli origin and terminus of replication

Escherichia coli chromosome replication forks are tethered to the cell centre. Two opposing models describe how the chromosomes segregate. In the extrusion-capture model, newly replicated DNA is fed bi-directionally from the forks toward the cell poles, forming new chromosomes in each cell half. Starting with the origins, chromosomal regions segregate away from their sisters progressively as they are replicated. The termini segregate last. In the sister chromosome cohesion model, replication produces sister chromosomes that are paired along much of their length. The origins and most other chromosomal regions remain paired until late in the replication cycle, and all segregate together. We use a combination of microscopy and flow cytometry to determine the relationship of origin and terminus segregation to the cell cycle. Origin segregation frequently follows closely after initiation, in strong support of the extrusion-capture model. The spatial disposition of the origin and terminus sequences also fits this model. Terminus segregation occurs extremely late in the cell cycle as the daughter cells separate. As the septum begins to invaginate, the termini of the completed sister chromosomes are transiently held apart at the cell centre, on opposite sides of the cell. This may facilitate the resolution of topological linkages between the chromosomes.     

A polymeric protein anchors the chromosomal origin/ParB complex at a bacterial cell pole

Bacterial replication origins move towards opposite ends of the cell during DNA segregation. We have identified a proline-rich polar protein, PopZ, required to anchor the separated Caulobacter crescentus chromosome origins at the cell poles, a function that is essential for maintaining chromosome organization and normal cell division. PopZ interacts directly with the ParB protein bound to specific DNA sequences near the replication origin. As the origin/ParB complex is being replicated and moved across the cell, PopZ accumulates at the cell pole and tethers the origin in place upon arrival. The polar accumulation of PopZ occurs by a diffusion/capture mechanism that requires the MreB cytoskeleton. High molecular weight oligomers of PopZ assemble in vitro into a filamentous network with trimer junctions, suggesting that the PopZ network and ParB-bound DNA interact in an adhesive complex, fixing the chromosome origin at the cell pole.     

Bacterial DNA segregation: its motors and positional control

A model for DNA segregation in bacteria is proposed which involves not merely growth of the cell membrane and wall, as previously assumed, but also the active movement of one of the two chromosome sister origins by a DNA helicase enzyme and of the chromosome termini and the bulk of the chromosomes by supercoiling tension exerted by DNA gyrase. This provides a unified mechanism for DNA chromosome movement in prosthecate budding bacteria as well as for bacteria that undergo binary fission. The positional control of DNA segregation and the plane of cell division depend, I suggest, on four things: (1) the attachment of the daughter chromosome termini to the cell wall in a position adjacent to the new cell poles at about the time of septation, (2) the displacement of the parental chromosome terminus from this attachment site by the mobile origin, which attaches itself instead to the wall at that point, (3) the movement of the chromosome terminus to a new location in between the daughter origins by the tension of supercoiling, and (4) the determination of the location of the future septum at the position occupied by the chromosome terminus at the time of septal initiation; septum-initiation proteins are postulated to achieve this by binding directly or indirectly to the chromosome terminus. This mechanism automatically ensures ordered DNA segregation in rapidly growing bacteria with more than two sister origins of replication.