Atypical low-copy number plasmid segregation systems, all in one?

Plasmid families harbor different maintenances functions, depending on their size and copy number. Low copy number plasmids rely on active partition systems, organizing a partition complex at specific centromere sites that is actively positioned using NTPase proteins. Some low copy number plasmids lack an active partition system, but carry atypical intracellular positioning systems using a single protein that binds to the centromere site but without an associated NTPase. These systems have been studied in the case of the Escherichia coli R388 and of the Staphylococcus aureus pSK1 plasmids. Here we review these two systems, which appear to be unrelated but share common features, such as their distribution on plasmids of medium size and copy number, certain activities of their centromere-binding proteins, StbA and Par, respectively, as well as their mode of action, which may involve dynamic interactions with the nucleoid-packed chromosome of their hosts.

A subclass of the IS1202 family of bacterial insertion sequences targets XerCD recombination sites.

We describe here a new family of IS which are related to IS1202, originally isolated from Streptococcus pneumoniae in the mid-1990s and previously tagged as an emerging IS family in the ISfinder database. Members of this family have impacted some important properties of their hosts. We describe here another potentially important property of certain family members: specific targeting of xrs recombination sites. The family could be divided into three subgroups based on their transposase sequences and the length on the target repeats (DR) they generate on insertion: subgroup IS1202 (24-29 bp); ISTde1 (15-18 bp); and ISAba32 (5-6 bp). Members of the ISAba32 subgroup were repeatedly found abutting Xer recombinase recombination sites (xrs), separated by an intervening copy of a DR. These xrs sites, present in multiple copies in a number of Acinetobacter plasmids flanking antibiotic resistance genes, were proposed to form a new type of mobile genetic element using the chromosomally-encoded XerCD recombinase for mobility. Transposase alignments identified subgroup-specific indels which may be responsible for the differences in the transposition properties of the three subgroups (i.e. DR length and target specificity). We propose that this collection of IS be classed as a new insertion sequence family: the IS1202 family composed of three subgroups, only one of which specifically targets plasmid-borne xrs. We discuss the implications of xrs targeting for gene mobility.

DNA Segregation in Enterobacteria.

DNA segregation ensures that cell offspring receive at least one copy of each DNA molecule, or replicon, after their replication. This important cellular process includes different phases leading to the physical separation of the replicons and their movement toward the future daughter cells. Here, we review these phases and processes in enterobacteria with emphasis on the molecular mechanisms at play and their controls.

Characterization of the DNA Binding Domain of StbA, A Key Protein of A New Type of DNA Segregation System.

Low-copy-number plasmids require sophisticated genetic devices to achieve efficient segregation of plasmid copies during cell division. Plasmid R388 uses a unique segregation mechanism, based on StbA, a small multifunctional protein. StbA is the key protein in a segregation system not involving a plasmid-encoded NTPase partner, it regulates the expression of several plasmid operons, and it is the main regulator of plasmid conjugation. The mechanisms by which StbA, together with the centromere-like sequence stbS, achieves segregation, is largely uncharacterized. To better understand the molecular basis of R388 segregation, we determined the crystal structure of the conserved N-terminal domain of StbA to 1.9 Å resolution. It folds into an HTH DNA-binding domain, structurally related to that of the PadR subfamily II of transcriptional regulators. StbA is organized in two domains. Its N-terminal domain carries the specific stbS DNA binding activity. A truncated version of StbA, deleted of its C-terminal domain, displays only partial activities in vivo, indicating that the non-conserved C-terminal domain is required for efficient segregation and subcellular plasmid positioning. The structure of StbA DNA-binding domain also provides some insight into how StbA monomers cooperate to repress transcription by binding to the stbDR and to form the segregation complex with stbS.

Complete Genome Sequence of Staphylococcus epidermidis PH1-28, Isolated from the Forehead of a Hyperseborrheic Donor.

We report the complete genome sequence of Staphylococcus epidermidis commensal strain PH1-28, isolated from the forehead of a healthy donor. The assembled 2.6-Mbp genome consisted of one chromosome and five plasmids. These data will provide valuable information and important insights into the physiology and metabolism of this skin flora microorganism.

Bacterial cell proliferation: from molecules to cells.

Bacterial cell proliferation is highly efficient, both because bacteria grow fast and multiply with a low failure rate. This efficiency is underpinned by the robustness of the cell cycle and its synchronization with cell growth and cytokinesis. Recent advances in bacterial cell biology brought about by single-cell physiology in microfluidic chambers suggest a series of simple phenomenological models at the cellular scale, coupling cell size and growth with the cell cycle. We contrast the apparent simplicity of these mechanisms based on the addition of a constant size between cell cycle events (e.g. two consecutive initiation of DNA replication or cell division) with the complexity of the underlying regulatory networks. Beyond the paradigm of cell cycle checkpoints, the coordination between the DNA and division cycles and cell growth is largely mediated by a wealth of other mechanisms. We propose our perspective on these mechanisms, through the prism of the known crosstalk between DNA replication and segregation, cell division and cell growth or size. We argue that the precise knowledge of these molecular mechanisms is critical to integrate the diverse layers of controls at different time and space scales into synthetic and verifiable models.

Post-replicative pairing of sister ter regions in Escherichia coli involves multiple activities of MatP.

The ter region of the bacterial chromosome, where replication terminates, is the last to be segregated before cell division in Escherichia coli. Delayed segregation is controlled by the MatP protein, which binds to specific sites (matS) within ter, and interacts with other proteins such as ZapB. Here, we investigate the role of MatP by combining short-time mobility analyses of the ter locus with biochemical approaches. We find that ter mobility is similar to that of a non ter locus, except when sister ter loci are paired after replication. This effect depends on MatP, the persistence of catenanes, and ZapB. We characterise MatP/DNA complexes and conclude that MatP binds DNA as a tetramer, but bridging matS sites in a DNA-rich environment remains infrequent. We propose that tetramerisation of MatP links matS sites with ZapB and/or with non-specific DNA to promote optimal pairing of sister ter regions until cell division.

Intracellular Positioning Systems Limit the Entropic Eviction of Secondary Replicons Toward the Nucleoid Edges in Bacterial Cells.

Bacterial genomes, organized intracellularly as nucleoids, are composed of the main chromosome coexisting with different types of secondary replicons. Secondary replicons are major drivers of bacterial adaptation by gene exchange. They are highly diverse in type and size, ranging from less than 2 to more than 1000 kb, and must integrate with bacterial physiology, including to the nucleoid dynamics, to limit detrimental costs leading to their counter-selection. We show that large DNA circles, whether from a natural plasmid or excised from the chromosome tend to localize in a dynamic manner in a zone separating the nucleoid from the cytoplasm at the edge of the nucleoid. This localization is in good agreement with silico simulations of DNA circles in the nucleoid volume. Subcellular positioning systems counteract this tendency, allowing replicons to enter the nucleoid space. In enterobacteria, these systems are found in replicons above 25 kb, defining the limit with small randomly segregated plasmids. Larger replicons carry at least one of the three described family of systems, ParAB, ParRM, and StbA. Replicons above 180 kb all carry a ParAB system, suggesting this system is specifically required in the cases of large replicons. Simulations demonstrated that replicon size profoundly affects localization, compaction, and dynamics of DNA circles in the nucleoid volume. The present work suggests that presence of partition systems on the larger plasmids or chromids is not only due to selection for accurate segregation but also to counteract their unmixing with the chromosome and consequent exclusion from the nucleoid.

Genomewide phenotypic analysis of growth, cell morphogenesis, and cell cycle events in Escherichia coli.

Cell size, cell growth, and cell cycle events are necessarily intertwined to achieve robust bacterial replication. Yet, a comprehensive and integrated view of these fundamental processes is lacking. Here, we describe an image-based quantitative screen of the single-gene knockout collection of Escherichia coli and identify many new genes involved in cell morphogenesis, population growth, nucleoid (bulk chromosome) dynamics, and cell division. Functional analyses, together with high-dimensional classification, unveil new associations of morphological and cell cycle phenotypes with specific functions and pathways. Additionally, correlation analysis across ~4,000 genetic perturbations shows that growth rate is surprisingly not predictive of cell size. Growth rate was also uncorrelated with the relative timings of nucleoid separation and cell constriction. Rather, our analysis identifies scaling relationships between cell size and nucleoid size and between nucleoid size and the relative timings of nucleoid separation and cell division. These connections suggest that the nucleoid links cell morphogenesis to the cell cycle.

FtsK translocation permits discrimination between an endogenous and an imported Xer/dif recombination complex.

In bacteria, the FtsK/Xer/dif (chromosome dimer resolution site) system is essential for faithful vertical genetic transmission, ensuring the resolution of chromosome dimers during their segregation to daughter cells. This system is also targeted by mobile genetic elements that integrate into chromosomal dif sites. A central question is thus how Xer/dif recombination is tuned to both act in chromosome segregation and stably maintain mobile elements. To explore this question, we focused on pathogenic Neisseria species harboring a genomic island in their dif sites. We show that the FtsK DNA translocase acts differentially at the recombination sites flanking the genomic island. It stops at one Xer/dif complex, activating recombination, but it does not stop on the other site, thus dismantling it. FtsK translocation thus permits cis discrimination between an endogenous and an imported Xer/dif recombination complex.

Mapping Topoisomerase IV Binding and Activity Sites on the E. coli Genome.

Catenation links between sister chromatids are formed progressively during DNA replication and are involved in the establishment of sister chromatid cohesion. Topo IV is a bacterial type II topoisomerase involved in the removal of catenation links both behind replication forks and after replication during the final separation of sister chromosomes. We have investigated the global DNA-binding and catalytic activity of Topo IV in E. coli using genomic and molecular biology approaches. ChIP-seq revealed that Topo IV interaction with the E. coli chromosome is controlled by DNA replication. During replication, Topo IV has access to most of the genome but only selects a few hundred specific sites for its activity. Local chromatin and gene expression context influence site selection. Moreover strong DNA-binding and catalytic activities are found at the chromosome dimer resolution site, dif, located opposite the origin of replication. We reveal a physical and functional interaction between Topo IV and the XerCD recombinases acting at the dif site. This interaction is modulated by MatP, a protein involved in the organization of the Ter macrodomain. These results show that Topo IV, XerCD/dif and MatP are part of a network dedicated to the final step of chromosome management during the cell cycle.

Mechanisms for chromosome segregation.

Bacteria face the problem of segregating their gigantic chromosomes without a segregation period restricted in time and space, as Eukaryotes do. Segregation thus involves multiple activities, general or specific of a chromosome region and differentially controlled. Recent advances show that these various mechanisms conform to a "pair and release" rule, which appears as a general rule in DNA segregation. We describe the latest advances in segregation of bacterial chromosomes with emphasis on the different pair and release mechanisms.

The FtsK family of DNA translocases finds the ends of circles.

A global view of bacterial chromosome choreography during the cell cycle is emerging, highlighting as a next challenge the description of the molecular mechanisms and factors involved. Here, we review one such factor, the FtsK family of DNA translocases. FtsK is a powerful and fast translocase that reads chromosome polarity. It couples segregation of the chromosome with cell division and controls the last steps of segregation in time and space. The second model protein of the family SpoIIIE acts in the transfer of the Bacillus subtilis chromosome during sporulation. This review focuses on the molecular mechanisms used by FtsK and SpoIIIE to segregate chromosomes with emphasis on the latest advances and open questions.

Resolution of Multimeric Forms of Circular Plasmids and Chromosomes.

One of the disadvantages of circular plasmids and chromosomes is their high sensitivity to rearrangements caused by homologous recombination. Odd numbers of crossing-over occurring during or after replication of a circular replicon result in the formation of a dimeric molecule in which the two copies of the replicon are fused. If they are not converted back to monomers, the dimers of replicons may fail to correctly segregate at the time of cell division. Resolution of multimeric forms of circular plasmids and chromosomes is mediated by site-specific recombination, and the enzymes that catalyze this type of reaction fall into two families of proteins: the serine and tyrosine recombinase families. Here we give an overview of the variety of site-specific resolution systems found on circular plasmids and chromosomes.

TPM analyses reveal that FtsK contributes both to the assembly and the activation of the XerCD-dif recombination synapse.

Circular chromosomes can form dimers during replication and failure to resolve those into monomers prevents chromosome segregation, which leads to cell death. Dimer resolution is catalysed by a highly conserved site-specific recombination system, called XerCD-dif in Escherichia coli. Recombination is activated by the DNA translocase FtsK, which is associated with the division septum, and is thought to contribute to the assembly of the XerCD-dif synapse. In our study, direct observation of the assembly of the XerCD-dif synapse, which had previously eluded other methods, was made possible by the use of Tethered Particle Motion, a single molecule approach. We show that XerC, XerD and two dif sites suffice for the assembly of XerCD-dif synapses in absence of FtsK, but lead to inactive XerCD-dif synapses. We also show that the presence of the γ domain of FtsK increases the rate of synapse formation and convert them into active synapses where recombination occurs. Our results represent the first direct observation of the formation of the XerCD-dif recombination synapse and its activation by FtsK.

FtsK actively segregates sister chromosomes in Escherichia coli.

Bacteria use the replication origin-to-terminus polarity of their circular chromosomes to control DNA transactions during the cell cycle. Segregation starts by active migration of the region of origin followed by progressive movement of the rest of the chromosomes. The last steps of segregation have been studied extensively in the case of dimeric sister chromosomes and when chromosome organization is impaired by mutations. In these special cases, the divisome-associated DNA translocase FtsK is required. FtsK pumps chromosomes toward the dif chromosome dimer resolution site using polarity of the FtsK-orienting polar sequence (KOPS) DNA motifs. Assays based on monitoring dif recombination have suggested that FtsK acts only in these special cases and does not act on monomeric chromosomes. Using a two-color system to visualize pairs of chromosome loci in living cells, we show that the spatial resolution of sister loci is accurately ordered from the point of origin to the dif site. Furthermore, ordered segregation in a region ∼200 kb long surrounding dif depended on the oriented translocation activity of FtsK but not on the formation of dimers or their resolution. FtsK-mediated segregation required the MatP protein, which delays segregation of the dif-surrounding region until cell division. We conclude that FtsK segregates the terminus region of sister chromosomes whether they are monomeric or dimeric and does so in an accurate and ordered manner. Our data are consistent with a model in which FtsK acts to release the MatP-mediated cohesion and/or interaction with the division apparatus of the terminus region in a KOPS-oriented manner.

Co-evolution of segregation guide DNA motifs and the FtsK translocase in bacteria: identification of the atypical Lactococcus lactis KOPS motif.

Bacteria use the global bipolarization of their chromosomes into replichores to control the dynamics and segregation of their genome during the cell cycle. This involves the control of protein activities by recognition of specific short DNA motifs whose orientation along the chromosome is highly skewed. The KOPS motifs act in chromosome segregation by orienting the activity of the FtsK DNA translocase towards the terminal replichore junction. KOPS motifs have been identified in γ-Proteobacteria and in Bacillus subtilis as closely related G-rich octamers. We have identified the KOPS motif of Lactococcus lactis, a model bacteria of the Streptococcaceae family harbouring a compact and low GC% genome. This motif, 5'-GAAGAAG-3, was predicted in silico using the occurrence and skew characteristics of known KOPS motifs. We show that it is specifically recognized by L. lactis FtsK in vitro and controls its activity in vivo. L. lactis KOPS is thus an A-rich heptamer motif. Our results show that KOPS-controlled chromosome segregation is conserved in Streptococcaceae but that KOPS may show important variation in sequence and length between bacterial families. This suggests that FtsK adapts to its host genome by selecting motifs with convenient occurrence frequencies and orientation skews to orient its activity.

A defined terminal region of the E. coli chromosome shows late segregation and high FtsK activity.

BACKGROUND: The FtsK DNA-translocase controls the last steps of chromosome segregation in E. coli. It translocates sister chromosomes using the KOPS DNA motifs to orient its activity, and controls the resolution of dimeric forms of sister chromosomes by XerCD-mediated recombination at the dif site and their decatenation by TopoIV. METHODOLOGY: We have used XerCD/dif recombination as a genetic trap to probe the interaction of FtsK with loci located in different regions of the chromosome. This assay revealed that the activity of FtsK is restricted to a ∼400 kb terminal region of the chromosome around the natural position of the dif site. Preferential interaction with this region required the tethering of FtsK to the division septum via its N-terminal domain as well as its translocation activity. However, the KOPS-recognition activity of FtsK was not required. Displacement of replication termination outside the FtsK high activity region had no effect on FtsK activity and deletion of a part of this region was not compensated by its extension to neighbouring regions. By observing the fate of fluorescent-tagged loci of the ter region, we found that segregation of the FtsK high activity region is delayed compared to that of its adjacent regions. SIGNIFICANCE: Our results show that a restricted terminal region of the chromosome is specifically dedicated to the last steps of chromosome segregation and to their coupling with cell division by FtsK.