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.

Analysis of ParB-centromere interactions by multiplex SPR imaging reveals specific patterns for binding ParB in six centromeres of Burkholderiales chromosomes and plasmids.

Bacterial centromeres-also called parS, are cis-acting DNA sequences which, together with the proteins ParA and ParB, are involved in the segregation of chromosomes and plasmids. The specific binding of ParB to parS nucleates the assembly of a large ParB/DNA complex from which ParA-the motor protein, segregates the sister replicons. Closely related families of partition systems, called Bsr, were identified on the chromosomes and large plasmids of the multi-chromosomal bacterium Burkholderia cenocepacia and other species from the order Burkholeriales. The centromeres of the Bsr partition families are 16 bp palindromes, displaying similar base compositions, notably a central CG dinucleotide. Despite centromeres bind the cognate ParB with a narrow specificity, weak ParB-parS non cognate interactions were nevertheless detected between few Bsr partition systems of replicons not belonging to the same genome. These observations suggested that Bsr partition systems could have a common ancestry but that evolution mostly erased the possibilities of cross-reactions between them, in particular to prevent replicon incompatibility. To detect novel similarities between Bsr partition systems, we have analyzed the binding of six Bsr parS sequences and a wide collection of modified derivatives, to their cognate ParB. The study was carried out by Surface Plasmon Resonance imaging (SPRi) mulitplex analysis enabling a systematic survey of each nucleotide position within the centromere. We found that in each parS some positions could be changed while maintaining binding to ParB. Each centromere displays its own pattern of changes, but some positions are shared more or less widely. In addition from these changes we could speculate evolutionary links between these centromeres.

Orderly Replication and Segregation of the Four Replicons of Burkholderia cenocepacia J2315.

Bacterial genomes typically consist of a single chromosome and, optionally, one or more plasmids. But whole-genome sequencing reveals about ten per-cent of them to be multipartite, with additional replicons which by size and indispensability are considered secondary chromosomes. This raises the questions of how their replication and partition is managed without compromising genome stability and of how such genomes arose. Vibrio cholerae, with a 1 Mb replicon in addition to its 3 Mb chromosome, is the only species for which maintenance of a multipartite genome has been investigated. In this study we have explored the more complex genome of Burkholderia cenocepacia (strain J2315). It comprises an extra replicon (c2) of 3.21 Mb, comparable in size to the3.87Mb main chromosome (c1), another extra replicon(c3) of 0.87 Mb and a plasmid of 0.09 Mb. The replication origin of c1 is typically chromosomal and those of c2 and c3 are plasmid-like; all are replicated bidirectionally. Fluorescence microscopy of tagged origins indicates that all initiate replication at mid-cell and segregate towards the cell quarter positions sequentially, c1-c2-p1/c3. c2 segregation is as well-phased with the cell cycle as c1, implying that this plasmid-like origin has become subject to regulation not typical of plasmids; in contrast, c3 segregates more randomly through the cycle. Disruption of individual Par systems by deletion of parAB or by addition of parS sites showed each Par system to govern the positioning of its own replicon only. Inactivation of c1, c2 and c3 Par systems not only reduced growth rate, generated anucleate cells and compromised viability but influenced processes beyond replicon partition, notably regulation of replication, chromosome condensation and cell size determination. In particular, the absence of the c1 ParA protein altered replication of all three chromosomes, suggesting that the partition system of the main chromosome is a major participant in the choreography of the cell cycle.

Centromere binding and evolution of chromosomal partition systems in the Burkholderiales.

How split genomes arise and evolve in bacteria is poorly understood. Since each replicon of such genomes encodes a specific partition (Par) system, the evolution of Par systems could shed light on their evolution. The cystic fibrosis pathogen Burkholderia cenocepacia has three chromosomes (c1, c2, and c3) and one plasmid (pBC), whose compatibility depends on strictly specific interactions of the centromere sequences (parS) with their cognate binding proteins (ParB). However, the Par systems of B. cenocepacia c2, c3, and pBC share many features, suggesting that they arose within an extended family. Database searching revealed seven subfamilies of Par systems like those of B. cenocepacia. All are from plasmids and secondary chromosomes of the Burkholderiales, which reinforces the proposal of an extended family. The subfamily of the Par system of B. cenocepacia c3 includes plasmid variants with parS sequences divergent from that of c3. Using electrophoretic mobility shift assay (EMSA), we found that ParB-c3 binds specifically to centromeres of these variants, despite high DNA sequence divergence. We suggest that the Par system of B. cenocepacia c3 has preserved the features of an ancestral system. In contrast, these features have diverged variably in the plasmid descendants. One such descendant is found both in Ralstonia pickettii 12D, on a free plasmid, and in Ralstonia pickettii 12J, on a plasmid integrated into the main chromosome. These observations suggest that we are witnessing a plasmid-chromosome interaction from which a third chromosome will emerge in a two-chromosome species.

Improved electrotransformation and decreased antibiotic resistance of the cystic fibrosis pathogen Burkholderia cenocepacia strain J2315.

The bacterium Burkholderia cenocepacia is pathogenic for sufferers from cystic fibrosis (CF) and certain immunocompromised conditions. The B. cenocepacia strain most frequently isolated from CF patients, and which serves as the reference for CF epidemiology, is J2315. The J2315 genome is split into three chromosomes and one plasmid. The strain was sequenced several years ago, and its annotation has been released recently. This information should allow genetic experimentation with J2315, but two major impediments appear: the poor potential of J2315 to act as a recipient in transformation and conjugation and the high level of resistance it mounts to nearly all antibiotics. Here, we describe modifications to the standard electroporation procedure that allow routine transformation of J2315 by DNA. In addition, we show that deletion of an efflux pump gene and addition of spermine to the medium enhance the sensitivity of J2315 to certain commonly used antibiotics and so allow a wider range of antibiotic resistance genes to be used for selection.

Dual role of DNA in regulating ATP hydrolysis by the SopA partition protein.

In bacteria, mitotic stability of plasmids and many chromosomes depends on replicon-specific systems, which comprise a centromere, a centromere-binding protein and an ATPase. Dynamic self-assembly of the ATPase appears to enable active partition of replicon copies into cell-halves, but for Walker-box partition ATPases the molecular mechanism is unknown. ATPase activity appears to be essential for this process. DNA and centromere-binding proteins are known to stimulate the ATPase activity but molecular details of the stimulation mechanism have not been reported. We have investigated the interactions which stimulate ATP hydrolysis by the SopA partition ATPase of plasmid F. By using SopA and SopB proteins deficient in DNA binding, we have found that the intrinsic ability of SopA to hydrolyze ATP requires direct DNA binding by SopA but not by SopB. Our results show that two independent interactions of SopA act in synergy to stimulate its ATPase. SopA must interact with (i) DNA, through its ATP-dependent nonspecific DNA binding domain and (ii) SopB, which we show here to provide an arginine-finger motif. In addition, the latter interaction stimulates ATPase maximally when SopB is part of the partition complex. Hence, our data demonstrate that DNA acts on SopA in two ways, directly as nonspecific DNA and through SopB as centromeric DNA, to fully activate SopA ATP hydrolysis.

ParABS systems of the four replicons of Burkholderia cenocepacia: new chromosome centromeres confer partition specificity.

Most bacterial chromosomes carry an analogue of the parABS systems that govern plasmid partition, but their role in chromosome partition is ambiguous. parABS systems might be particularly important for orderly segregation of multipartite genomes, where their role may thus be easier to evaluate. We have characterized parABS systems in Burkholderia cenocepacia, whose genome comprises three chromosomes and one low-copy-number plasmid. A single parAB locus and a set of ParB-binding (parS) centromere sites are located near the origin of each replicon. ParA and ParB of the longest chromosome are phylogenetically similar to analogues in other multichromosome and monochromosome bacteria but are distinct from those of smaller chromosomes. The latter form subgroups that correspond to the taxa of their hosts, indicating evolution from plasmids. The parS sites on the smaller chromosomes and the plasmid are similar to the "universal" parS of the main chromosome but with a sequence specific to their replicon. In an Escherichia coli plasmid stabilization test, each parAB exhibits partition activity only with the parS of its own replicon. Hence, parABS function is based on the independent partition of individual chromosomes rather than on a single communal system or network of interacting systems. Stabilization by the smaller chromosome and plasmid systems was enhanced by mutation of parS sites and a promoter internal to their parAB operons, suggesting autoregulatory mechanisms. The small chromosome ParBs were found to silence transcription, a property relevant to autoregulation.

The parAB gene products of Pseudomonas putida exhibit partition activity in both P. putida and Escherichia coli.

The bacteria for which there is evidence that proteins of the ParAB family act in chromosome segregation also undergo developmental transitions that involve the ParAB homologues, raising the question of whether the partition activity is equivalent to that of plasmid partition systems. We have investigated the role in partition of the parAB locus of a free-living bacterium, Pseudomonas putida, not known to pass through developmental phases. A parAB deletion mutant, compared with wild type, showed slightly higher frequencies of anucleate cells in exponentially growing cultures but much higher frequencies in deceleration phase. This increase was growth medium dependent. Oversupply of ParA and ParB proteins also raised anucleate cell levels, specifically in the deceleration phase, in wild-type and mutant strains and regardless of medium, as well as generating abnormal cell morphologies. Absence or oversupply of ParAB function had either slight or considerable effects on growth rate, depending on temperature and medium. The need for the Par proteins in chromosome partition thus appears to be subject to the cell's physiological state. Three sequences similar to cis-acting stabilization sites of Bacillus subtilis are present in the P. putida oriC-parAB region. One was inserted into an unstable mini-F and shown to stabilize it in E. coli in a ParAB-dependent manner.

Exclusion of long heterologous insertions and deletions from the pairing synapsis in pneumococcal transformation.

We have studied the mode of recombination of six insertions during genetic transformation of Streptococcus pneumoniae. The six heterologous insertions are located at the same site in the ami locus of the pneumococcal chromosome; insertion sizes range from 4 to 1374 bp. With respect to single-point markers we found that the number of transformants in one-point crosses is reduced, while the number of wild-type transformants in two-point crosses is drastically increased, what we call hyper-recombination. The magnitude of the shift is correlated with the size of the insert. This effect could result either from a special repair pathway of multibase heteroduplexes or from the exclusion of multibase heterologous insertions out of the pairing synapsis. To test these hypotheses we have used insertions in two kinds of three-point crosses. The repair model predicts that the excess of wild-type transformants remains in one set of crosses but is suppressed in the second set. The results we obtained are reversed, ruling out the hypothesis of a repair process, but in agreement with predictions based on the exclusion model. Moreover, we have re-examined the situation of deletions, our previous results suggesting that deletions were likely to be converted at the heteroduplex step. Genetic evidence we obtained in this work no longer supports this hypothesis. Thus, long heterologous insertions are partly excluded at the pairing step.