New genes with old modus operandi. The connection between supercoiling and partitioning of DNA in Escherichia coli

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.

Local sensing of global DNA topology: from crossover geometry to type II topoisomerase processivity

Type II topoisomerases are ubiquitous enzymes that control the topology and higher order structures of DNA. Type IIA enzymes have the remarkable property to sense locally the global DNA topology. Although many theoretical models have been proposed, the molecular mechanism of chiral discrimination is still unclear. While experimental studies have established that topoisomerases IIA discriminate topology on the basis of crossover geometry, a recent single-molecule experiment has shown that the enzyme has a different processivity on supercoiled DNA of opposite sign. Understanding how cross-over geometry influences enzyme processivity is, therefore, the key to elucidate the mechanism of chiral discrimination. Analysing this question from the DNA side reveals first, that the different stability of chiral DNA cross-overs provides a way to locally sense the global DNA topology. Second, it shows that these enzymes have evolved to recognize the G- and T-segments stably assembled into a right-handed cross-over. Third, it demonstrates how binding right-handed cross-overs across their large angle imposes a different topological link between the topoIIA rings and the plectonemes of opposite sign thus directly affecting the enzyme freedom of motion and processivity. In bridging geometry and kinetic data, this study brings a simple solution for type IIA topoisomerase chiral discrimination.

Entropy as the driver of chromosome segregation

We present a new physical biology approach to understanding the relationship between the organization and segregation of bacterial chromosomes. We posit that replicated Escherichia coli daughter strands will spontaneously demix as a result of entropic forces, despite their strong confinement within the cell; in other words, we propose that entropy can act as a primordial physical force which drives chromosome segregation under the right physical conditions. Furthermore, proteins implicated in the regulation of chromosome structure and segregation may in fact function primarily in supporting such an entropy-driven segregation mechanism by regulating the physical state of chromosomes. We conclude that bacterial chromosome segregation is best understood in terms of spontaneous demixing of daughter strands. Our concept may also have important implications for chromosome segregation in eukaryotes, in which spindle-dependent chromosome movement follows an extended period of sister chromatid demixing and compaction.

Coiled-coil protein composition of 22 proteomes--differences and common themes in subcellular infrastructure and traffic control

Background

Long alpha-helical coiled-coil proteins are involved in diverse organizational and regulatory processes in eukaryotic cells. They provide cables and networks in the cyto- and nucleoskeleton, molecular scaffolds that organize membrane systems and tissues, motors, levers, rotating arms, and possibly springs. Mutations in long coiled-coil proteins have been implemented in a growing number of human diseases. Using the coiled-coil prediction program MultiCoil, we have previously identified all long coiled-coil proteins from the model plant Arabidopsis thaliana and have established a searchable Arabidopsis coiled-coil protein database.

Results

Here, we have identified all proteins with long coiled-coil domains from 21 additional fully sequenced genomes. Because regions predicted to form coiled-coils interfere with sequence homology determination, we have developed a sequence comparison and clustering strategy based on masking predicted coiled-coil domains. Comparing and grouping all long coiled-coil proteins from 22 genomes, the kingdom-specificity of coiled-coil protein families was determined. At the same time, a number of proteins with unknown function could be grouped with already characterized proteins from other organisms.

Conclusion

MultiCoil predicts proteins with extended coiled-coil domains (more than 250 amino acids) to be largely absent from bacterial genomes, but present in archaea and eukaryotes. The structural maintenance of chromosomes proteins and their relatives are the only long coiled-coil protein family clearly conserved throughout all kingdoms, indicating their ancient nature. Motor proteins, membrane tethering and vesicle transport proteins are the dominant eukaryote-specific long coiled-coil proteins, suggesting that coiled-coil proteins have gained functions in the increasingly complex processes of subcellular infrastructure maintenance and trafficking control of the eukaryotic cell.

Effects of perturbing nucleoid structure on nucleoid occlusion-mediated toporegulation of FtsZ ring assembly

In Escherichia coli, assembly of the FtsZ ring (Z ring) at the cell division site is negatively regulated by the nucleoid in a phenomenon called nucleoid occlusion (NO). Previous studies have indicated that chromosome packing plays a role in NO, as mukB mutants grown in rich medium often exhibit FtsZ rings on top of diffuse, unsegregated nucleoids. To address the potential role of overall nucleoid structure on NO, we investigated the effects of disrupting chromosome structure on Z-ring positioning. We found that NO was mostly normal in cells with inactivated DNA gyrase or in mukB-null mutants lacking topA, although some suppression of NO was evident in the latter case. Previous reports suggesting that transcription, translation, and membrane insertion of proteins ("transertion") influence nucleoid structure prompted us to investigate whether disruption of these activities had effects on NO. Blocking transcription caused nucleoids to become diffuse, and FtsZ relocalized to multiple bands on top of these nucleoids, biased towards midcell. This suggested that these diffuse nucleoids were defective in NO. Blocking translation with chloramphenicol caused characteristic nucleoid compaction, but FtsZ rarely assembled on top of these centrally positioned nucleoids. This suggested that NO remained active upon translation inhibition. Blocking protein secretion by thermoinduction of a secA(Ts) strain caused a chromosome segregation defect similar to that in parC mutants, and NO was active. Although indirect effects are certainly possible with these experiments, the above data suggest that optimum NO activity may require specific organization and structure of the nucleoid.

Bacterial Mitotic Machineries

Here, we review recent progress that yields fundamental new insight into the molecular mechanisms behind plasmid and chromosome segregation in prokaryotic cells. In particular, we describe how prokaryotic actin homologs form mitotic machineries that segregate DNA before cell division. Thus, the ParM protein of plasmid R1 forms F actin-like filaments that separate and move plasmid DNA from mid-cell to the cell poles. Evidence from three different laboratories indicate that the morphogenetic MreB protein may be involved in segregation of the bacterial chromosome.

Replication restart in gyrB Escherichia coli mutants

Gyrase is an essential topoisomerase in bacteria that introduces negative supercoils in DNA and relaxes the positive supercoils that form downstream of proteins tracking on DNA, such as DNA or RNA polymerases. Two gyrase mutants that suffer partial loss of function were used here to study the need for replication restart in conditions in which gyrase activity is affected. We show that the preprimosomal protein PriA is essential for the viability of these gyrB mutants. The helicase function of PriA is not essential. The lethality of the gyrB priA double mutants is suppressed by a dnaC809 mutation, indicating a requirement for primosome assembly in gyrB strains. The lethality of gyrB priA combination of mutations is independent of the level of DNA supercoiling, as gyrB and priA were also co-lethal in the presence of a DeltatopA mutation. Inactivation of homologous recombination did not affect the viability of gyrB mutants, indicating that replication restart does not require the formation of a recombination intermediate. We propose that the replisome is disassembled from replication forks when replication progression is blocked by the accumulation of positive supercoils in gyrase mutants, and that replication restarts via PriA-dependent primosome assembly, directly on the in-activated replication forks, without the formation of a recombination intermediate.

The role of co-transcriptional translation and protein translocation (transertion) in bacterial chromosome segregation

Many recent reviews in the field of bacterial chromosome segregation propose that newly replicated DNA is actively separated by the functioning of specific proteins. This view is primarily based on an interpretation of the position of fluorescently labelled DNA regions and proteins in analogy to the active segregation mechanism in eukaryotic cells, i.e. to mitosis. So far, physical aspects of DNA organization such as the diffusional movement of DNA supercoil segments and their interaction with soluble proteins, leading to a phase separation between cytoplasm and nucleoid, have received relatively little attention. Here, a quite different view is described taking into account DNA-protein interactions, the large variation in the cellular position of fluorescent foci and the compaction and fusion of segregated nucleoids upon inhibition of RNA or protein synthesis. It is proposed that the random diffusion of DNA supercoil segments is transiently constrained by the process of co- transcriptional translation and translocation (transertion) of membrane proteins. After initiation of DNA replication, a bias in the positioning of transertion areas creates a bidirectionality in chromosome segregation that becomes self-enhanced when neighbouring genes on the same daughter chromosome are expressed. This transertion-mediated segregation model is applicable to multifork replication during rapid growth and to multiple chromosomes and plasmids that occur in many bacteria.

Eclipse period without sequestration in Escherichia coli

The classical Meselson-Stahl density shift experiment was used to determine the length of the eclipse period in Escherichia coli, the minimum time period during which no new initiation is allowed from a newly replicated origin of chromosome replication, oriC. Populations of bacteria growing exponentially in heavy ((15)NH(4)+ and (13)C(6)-glucose) medium were shifted to light ((14)NH(4)+ and (12)C(6)-glucose) medium. The HH-, HL- and LL-DNA were separated by CsCl density gradient centrifugation, and their relative amounts were determined using radioactive gene-specific probes. The eclipse period, estimated from the kinetics of conversion of HH-DNA to HL- and LL-DNA, turned out to be 0.60 generation times for the wild-type strain. This was invariable for widely varying doubling times (35, 68 and 112 min) and was independent of the chromosome locus at which the eclipse period was measured. For strains with seqA, dam and damseqA mutants, the length of the eclipse period was 0.16, 0.40 and 0.32 generation times respectively. Thus, initiations from oriC were repressed for a considerable proportion of the generation time even when the sequestration function seemed to be severely compromised. The causal relationship between the length of the eclipse period and the synchrony of initiations from oriC is discussed.

Compatible bacterial plasmids are targeted to independent cellular locations in Escherichia coli

Targeting of DNA molecules to specific subcellular positions is essential for efficient segregation, but the mechanisms underlying these processes are poorly understood. In Escherichia coli, several plasmids belonging to different incompatibility groups (F, P1 and RK2) localize preferentially near the midcell and quartercell positions. Here we compare the relative positions of these three plasmids using fluorescence in situ hybridization. When plasmids F and P1 were localized simultaneously using differentially labeled probes, the majority of foci (approximately 75%) were well separated from each other. Similar results were found when we compared the subcellular localization of F with RK2, and RK2 with P1: regardless of the number of foci per cell or growth conditions, most of the foci (70-80%) were not in close proximity to one another. We also localized RK2 in Pseudomonas aeruginosa and Vibrio cholerae, and found that plasmid RK2 localization is conserved across bacterial species. Our results suggest that each plasmid has its own unique subcellular address, implying a mechanism for the stable co-existence of plasmids in which subcellular targeting plays a major role.

Partitioning of the Escherichia coli chromosome: superhelicity and condensation

Segregation in Escherichia coli, the process of separating the replicated chromosomes into daughter progeny cells, seems to start long before the duplication of the genome reaches completion. Soon after initiation in mid-cell region, the daughter oriCs rapidly move apart to fixed positions inside the cell (quarter length positions from each pole) and are anchored there by yet unknown mechanism(s). As replication proceeds, the rest of the chromosome is sequentially unwound and then refolded. At termination, the two sister chromosomes are unlinked by decatenation and separated by supercoiling and/or condensation. Muk and Seq proteins are involved in different stages of this replication-cum-partition process and thus can be categorized as important partition proteins along with topoisomerases. E. coli strains, lacking mukB or seqA functions, are defective in segregation and cell division. The nucleoids in these mutant strains exhibit altered condensation and superhelicity as can be demonstrated by sedimentation analysis and by fluorescence microscopy. As the supercoiling of an extrachromosomal element (a plasmid DNA) was also influenced by the mukB and seqA mutations we concluded that the MukB and SeqA proteins are possibly involved in maintaining the general supercoiling activity in the cell. The segregation of E. coli chromosome might therefore be predominantly driven by factors that operate by affecting the superhelicity and condensation of the nucleoid (MukB, SeqA, topoisomerases and additional unknown proteins). A picture thus emerges in which replication and partition are no longer compartmentalized into separable stages with clear gaps (S and M phases in eukaryotes) but are parallel processes that proceed concomitantly through a cell cycle continuum.

Role of the mukB gene in chromosome and plasmid partition in Escherichia coli

The intracellular locations of oriC and oriR1, the replication origins of the chromosome and plasmid R1, respectively, were visualized by fluorescence in situ hybridization (FISH) in exponentially growing populations of Escherichia coli. The locations of oriC and oriR1 (from a Par+ R1 plasmid) were unique and different in the wild-type host. In a mukB mutant, the positions were perturbed for both origins. The position of oriR1 from a plasmid with active partition (Par+) in the mukB host was as randomized as that of oriR1 from the Par- plasmid in a wild-type host. However, this mukB-induced randomization did not result in unstable inheritance of the Par+ plasmid, as measured by the conventional segregation assay. This might result from the preferential association of the Par+ plasmid with the bigger, decondensed nucleoid-containing daughters during cell division of MukB- cells, whereas the Par- plasmids were distributed at random and were lost by frequently ending up in anucleate cells.