The 2 micron plasmid purloins the yeast cohesin complex: a mechanism for coupling plasmid partitioning and chromosome segregation?

The yeast 2 micron plasmid achieves high fidelity segregation by coupling its partitioning pathway to that of the chromosomes. Mutations affecting distinct steps of chromosome segregation cause the plasmid to missegregate in tandem with the chromosomes. In the absence of the plasmid stability system, consisting of the Rep1 and Rep2 proteins and the STB DNA, plasmid and chromosome segregations are uncoupled. The Rep proteins, acting in concert, recruit the yeast cohesin complex to the STB locus. The periodicity of cohesin association and dissociation is nearly identical for the plasmid and the chromosomes. The timely disassembly of cohesin is a prerequisite for plasmid segregation. Cohesin-mediated pairing and unpairing likely provides a counting mechanism for evenly partitioning plasmids either in association with or independently of the chromosomes

The selfish yeast plasmid uses the nuclear motor Kip1p but not Cin8p for its localization and equal segregation

The 2 micron plasmid of Saccharomyces cerevisiae uses the Kip1 motor, but not the functionally redundant Cin8 motor, for its precise nuclear localization and equal segregation. The timing and lifetime of Kip1p association with the plasmid partitioning locus STB are consistent with Kip1p being an authentic component of the plasmid partitioning complex. Kip1–STB association is not blocked by disassembling the mitotic spindle. Lack of Kip1p disrupts recruitment of the cohesin complex at STB and cohesion of replicated plasmid molecules. Colocalization of a 2 micron reporter plasmid with Kip1p in close proximity to the spindle pole body is reminiscent of that of a CEN reporter plasmid. Absence of Kip1p displaces the plasmid from this nuclear address, where it has the potential to tether to a chromosome or poach chromosome segregation factors. Exploiting Kip1p, which is subsidiary to Cin8p for chromosome segregation, to direct itself to a “partitioning center” represents yet another facet of the benign parasitism of the yeast plasmid

Genomic Plasticity of Vibrio cholerae

Vibrio cholerae is one of the deadliest pathogens in the history of humankind. It is the causative agent of cholera, adisease characterized by a profuse and watery diarrhoea that still today causes 95.000 deaths worldwide every year. V. choleraeis a free living marine organism that interacts with and infects a variety of organisms, from amoeba to humans, including insectsand crustaceans. The complexity of the lifestyle and ecology of V. cholerae suggests a high genetic and phenotypic plasticity. Inthis review, we will focus on two peculiar genomic features that enhance genetic plasticity in this bacterium: the division of itsgenome in two different chromosomes and the presence of the superintegron, a gene capture device that acts as a large, low-costmemory of adaptive functions, allowing V. cholerae to adapt rapidly

The stabilization of repetitive tracts of DNA by variant repeats requires a functional DNA mismatch repair system

AbstractSimple repetitive tracts of DNA are unstable in all organisms thus far examined. In the yeast S. cerevisiae, we show that a 51 by poly(GT) tract alters length at a rate of about 10−5 per cell division. Insertion of a single variant repeat (either AT or CT) into the middle of the poly(GT) tract results in 100-fold stabilization. This stabilization requires the DNA mismatch repair system. Alterations within tracts with variant repeats occur more frequently on one side of the interruption than on the other. The stabilizing effects of variant repeats and polarity of repeat alterations have also been observed in trinucleotide repeats associated with certain human diseases

Stable propagation of the yeast 2 micron plasmid : equal segregation by hitchhiking on chromosomes.

textThe 2 micron plasmid of Saccharomyces cerevisiae resides in the nucleus as an extra-chromosomal element with a steady state copy number of 40-60 per cell. As a benign but selfish DNA element, the plasmid utilizes a self-encoded partitioning system and an amplification system to ensure its stable, high-copy propagation. The partitioning system consists of the plasmid encoded proteins, Rep1 and Rep2 and a cis-acting partitioning locus STB. The Rep proteins, together with several host factors, assembled at STB couple plasmid segregation to chromosome segregation. A plasmid lacking an active partitioning system is subject to a ‘diffusion barrier’, which causes it to be retained in the mother cell with a strong bias (mother bias). Currently available evidence favors the hitchhiking model for plasmid segregation, in which the tethering of plasmids to chromosome provides the basis for faithful plasmid partitioning. However, direct evidence to support this hypothesis has been difficult to obtain because of the small size of the budding yeast nucleus and the poor resolution of chromosomes in live cells or in chromosome spreads. In this study, we have attempted to verify the hitchhiking model using single copy derivatives of the 2 micron plasmid as reporters. We demonstrate, using two single copy reporters present in the same nucleus, that plasmid association with chromosome spreads is authentic, and is dependent on the partitioning system. By using a strategy that forces all chromosomes to stay in either the mother or the daughter compartment, we show that plasmid segregation can be uncoupled from nuclear envelope segregation. However, plasmid segregation cannot be uncoupled from chromosome segregation under this condition. This tight coupling between plasmid and chromosome segregation is consistent with the hitchhiking model for plasmid segregation. The plasmid partitioning complex is assembled de novo at STB during each cell cycle during the G1-S window. Plasmid replication or cell cycle cues that signal cellular DNA replication appear to trigger this assembly. Furthermore, there is an apparent temporal hierarchy in the association and dissociation of protein factors at STB. When DNA replication is delayed or blocked, the dissociation of factors from STB from the previous portioning cycle and the association of factors for the new partitioning cycle are delayed or blocked, respectively. The precise role of replication in plasmid segregation has not been elucidated. We have addressed this question by blocking either plasmid replication or all cellular DNA replication. We find that replication is not required for plasmid to overcome mother bias. However, replication is critical for the equal segregation of sister plasmid copies. These results provide a refinement of the hitchhiking model by suggesting that sister plasmids tether to sister chromatids in a replication-dependent manner and hitchhike on them during chromosome segregation. Finally, we have attempted to reconstitute the 2 micron plasmid partitioning system in mammalian cells with the goal of exploiting their larger nuclear size and the considerably higher chromosome resolution they provide. In experiments completed so far, we show that Rep2 expressed in COS7 cells localizes to chromosomes, and Rep1 does so in the presence of Rep2. Furthermore, they show co-localization on sister chromatids in a symmetric fashion, implying that plasmids associated with them are likely to follow suit. These observations suggest, by extrapolation, the Rep1-Rep2 assisted association of sister plasmids with sister chromatids in yeast as well, and are consistent with the refined hitchhiking model for plasmid segregation.Microbiolog

Evolution of a selfish genetic element: the 2 micron plasmid of saccharomyces spp.

The 2 Micron plasmid is a multicopy DNA circle inhabiting the genome of the budding yeasts, Sacchormyces spp. The plasmid confers no known benefits to the host, but imposes a small fitness cost. However the plasmid is able to drive, i.e. to transmit to >50% of sexual offspring, which allows the element to spread through an outcrossing host population. Therefore we can consider the plasmid a selfish genetic element of yeast. Here we draw on a number of approaches to improve our understanding of this element. Firstly, we examined the relationship between the cost of plasmid carriage and copy number by experimentally manipulating the number of plasmids in the host. We find that host fitness decreases at a rate of ~0.09% per additional plasmid. Secondly we use experimentally evolving yeast populations to test the hypothesis that sexual reproduction, which is fundamental to the evolution of selfish genetic elements, will drive increasing virulence in the plasmid. We find that 2 Micron copy number increased in outcrossing populations but remained constant in asexual populations. We also find that sex allowed the invasion of non-functional mitochondria in to the populations, showing that sex has the capacity to generate a driving selfish genetic element from one of the most fundamental endosymbionts of the eukaryotic cell. In addition, we have investigated plasmid variation from global populations of Saccharromyces spp. in order to better understand the population biology and evolution of this plasmid. Here we find evidence that the plasmid is able to move between species, recombine with other plasmids within the cell, and exist at a surprisingly wide range of copy numbers in different host populations. Understanding the population structure and evolution of this element allows us to view the plasmid as an autonomous unit evolving in its own right in the genomes of its hosts

Evolution of long centromeres in fire ants

Background: Centromeres are essential for accurate chromosome segregation, yet sequence conservation is low even among closely related species. Centromere drive predicts rapid turnover because some centromeric sequences may compete better than others during female meiosis. In addition to sequence composition, longer centromeres may have a transmission advantage. Results: We report the first observations of extremely long centromeres, covering on average 34 % of the chromosomes, in the red imported fire ant Solenopsis invicta. By comparison, cytological examination of Solenopsis geminata revealed typical small centromeric constrictions. Bioinformatics and molecular analyses identified CenSol, the major centromeric satellite DNA repeat. We found that CenSol sequences are very similar between the two species but the CenSol copy number in S. invicta is much greater than that in S. geminata. In addition, centromere expansion in S. invicta is not correlated with the duplication of CenH3. Comparative analyses revealed that several closely related fire ant species also possess long centromeres. Conclusions: Our results are consistent with a model of simple runaway centromere expansion due to centromere drive. We suggest expanded centromeres may be more prevalent in hymenopteran insects, which use haplodiploid sex determination, than previously considered