RSC2, Encoding a Component of the RSC Nucleosome Remodeling Complex, Is Essential for 2μm Plasmid Maintenance in Saccharomyces cerevisiae

Abstract

The stable maintenance of the 2m circle plasmid depends on its ability to overcome intrinsic maternal inheritance bias, which in yeast normally results in the failure to transmit DNA molecules efficiently to daughter cells. In addition to the plasmid proteins Rep1 and Rep2 acting on the plasmid DNA locus STB, it is likely that other chromosomally encoded yeast proteins are required. We have isolated mutants of yeast unable to maintain 2m and found that RSC2 is essential for 2m to overcome maternal inheritance bias. Rsc2 is part of a multisubunit RSC chromatin remodeling complex, and we show that in the absence of Rsc2 the chromatin structure of the STB region is significantly altered and the Rep1 protein loses its normal localization to subnuclear foci. Rsc1, a closely related homolog of Rsc2 present in an alternative form of the RSC complex, is not required for 2m maintenance and does not replace the requirement for Rsc2 when overexpressed. This represents the first specific role for Rsc2 that has been related to a change in chromatin structure, as well as the first direct evidence linking chromatin structure to 2m segregation.The 2m circle is a high-copy-number plasmid widespread in laboratory and industrial strains of Saccharomyces cerevisiae that has been used as a molecular model for aspects of cellular function such as DNA replication, gene transcription, and recombination, as well as the basis of yeast cloning vectors. High-copy-number plasmids carrying only replication origins show maternal inheritance bias (MIB) and are poorly segregated to daughter cells (31), whereas 2m and its derivatives are effectively segregated between mother and daughter cells at division (22). However, the mechanism by which 2m escapes MIB resulting in equal segregation is not fully understood, nor have all of the host components involved been identified. Of the four genes carried by the plasmid, REP1 and REP2 are necessary for MIB to be overcome, and their products act at the plasmid locus STB to confer plasmid stability (2,10,18,22,33,36). Within the nucleus, Rep1 and Rep2 proteins are localized into discrete foci (39) in which the plasmid is also colocalized (39, 52). These plasmid and protein foci are segregated in a manner similar to that of the chromosomes themselves, a finding consistent with the observation that the 2m plasmid is packaged into nucleosomes (50). Thus, the plasmid may be exploiting the chromosomal segregation machinery indirectly by associating with specific or nonspecific chromosomal sites. It has been therefore suggested that the segregation of 2m, and hence its overcoming of MIB, likely requires host (non-plasmid-encoded) genes (18). One candidate is the product of the SHF1 (CST6) gene, which has been shown to bind to STB in vivo and in vitro (51).Nucleosomes regulate and modify the ability of proteins to interact with DNA, and the remodeling of chromatin and repositioning of nucleosomes is important in gene regulation and other functions of DNA. There is evidence that specific chromatin structures are...

Mutations in a Partitioning Protein and Altered Chromatin Structure at the Partitioning Locus Prevent Cohesin Recruitment by the Saccharomyces cerevisiae Plasmid and Cause Plasmid Missegregation

“…4C, lane 2). These data are consistent with the fact that, unlike the rsc2⌬ mutation, an rsc1⌬ mutation does not lead to an elevated loss of the 2m plasmid (34).…”

“…A recent discovery by Wong et al (34) revealed a strong correlation between the nucleosome architecture at STB and the stable propagation of the 2m plasmid. In the absence of the Rsc2 protein, a component of one of the nucleosome remodeling complexes in yeast, there is a high rate of loss of the 2m circle (34). Concomitantly, there is a finite alteration of the chromatin structure of the STB region.…”

“…Furthermore, the cycling of Rep1 and Rep2 proteins at the STB DNA as a function of the yeast cell cycle specifies the timing of cohesin recruitment to the plasmid and synchronizes the plasmid with chromosomes in cohesin assembly. In the absence of the RSC2 gene function, the chromatin structure at the STB DNA is altered (34) and Rep1p (but not Rep2p) fails to bind to this locus. Consequently, plasmid-cohesin association cannot occur in an rsc2⌬ background.…”

“…A recent observation suggests that the chromatin structure of the STB locus has a pronounced effect on its functionality in plasmid maintenance. In an rsc2⌬ strain lacking one of the RSC (remodel the structure of chromatin) complexes involved in chromatin remodeling, the 2m plasmid becomes highly unstable (34).…”

Histone H3-variant Cse4-induced positive DNA supercoiling in the yeast plasmid has implications for a plasmid origin of a chromosome centromere

“…These factors include the mitotic spindle, the spindleassociated motor Kip1, the RSC2 chromatin-remodeling complex, the centromere-specific histone H3-variant Cse4 (CenH3), and the yeast cohesin complex (9)(10)(11)(12)(13)(14)(15). The de novo assembly of the plasmid-partitioning complex at STB during the G1-S window of each cell cycle (9,12,14,16) is reminiscent of the assembly of the kinetochore complex at centromeres.…”