Academic journal article Genetics

Cohesin and Recombination Proteins Influence the G^sub 1^-to-S Transition in Azygotic Meiosis in Schizosaccharomyces Pombe

Academic journal article Genetics

Cohesin and Recombination Proteins Influence the G^sub 1^-to-S Transition in Azygotic Meiosis in Schizosaccharomyces Pombe

Article excerpt

ABSTRACT

To determine whether recombination and/or sister-chromatid cohesion affect the timing of meiotic prophase events, the horsetail stage and S phase were analyzed in Schizosaccharomyces pombe strains carrying mutations in the cohesin genes rec8 or rec11, the linear element gene rec10, the pairing gene meu13, the double-strand-break formation genes rec6, rec7, rec12, rec14, rec15, and mde2, and the recombination gene dmc1. The double-mutant strains rec8 rec11 and rec8 rec12 were also assayed. Most of the single and both double mutants showed advancement of bulk DNA synthesis, start of nuclear movement (horsetail stage), and meiotic divisions by up to 2 hr. Only mde2 and dmc1 deletion strains showed wild-type timing. Contrasting behavior was observed for rec8 deletions (delayed by 1 hr) compared to a rec8 point mutation (advanced by 1 hr). An hypothesis for the role of cohesin and recombination proteins in the control of the G^sub 1^-to-S transition is proposed. Finally, differences between azygotic meiosis and two other types of fission yeast meiosis (zygotic and pat1-114 meiosis) are discussed with respect to possible control steps in meiotic G^sub 1^.

IN sexually reproducing eukaryotes, meiosis results in haploid gametes, which fuse to form the diploid zygote. In organisms with a diploid life cycle, this zygote will give rise to a colony of diploid vegetative cells, or a multicellular body consisting of somatic cells. In Schizosaccharomyces pombe (haploid life cycle), the gametes differentiate into spores (endurance state), which germinate on nutrient-rich media to form colonies of haploid cells. Under nutritional stress (especially nitrogen starvation), haploid cells of opposite mating type differentiate and fuse, and the resulting zygote usually undergoes meiosis immediately (zygoticmeiosis; see Figure 1A). However, when returned to rich media before commitment to meiosis, zygotes can resume vegetative growth and form colonies of diploid cells. Under nitrogen starvation, diploid cells heterozygous for mating type willundergoazygotic meiosis (Figure 1A) (Egel 1973; Egel and Egel-Mitani 1974). Azygotic meiosis is more synchronous than zygotic meiosis, but less synchronous than pat1-114 meiosis (Figure 1A).

A single round of DNA replication followed by two divisions is one major difference between meiosis and mitosis (for review see Page and Hawley 2003). The second meiotic division (MII) is equational, comparable to mitosis. During prophase I, homologous chromosomes pair and recombine. In many organisms, a proteinaceous structure called the synaptonemal complex (SC) is formed between homologous chromosomes. Recombination is initiated by DNA double-strand-break (DSB) formation, catalyzed by a topoisomerase-like protein called Spo11 in Saccharomyces cerevisiae and other eukaryotes and Rec12 in S. pombe (Bergerat et al. 1997; Keeney et al. 1997; Davis and Smith 2001). In both fission and budding yeast, proteins in addition to Spo11/Rec12 are required for DSB formation, several of them without conservation of amino acid sequences between species (Davis and Smith 2001; Keeney 2001). Some of the DSBs are processed into crossovers, which, in combination with sister-chromatid cohesion (SCC), physically link homologous chromosomes. After SC disassembly, the microscopically discernible chiasmata form. They are required for proper segregation of the homologous chromosomes duringmeiosis I (MI), the reductional division. The sister chromatids of the chromosomes are then segregated during MII.

In contrast to other eukaryotes, S. pombe forms linear elements (LE), which resemble the axial elements, but not SCs (Bahler et al. 1993). During prophase I, the fission yeast nucleus elongates to form the "horsetail" (HT) nucleus, which moves back and forth in the cell (Chikashige et al. 1994; Svoboda et al. 1995). The bouquet structure involves clustering of the telomeres at the spindle pole body at the leading end of the HT nucleus. …

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