Saccharomyces cerevisiae cells lacking the homologous pairing protein p175 SEP1 arrest at pachytene during meiotic prophase

Abstract

Saccharomyces cerevisiae cells containing null mutations in the SEP1 gene, which encodes the homologous pairing and strand exchange protein p175SEP1, enter pachytene with a delay. They arrest uniformly at this stage of meiotic prophase, probably revealing a checkpoint in the transition from pachytene to meiosis I. At the arrest point, the cells remain largely viable and are cytologically characterized by the duplicated but unseparated spindle pole bodies of equal size and by the persistence of the synaptonemal complex, a cytological marker for pachytene. In addition, fluorescence in situ hybridization revealed that in arrested mutant cells maximal chromatin condensation and normal homolog pairing is achieved, typical for pachytene in wild type. A hallmark of meiosis is the high level of homologous recombination, which was analyzed both genetically and physically. Formation and processing of the double-strand break intermediate in meiotic recombination is achieved prior to arrest. Physical intragenic (conversion) and intergenic (crossover) products are formed just prior to, or directly at, the arrest point. Structural deficits in synaptonemal complex morphology, failure to separate spindle pole bodies, and/or defects in prophase DNA metabolism might be responsible for triggering the observed arrest. The pachytene arrest in sep1 cells is likely to be regulatory, but is clearly different from the RAD9 checkpoint in meiotic prophase, which occurs prior to the pachytene stage.

MUS81 encodes a novel Helix-hairpin-Helix protein involved in the response to UV- and methylation-induced DNA damage in Saccharomyces cerevisiae

“…Asci and cells (at least 200 per culture) were counted and four-spored asci dissected on YPD plates to determine spore viability. Meiotic time-course analysis (return-to-growth) and physical recombination assays were performed as described by BaÈ hler et al (1994) using the assay developed in N. Kleckner's laboratory (Cao et al 1990). …”

Regulation and Intracellular Localization of Saccharomyces cerevisiae Strand Exchange Protein 1 (Sep1/Xrn1/Kem1), a Multifunctional Exonuclease

“…Mating, sporulation, and tetrad analysis were performed as described previously (50). Meiotic time course analysis has been described elsewhere (3,48,62). ␣-Factor arrest-release experiments were performed as described previously (12), using strain L42 (Table 1).…”

“…These properties have suggested a role in DNA recombination. Although sep1 mutants do not show significant defects in mitotic recombination (3,11,35,62) or in mating-type switching (57), the meiotic recombination defects observed in sep1 mutants have been interpreted to indicate that Sep1 plays a direct role in meiotic DNA recombination (3,11,62,63). Kem1 (Sep1) has recently been described as a nuclease specific for G4 tetrastranded DNA (45).…”

“…Kem1 (Sep1) has recently been described as a nuclease specific for G4 tetrastranded DNA (45). It was proposed that the meiotic arrest observed in sep1 mutants (3,63) arises from an inability to initiate pairing of homologous chromosomes via the formation of interchromosomal G4 tetrastranded DNA (45). In addition, Sep1 could play a role in the function of telomeres which may contain G4 tetrastranded DNA.…”

“…However, the correlation of in vitro G4 tetrastranded DNA binding with in vivo function remains to be demonstrated. Genetic arguments have suggested that Sep1 carries out other essential meiotic functions in addition to recombination (3,63). Consequently, it is possible that the meiotic arrest is regulatory in nature and that the resulting recombination defects are a consequence of the arrest point (3,62,63).…”