SACCHAROMYCES-CEREVISIAE CELLS LACKING THE HOMOLOGOUS PAIRING PROTEINP175(SEP1) ARREST AT PACHYTENE DURING MEIOTIC PROPHASE

Saccharomyces cerevisiae cells containing null mutations in the SEP1 g ene, which encodes the homologous pairing and strand exchange protein p175(SEP1), enter pachytene with a delay. They arrest uniformly at thi s stage of meiotic prophase, probably revealing a checkpoint in the tr ansition from pachytene to meiosis I. At the arrest point, the cells r emain largely viable and are cytologically characterized by the duplic ated but unseparated spindle pole bodies of equal size and by the pers istence of the synaptonemal complex, a cytological marker for pachyten e. In addition, fluorescence in situ hybridization revealed that in ar rested 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 anal yzed 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 (cros sover) products are formed just prior to, or directly at, the arrest p oint. Structural deficits in synaptonemal complex morphology, failure to separate spindle pole bodies, and/or defects in prophase DNA metabo lism might be responsible for triggering the observed arrest. The pach ytene arrest in sep1 cells is likely to be regulatory, but is clearly different from the RAD9 checkpoint in meiotic prophase, which occurs p rior to the pachytene stage.