HETERODUPLEX DNA FORMATION AND HOMOLOG PAIRING IN YEAST MEIOTIC MUTANTS

Previous studies of Saccharomyces cerevisiae have identified several m eiosis-specific genes whose products are required for wild-type levels of meiotic recombination and for normal synaptonemal complex (SC) for mation. Several of these mutants were examined in a physical assay des igned to detect heteroduplex DNA (hDNA) intermediates in meiotic recom bination. hDNA was not detected in the rec102, mei4 and hop1 mutants; it was observed at reduced levels in red1, mek1 and mer1 strains and a t greater than the wild-type level in zip1. These results indicate tha t the REC102, MEI4, HOP1, RED1, MEK1 and MER1 gene products act before hDNA formation in the meiotic recombination pathway, whereas ZIP1 act s later. The same mutants assayed for hDNA formation were monitored fo r meiotic chromosome pairing by in situ hybridization of chromosome-sp ecific DNA probes to spread meiotic nuclei. Homolog pairing occurs at wild-type levels in the zip1 and mek1 mutants, but is substantially re duced in mei4, rec102 hop1, red1 and merl strains. Even mutants that f ail to recombine or to make any SC or SC precursors undergo a signific ant amount of meiotic chromosome pairing. The in situ hybridization pr ocedure revealed defects in meiotic chromatin condensation in mer1, re d1 and hop1 strains.