ISOLATION OF COM1, A NEW GENE REQUIRED TO COMPLETE MEIOTIC DOUBLE-STRAND BREAK-INDUCED RECOMBINATION IN SACCHAROMYCES-CEREVISIAE

We have designed a screen to isolate mutants defective during a specif ic part of meiotic prophase I of the yeast Saccharomyces cerevisiae Ge nes required for the repair of meiotic double-strand breaks or for the separation of recombined chromosomes are targets of this mutant hunt. The specificity is achieved by selecting for mutants that produce via ble spores when recombination and reductional segregation are prevente d by mutations in SPO11 and SPO13 genes, but fail to yield viable spor es during a normal Rec(+) meiosis. We have identified and characterize d a mutation com1-1, which blocks processing of meiotic double-strand breaks and which interferes with synaptonemal complex formation, homol ogous pairing and, as a consequence, spore viability after induction o f meiotic recombination, The COM1/SAE2 gene was cloned by complementat ion, and the deletion mutant has a phenotype similar to com1-1. com1/s ae2 mutants closely resemble the phenotype of rad50S, as assayed by ph ase-contrast microscopy for spore formation, physical and genetic anal ysis of recombination, fluorescence in situ hybridization to quantify homologous pairing and immunofluorescence and electron microscopy to d etermine the capability to synapse axial elements.