THE YEAST MER2 GENE IS REQUIRED FOR CHROMOSOME SYNAPSIS AND THE INITIATION OF MEIOTIC RECOMBINATION

Mutation of the MER2 gene of Saccharomyces cerevisiae confers meiotic lethality. To gain insight into the function of the Mer2 protein, we h ave carried out a detailed characterization of the mer2 null mutant. G enetic analysis indicates that mer2 completely eliminates meiotic inte rchromosomal gene conversion and crossing over. In addition, mer2 abol ishes intrachromosomal meiotic recombination, both in the ribosomal DN A array and in an artificial duplication. The results of a physical as say demonstrate that the mer2 mutation prevents the formation of meios is-specific, double-strand breaks, indicating that the Mer2 protein ac ts at or before the initiation of meiotic recombination. Electron micr oscopic analysis reveals that the mer2 mutant makes axial elements, wh ich are precursors to the synaptonemal complex, but homologous chromos omes fail to synapse. Fluorescence in situ hybridization of chromosome -specific DNA probes to spread meiotic chromosomes demonstrates that h omolog alignment is also significantly reduced in the mer2 mutant. Alt hough the MERE gene is transcribed during vegetative growth, deletion or overexpression of the MER2 gene has no apparent effect on mitotic r ecombination or DNA damage repair. We suggest that the primary defect in the mer2 mutant is in the initiation of meiotic genetic exchange.