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Abstract
A search for yeast mutants defective in meiotic recombination has been carried out. The spo13 starting strains are disomic for chromosome III, carry mutant heteroalleles at HIS4 and LEU2, and are heterozygous at MAT. These strains were mutagenized with UV light or EMS and the survivors were screened for reduced meiotic frequencies of His$\sp+$ and Leu$\sp+$ prototrophs. Seven sporulation-competent isolates that fail to exhibit wild-type levels of meiotic gene conversion have been identified. Two alleles of one recombination-defective mutant, designated mei4-1, were isolated and represent mutations in a previously unidentified gene. The MEI4 gene has been cloned from a yeast genomic library by complementation of the recombination defect. Strains carrying a deletion/insertion mutation of the MEI4 gene exhibit decreased sporulation and produce inviable spores. The spore inviability phenotype is rescued by a spo13 mutation, which causes cells to bypass the meiosis I division. mei4 mutants display no meiotic gene conversion but normal mitotic conversion frequencies. Both meiotic interchromosomal and intrachromosomal crossing over are completely abolished in mei4 strains. The mei4 mutation is able to rescue the spore-inviability phenotype of spo13 rad52 strains (i.e., mei4 spo13 rad52 mutants produce viable spores), indicating that MEI4 acts before RAD52 in the meiotic recombination pathway. DNA sequence analysis predicts that the MEI4 gene encodes a 450 amino acid protein weighing 52.9 kDa. The predicted protein is not significantly similar to proteins in the GenBank data base. The MEI4 message is spliced, and splicing is not dependent on MER1, a gene shown to be involved in splicing the message of another meiotic gene, MER2. MEI4 expression is meiosis-specific and occurs in a pattern similar to RED1, a gene expressed prior to the first meiotic division when chromosome pairing and high levels of recombination are occurring.
