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Genetic studies have previously demonstrated that the RED1 gene of Saccharomyces cerevisiae is required for chromosome segregation at the first meiotic division. Northern blot hybridization analysis indicates that the RED1 gene produces two transcripts of 2.75 and 3.2 kilobases. The major 2.75 kb transcript is not present in mitotic cells and is meiotically induced to accumulate maximally just prior to the meiosis I division. The DNA sequence of the RED1 gene was determined and used to predict the amino acid sequence of the encoded gene product. The RED1 protein is 827 amino acids in length and has a molecular weight of 95.5 kilodaltons. There is no significant homology between the RED1 amino acid sequence and other known protein sequences, including those encoded by genes essential for meiosis.

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The synaptonemal complex (SC) is a meiosis-specific proteinaceous structure that holds homologous chromosomes close together along their length during the pachytene stage of meiotic prophase. The SC is observed in sexually reproducing fungi, plants and animals and is highly conserved at the cytological level. Despite this striking conservation of structure, however, the known protein components of the SC do not appear to be highly conserved across species. In Saccharomyces cerevisiae, the products of the RED1 and HOP1 genes are associated with the lateral elements of the SC. Using a functional complementation strategy, we have isolated homologs of these genes from the related yeast, Kluyveromyces lactis. The predicted K. lactis Red1 protein is 26% identical to the S. cerevisiae Red1 protein, and the K. lactis Hop1 protein is 40% identical to the S. cerevisiae Hop1 protein. The K. lactis RED1 gene fully complements the S. cerevisiae red1 mutant, both when overexpressed and when present in two copies in a diploid. However, the K. lactis HOP1 gene complements a hop1 mutant poorly when overproduced and not at all when present in two copies in a diploid. Unlike the S. cerevisiae RED1 gene, the K. lactis RED1 contains an intron; the transcript of the K. lactis gene is efficiently spliced during meiosis in S. cerevisiae.