The a2 protein, the product of the MATa2 cistron, represses various genes specific to the a mating type (a2 repression), and when combined with the MATal gene product, it represses MATaJ and various haploidspecific genes (al-a2 repression). One target of al-a2 repression is RME1, which is a negative regulator of a/a-specific genes. We have isolated 13 recessive mutants whose al-a2 repression is defective but which retain a2 repression in a genetic background of ho MATa HMLa HMRa sir3 or ho MATa HMRa HMRa sir3. These mutations can be divided into three different classes. One class contains a missense mutation, designated hmla2-102, in the a2 cistron of HML, and another class contains two missense mutations, mata2-201 and mata2-202, in the MATa locus. These three mutants each have an amino acid substitution of tyrosine or phenylalanine for cysteine at the 33rd codon from the translation initiation codon in the a2 cistron of HMLa or MATa. The remaining 10 mutants make up the third class and form a single complementation group, having mutations designated aarl (al-a2 repression), at a gene other than MAT, HML, HMR, RMEI, or the four SIR genes. Although a diploid cell homozygous for the aarl and sir3 mutations and for the MATa, HMLa, and HMRa alleles showed a mating type, it could sporulate and gave rise to asci containing four a mating-type spores. These facts indicate that the domain for a2 repression is separable from that for al-a2 protein interaction or complex formation in the a2 protein and that an additional regulatory gene, AAR], is associated with the al-a2 repression of the al cistron and haploid-specific genes.Cell types of Saccharomyces cerevisiae are determined by the codominant MATa and MATa alleles at the mating-type locus, MAT, on the right arm of chromosome III (for reviews, see references 16 and 37). A haploid MATa cell has the a mating type and can mate with a cell having the MATa genotype. Conjugation of a and a cells produces an ala diploid cell which has the third cell type, nonmater. The a/a diploid cells undergo meiosis and form asci, and each ascus contains four ascospores bearing a haploid tetrad nucleus. Besides cell-type control, the MAT locus regulates various other genes such as HO for homothallic switching of the MAT locus (19) and the transcription of Ty elements (8,9).In addition to the MAT locus, complete but unexpressed copies of MAT information are encoded at two other loci, HML and HMR, on the same chromosome (16, 37). Homothallic switching of information at the MATa locus to a or MATa to a is achieved by transposition of the a or a cassette from the HMR or HML locus to the MAT locus. In general, HMR encodes the a information and HML has a, but their expression is normally repressed by the function of the SIR genes.A hypothesis concerning the regulatory function of the MAT genes, known as the al-a2 hypothesis (39), proposes that MATa consists of two cistrons, al and a2. The al cistron encodes a positive regulator for a-specific genes, and the a2 cistron encodes a negative regulat...
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