Mutations in rik1, clr2, clr3 and clr4 genes asymmetrically derepress the silent mating-type loci in fission yeast.
In Schizosaccharomyces pombe the mating-type information is stored at two transcriptionally silent loci (mat2 and mat3). The region between these sites (K region) is inert for meiotic crossing over. The mating-type genes (M or P) are expressed only when present at a third, active locus (mat1). We have earlier shown that the positional regulation of P genes is based on repression at the silent site, caused by elements in the flanking DNA sequences. In this study we have mutagenized a sterile mat1 deleted strain and selected for cells that are able to conjugate. Recessive mutations of this type should define genes encoding trans-acting factors involved in repression of the silent mating-type loci. Before this work mutations in two genes, clr1 and swi6, had been shown to allow both expression of the silent loci and recombination in the K region. The sensitivity of the present selection is demonstrated by the isolation of new mutations that derepress one or both of the silent loci (M-mating or bi-mating). The frequency of M-mating mutants was almost two orders of magnitude higher than that of bi-mating mutants and in all mutants analyzed mat3-M expression was significantly higher than mat2-P expression. The mutations define three new genes, clr2, clr3 and clr4. In addition we show that the rik1 mutant previously known to allow recombination in the K region also depresses the silent loci.
Ekwall
K
K
Department of Molecular Biology, University of Uppsala Biomedical Center, Sweden.
Ruusala
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T
D000483:Alleles; D001483:Base Sequence; D002874:Chromosome Mapping; D015825:Chromosomes, Fungal; D003227:Conjugation, Genetic; D017931:DNA Primers; D018076:DNA, Complementary / Q000032:analysis; D005799:Genes, Dominant; D005800:Genes, Fungal; D049770:Genes, Mating Type, Fungal; D005808:Genes, Recessive; D005816:Genetic Complementation Test; D005838:Genotype; D008969:Molecular Sequence Data; D016296:Mutagenesis; D016133:Polymerase Chain Reaction; D012568:Schizosaccharomyces / Q000235:genetics*; D014158:Transcription, Genetic
D017931:DNA Primers; D018076:DNA, Complementary