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. 1994 Jan;136(1):53–64. doi: 10.1093/genetics/136.1.53

Mutations in Rik1, Clr2, Clr3 and Clr4 Genes Asymmetrically Derepress the Silent Mating-Type Loci in Fission Yeast

K Ekwall 1, T Ruusala 1
PMCID: PMC1205792  PMID: 8138176

Abstract

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 derepresses the silent loci.

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Selected References

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