Ekwall K, Ruusala T
Department of Molecular Biology, University of Uppsala Biomedical Center, Sweden.
Genetics. 1994 Jan;136(1):53-64. doi: 10.1093/genetics/136.1.53.
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.
在粟酒裂殖酵母中，交配型信息存储在两个转录沉默位点（mat2和mat3）。这两个位点之间的区域（K区域）在减数分裂交叉中是无活性的。交配型基因（M或P）仅当存在于第三个活性位点（mat1）时才会表达。我们之前已经表明，P基因的位置调控基于侧翼DNA序列中的元件导致的沉默位点的抑制。在本研究中，我们对一个缺失mat1的不育菌株进行了诱变，并筛选出能够接合的细胞。这种类型的隐性突变应该定义编码参与沉默交配型位点抑制的反式作用因子的基因。在这项工作之前，已经表明两个基因clr1和swi6中的突变会导致沉默位点的表达以及K区域的重组。通过分离使一个或两个沉默位点去抑制（M交配或双交配）的新突变，证明了当前筛选的敏感性。M交配突变体的频率几乎比双交配突变体高两个数量级，并且在所有分析的突变体中，mat3-M的表达明显高于mat2-P的表达。这些突变定义了三个新基因clr2、clr3和clr4。此外，我们表明先前已知允许K区域重组的rik1突变体也会抑制沉默位点。
