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The mating‐type region of fission yeast consists of three components, mat1, mat2‐P and mat3‐M, each separated by 15 kb. Cell‐type is determined by the alternate allele present at mat1, either P in an h+ or M in an h‐ cell. mat2‐P and mat3‐M serve as donors of information that is transposed to mat1 during a switch of mating type. We have determined the nucleotide sequence of each component of mat. The P and M specific regions are 1104 and 1128 bp, respectively, and bounded by sequences common to each mating‐type cassette (H1; 59 bp and H2; 135 bp). A third sequence is present at mat2‐P and mat3‐M but absent at mat1 (H3; 57 bp), and may be involved in transcriptional repression of these cassettes. mat1‐P and mat1‐M each encode two genes (Pc; 118 amino acids, Pi; 159 amino acids, Mc; 181 amino acids and Mi; 42 amino acids). Introduction of opal or frame‐shift mutations into the open‐reading‐frame of each gene revealed that Pc and Mc are necessary and sufficient for mating and confer an h+ or h‐ mating type respectively. All four genes are required for meiotic competence in an h+/h‐ diploid. The transcription of each mat gene is strongly influenced by nutritional conditions and full induction was observed only in nitrogen‐free medium. The predicted product of the Pi gene contains a region of homology with the homeobox sequence, suggesting that this gene encodes a DNA binding protein that directly regulates the expression of other genes.

Inheritance of the active and inactive states of gene expression by individual cells is crucial for development. In fission yeast, mating-type region consists of three loci called mat1, mat2, and mat3. Transcriptionally silent mat2 and mat3 loci are separated by a 15 kb interval, designated the K-region, and serve as donors of information for transcriptionally active mat1 interconversion. In a strain carrying replacement of 7.5 kb of the K-region with the ura4 gene, we discovered that ura4 silencing and efficiency of mating-type switching were covariegated and were regulated by an epigenetic mechanism. Genetic analyses demonstrated that epigenetic states were remarkably stable not only in mitosis but also in meiosis and were linked to the mating-type region. This study indicates that different epigenetic states are heritable forms of chromatin organization at the mat region.

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