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.

The two mating type loci MATa and MAT alpha each produce two mRNAs that are transcribed in opposite and diverging directions from central promoters. Silent copies of MATa (HMRa) and MAT alpha (HML alpha) contain identical DNA sequences throughout the transcribed region, yet are not transcribed. It is concluded that sequences to the left of HMRa (and probably HML alpha) must somehow affect transcription initiated at the centre of each locus 700 to 1,400 base pairs away. A possible mechanism for this position effect is discussed.

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