Three additional linkage groups that repress transcription and meiotic recombination in the mating-type region of Schizosaccharomyces pombe.

The mating-type genes of Schizosaccharomyces pombe are found at three locations in the same chromosomal region. These genes are in an active configuration at the mat1 locus and in an inactive configuration at the mat2 and mat3 loci. The mechanism that represses transcription of mat2 and mat3 also inactivates other promoters introduced nearby and is accompanied by a block to meiotic recombination in the mat2-mat3 interval, suggesting that this mechanism involves a particular chromatin structure. We present evidence that the transcription and recombination blocks require three newly defined trans-acting loci, clr2, clr3 and clr4, in addition to the previously identified clr1, rik1 and swi6 loci. We also investigated the role of mat2 cis-acting sequences in silencing. Four cis-acting elements that repress mat2 in a plasmid context were previously identified. Deletion of two of these elements proved to have little effect in a chromosomal context. However, when combined with mutations in trans-acting genes, deletion of the same two elements greatly enhanced mat2 expression. The observed cumulative effects suggest a redundancy in the silencing mechanism.