Trans-acting factors and properly positioned DNA elements repress mating-type genes in fission yeast

Repression of the mating-type P genes at the silent mat2-P locus in fission yeast is dependent on four cis-acting DNA elements, two on each side of the coding sequences. The mechanism by which these elements exert their influence on the mating-type promoter is studied here by insertion of a bacterial antibiotic resistance gene at several positions in the silent region. The behavior of the resistance gene itself, and the changes its insertion causes in mating-type expression, reveal that the repressive elements have a limited range of action and that the four elements have unequal effects on gene expression. Repression of the antibiotic resistance gene inside the silent region leads to an antibiotic-sensitive phenotype and facilitates the selection of resistant mutants. These mutants can de-repress the resistance gene at other positions than the one used for their selection. Strong antibiotic resistance correlates with derepression of the plasmid-borne mating-type cassette. These data argue that mat2-P repression is dependent on trans-acting factors and the positioning of the repressive DNA elements, but less dependent on the nature of the affected promoter.