The mating type in fission yeast is switched independently of its expression.

The mating type of fission yeast is determined by the mat1 locus on chromosome II. The sequence content of this locus, and hence the mating type, is switched in a strictly regular pattern by transposition from one of two unexpressed mating type sequences. The expressed and the two silent sequences are located on the same chromosome. It is not understood how one of the two donor sequences is selected in this reaction. Here I test the possibility that the selection is governed by gene expression from the mat1 locus. Such a mechanism could favor transposition of a donor sequence of opposite mating type to the one present at mat1. Alternatively it could disfavor transposition of a synonymous sequence. The present data argue strongly against any type of participation of mat1 gene products in the choice of donor during the mating type switch. Alternative steering mechanisms are discussed.