Control of yeast cell types by mobile genes: a test

The yeast Saccharomyces cerevisiae changes cell types by switching the alleles of the mating type locus (MAT) from a to alpha and vice versa. In the cassette model, these switches, --e.g, from a to alpha--occur when a replica of silent alpha information (an alpha "cassette") replaces the resident a cassette at the mating type locus and is thereby expressed. We have identified a mutation in the locus postulated to be the silent alpha information (HMLalpha) and find that a mutation is introduced into the mating type locus as a result of interconversion: HMLalpha(-) MATalpha cells switch to MATa and then to MATalpha(-). The MATalpha(-) mutation leads to defective mating and behaves like some previously identified MATalpha(-) mutations. These observations satisfy the prediction of the cassette and controlling element models that genetic information is transmitted from HMLalpha to the mating type locus.