Mating‐induced mating‐type cassette conversion in Saccharomyces cerevisiae

We have previously reported that the HMR a‐bearing restriction fragment of a ρ° sir4‐11 strain ( HML α‐ MAT α‐ HMR a), which acts as an α‐mater because of being ρ°, changes its electrophoretic mobility when the strain mates with a certain group of a‐mating strains ( HML α‐ MAT a‐ HMR a). In this study, we found that the sir4‐11 strain being ρ° was not essential for this phenomenon and also that the altered form of the fragment contained HMR α in place of HMR a. Furthermore, we observed conversion of HML a to HML α in the cross in which a sir4‐11 HML a‐ MAT α‐ HMR α strain was mated with a representative of the above‐mentioned a‐mating strain. In addition, when this a‐mating strain was mated with a SIR + HML α‐ MAT a‐ HMR α strain, the resultant diploid was found to be HML α/ HML α MAT a/ MAT α HMR a/ HMR α, indicating that conversion of MAT a to MAT α had taken place in the course of mating. From all these observations, we conclude that there is a group of S. cerevisiae strains that carries factor(s) that induces conversion of a mating‐type cassette of the mating partner to α mating‐type cassette and that this mating type cassette conversion takes place in all three mating type loci, HML, MAT and HMR , if the loci are in the non‐silenced condition. Copyright © 2004 John Wiley & Sons, Ltd.