Reversion analysis of [<i>psi</i>⁻] mutations in<i>Saccharomyces cerevisiae</i>

Abstract

SUMMARY Mutants of [ psi ] a cytoplasmically inherited factor in the yeast Saccharomyces cerevisiae were isolated after treatment with a variety of agents including conventional mutagens and a number of compounds which cause loss of [ psi ] at high frequencies, namely methanol, KCl, dimethyl sulphoxide and guanidine hydrochloride. In [ psi − ] mutants the suppressor SUQ5 does not suppress ochre mutations such as ade.2.1 . Reversion analysis of the [ psi − ] mutants revealed three classes: (1) a class of agents producing [ psi − ] mutations which could readily revert to [ psi + ] (methanol, KCl and dimethyl sulphoxide belong to this class), (2) those which could not be shown to revert (GuHCl) and (3) the conventional mutagens which produced both revertible and apparently non-revertible [ psi − ] mutations. We conclude that GuHCl causes a deletion or loss of the [ psi ] factor. Methanol may cause an alteration of ‘state’ for example, of a promoter, and KCl may be selecting or inducing low copy number variants of [ psi + ] strains. It is possible that DMSO may be involved in regulation of [ psi ].