REVERSION FROM SUPPRESSION TO NONSUPPRESSION IN SUQ5 [psi +] STRAINS OF YEAST: THE CLASSIFICATION OF MUTATIONS

Reversion from the suppressed to nonsuppressed phenotype in strains of genotype SUQ5 [psi  +] ade2-1 his5-2 lys1-1 can1-100 ura3-1 has been induced by treatment with ethyl methanesulphonate, nitrosoguanidine or UV (254 nm) light. Spontaneously occurring revertants have also been selected by two different methods. Reversion has been shown to occur through a variety of nuclear mutations and through mutation of [psi  +] to [psi  -]. Nuclear mutations included back-mutation of SUQS, antisuppressor mutations that were recessive, semi-dominant or dominant, and dominant or recessive mutations of genes required for the maintenance of the [psi  +] factor. Complementation tests by which the various kinds of mutations could be distin guished from one another were designed. The spectra of spontaneously occurring and induced mutations have been described.