A recessive lethal super-suppressor mutation in yeast and other Ψ phenomena

Abstract

1. This paper is an account of the isolation in a diploid strain of yeast of an ochre-mutation suppressor, 5Q2, which is then segregated in meiosis as a recessive lethal. 2. A genetic analysis has been carried out which demonstrates that it is the suppressor phenotype itself which leads to the death of haploid cells carrying the mutation.3. The possibilities that the lethal phenotype is due to a coincident, closely linked mutation, or that both the suppressing and the lethal phenotypes are the result of a deletion affecting two loci have been considered. 4. They have been ruled out by showing that various independently isolated alleles of the suppressor (including, in these strains, the supersuppressor SD, isolated by Hawthorne and Mortimer (1963)) are also recessive lethals, and that a number of independent reversions of the suppressor from ochre-specific to amber-specific suppression were accompanied by a loss of the lethal phenotype. 5. This interpretation is reinforced by the observation that the lethal phenotype can be lost without affecting the suppressor phenotype as a result of a modifying mutation. 6. The modifier is a cytoplasmically inherited determinent, Ψ+ The suppressor SQ2 is lethal in haploid Ψ− strains, but not so in Ψ strains. The determinant has previously been identified by the fact that it modifies the activity of another ochre-specific super-suppressor, SQ5, though in that case, it is the suppressing phenotype which is altered; in Ψ− strains ochre mutations are not suppressed by SQ5, while they are in Ψ+ strains. 7. The interpretation of these results takes into account certain other effects of the 1s determinant on ochre-specific super-suppressors. It is that all the effects, lethality and suppressor activity, can be related to the efficiency of suppression, that is the frequency with which ochre codons are misread and chain termination prevented. When this efficiency is too great, normal protein synthesis is seriously affected, and lethality results. When it is too small, no suppression is observed. 8. The phenomena reported would result if the rate of ochre-misreading were higher in Ψ+ strains than in Ψ− strains. © 1971, The Genetical Society of Great Britain. All rights reserved.