Allosuppressors in yeast

Abstract

SUMMARY: Five loci have been identified inSaccharomyces cerevisiaewhose function reduces suppressor activity in strains carrying ochre super-suppressor mutations. Recessive mutations which allow an increased level of suppression occur at these loci. In such mutants, termed allosuppressors, the serine-inserting suppressorSUPQ5suppresses ochre mutations in a [psi⁻] background and Class I tyrosine-inserting suppressors are lethal or have a reduced viability. Mutations at two allosuppressor loci,sal3 andsal4, have a lethal interaction with one another and with the extrachromosomal determinant [psi⁺]. This interaction is expressed in the absence of any suppressor mutation. All the mutant alleles of one allosuppressor locussal3 are cold sensitive. One allosuppressor mutation,sal4.2, is temperature-sensitive for growth, as well as for other aspects of its phenotypic expression; namely the expression ofSUPQ5and the lethal interactions with Class I super-suppressors, with [psi⁺] and withsal3. At low temperature (24 °C),sal3-sal4.2 double mutants weakly suppress the ochre mutationade2.1, but do not suppresshis5.2 orlys1.1. It is argued that the site of function of the products of these loci is ribosomal and that they are involved in chain termination at UAA codons. It is inferred that the [psi⁺] factor or its product affects protein synthesis by interaction with the ribosome.