Dominant and recessive informational suppressors of a missense mutation in Coprinus

Allosuppressors in yeast

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.

Allele specific, gene unspecific suppressors in Aspergillus nidulans

Seven suppressor mutations have been isolated in Aspergillus nidulans by coreversion of alleles in physiologically unrelated genes namely, alX, sB, alcA, putative structural genes for allantoinase, sulphate permease and alcohol dehydrogenase respectively. The suppressors are allele specific, gene unspecific. Those described map in four loci, suaA, B, C, D. suaA and suaB are on linkage group III, suaC and suaD on VII. suaB111, suaD103 and suaD108 are semi-dominant in their suppression of alX4 and sB43, suaA101, suaA105 and suaC10. are recessive and have a pleiotropic effect on morphology. SuaC109 is cold sensitive for growth as is sua115, an unmapped mutation on linkage group III which is similar in morphology to suaC109. The two mutations, suaA101 and suaA105 have different spectra of suppression and morphologies. suaA105 weakly suppresses alX4 and sB43 whereas suaA101 strongly suppresses these and alcA125. suaD103 and suaD108 have the same spectrum of suppression. The properties of these suppressors are consistent with their being informational suppressors are consistent with their being informational suppressors of the nonsense type.