Genetic analysis of a gene required for the expression of allele-specific missense suppression in Saccharomyces cerevisiae

Genetic and molecular analysis of the SOE1 gene: a tRNA(3Glu) missense suppressor of yeast cdc8 mutations

The CDC8 gene of Saccharomyces cerevisiae encodes deoxythymidylate (dTMP) kinase and is required for nuclear and mitochondrial DNA replication in both the mitotic and meiotic cell cycles. All cdc8 temperature-sensitive mutants are partially defective in meiotic and mitochondrial functions at the permissive temperature. In a study of revertants of temperature-sensitive cdc8 mutants, the SOE201 and SOE1 mutants were isolated. The SOE201 mutant is a disome of chromosome X to which the cdc8 gene maps. Using the chromosome X aneuploids to vary cdc8 gene dosage, we demonstrate that different levels of dTMP kinase activity are required for mitotic, meiotic or mitochondrial DNA replication. The SOE1 mutant contains a dominant suppressor that suppresses five different cdc8 alleles but does not suppress a complete cdc8 deletion. The SOE1 gene is located less than 1.5 cM from the CYH2 gene on chromosome VII and is adjacent to the TSM437-CYH2 region, with the gene order being SOE1-TSM437-CYH2. SOE1 is an inefficient suppressor that can neither suppress the cdc8 hypomorphic phenotype nor restore dTMP kinase activity in vitro. SOE1 is a single C to T mutation in the anticodon of a tRNA(3Glu) gene and thereby, produces a missense suppressor tRNA capable of recognizing AAA lysine codons. We propose that the resultant lysine to glutamate change stabilizes thermo-labile dTMP kinase molecules in the cell.

Extragenic revertants of rad50, a yeast mutation causing defects in recombination and repair

The RAD50 gene in yeast is required for recombination-repair (i.e., the double strand break repair pathway) in mitosis, and for meiotic recombination and sporulation. Both of these processes are complex and seem likely to require a relatively large number of gene products. In order to help define other genes required for recombination and repair processes in yeast, we have isolated extragenic revertants of rad50-4 which restore the ability to grow in the presence of MMS. Evidence from segregation indicates the extragenic revertants fall into at least five loci. Two of them reduce sporulation and spore viability at high temperature; another mutation confers a spontaneous hyperrec phenotype on mitotic cells. Thus, at least three revertants are candidates for mutations which affect recombination functions.

Genetic analysis of antisuppressor mutants in the fission yeast Schizosaccharomyces pombe

Fourteen unlinked sin genes could be mutated to recessive antisuppressor alleles preventing the expression of suppressors in the fission yeast Schizosaccharomyces pombe. cyh1 alleles, resistant to the ribosomal inhibitor cycloheximide, also have some antisuppressor effect. The genetical and physiological characterization of these mutants is consistent with the hypothesis that they affect components of the messenger RNA translation machinery such as tRNA modifying enzymes or ribosomal proteins.

Genetic evidence of ribosomal antisuppressors in Podospora anserina

Antisuppressors were screened for with the help of informational suppressors in Podospores anserina. Four mutations in the AS1 locus and two in the AS2 locus were isolated, using allele non specific suppressors supposed to be ribosomal ambiguity mutations. Four mutations in the AS3 locus and 45 in the AS4 locus were obtained, using a nonsense (t-RNA like) suppressor. All antisuppressors are partially dominant. Most mutations in the AS4 locus are lethal. The four mutants at the AS3 locus and 6 out of the 8 viable mutants at the AS4 locus are cold sensitive. Phenotypic properties and action spectra of the antisuppressors suggest that they are restrictive ribosomal mutations.