A MUTANT ALLELE OF THE SUP45 (SAL4) GENE OF SACCHAROMYCES-CEREVISIAE SHOWS TEMPERATURE-DEPENDENT ALLOSUPPRESSOR AND OMNIPOTENT SUPPRESSOR PHENOTYPES

Using a plasmid-based termination-readthrough assay, the sal4-2 conditional-lethal (temperature-sensitive) allele of the SUP45 (SALA) gene was shown to enhance the efficiency of the weak ochre suppressor tRNA SUQ5 some 10-fold at 30 degrees C. Additionally, this allele increased the suppressor efficiency of SRM2-2, a weak tRNA(Gln) ochre suppressor, indicating that the allosuppres sor phenotype is not SUQ5-specific. A sup(+) sal4-2 strain also showed a temperature-dependent omnipotent suppressor phenotype, enhancing readthrough of all three termination codons. Combining the sal4-2 allele with an efficient tRNA nonsense suppressor (SUP4) increased the temperature-sensitivity of that strain, indicating that enhanced nonsense suppressor levels contribute to the conditional-lethality conferred by the sal4-2 allele. However, UGA suppression levels in a sup(+) sal4-2 strain following a shift to the non-permissive temperature reached a maximum significantly below that exhibited by a non-temperature sensitive SUP4 suppressor strain. Enhanced nonsense suppression may not therefore be the primary cause of the conditional-lethality of this allele. These data indicate a role for Sup45p in translation termination, and possibly in an additional, as yet unidentified, cellular process.