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 condit ional-lethal (temperature-sensitive) allele of the SUP45 (SALA) gene w as shown to enhance the efficiency of the weak ochre suppressor tRNA S UQ5 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 c odons. Combining the sal4-2 allele with an efficient tRNA nonsense sup pressor (SUP4) increased the temperature-sensitivity of that strain, i ndicating that enhanced nonsense suppressor levels contribute to the c onditional-lethality conferred by the sal4-2 allele. However, UGA supp ression levels in a sup(+) sal4-2 strain following a shift to the non- permissive temperature reached a maximum significantly below that exhi bited by a non-temperature sensitive SUP4 suppressor strain. Enhanced nonsense suppression may not therefore be the primary cause of the con ditional-lethality of this allele. These data indicate a role for Sup4 5p in translation termination, and possibly in an additional, as yet u nidentified, cellular process.