Antisuppression of class I suppressors in an isopentenylated-transfer RNA deficient mutant of Saccharomyces cerevisiae.

The effect of a previously isolated antisuppressor mutation from bakers' yeast, that reduced the efficiency of the tyrosine-inserting ochre suppressor, SUP7-o, on other tyrosine-inserting ochre suppressors has been determined. As expected, the antisuppressor mutation, mod5-1, restricted the capacity of all eight tyrosine-inserting ochre suppressors to suppress nonsense mutations. Based on the suppression of five ochre alleles in the presence of mod5, the eight class I suppressors can be grouped into three subclasses. The most efficient subclass had only one member, SUP4-o. Members of the second group included SUP2-o, SUP3-o, SUP7-o, and SUP8-o. The third and least efficient subclass included SUP5-o, SUP6-o, and SUP1 1-o. These differences in efficiencies are a function of the relative expression of the eight genes encoding tRNA(TYR).