YEAST UAA SUPPRESSORS EFFECTIVE IN PSI+-STRAINS SERINE-INSERTING SUPPRESSORS

Over 200 revertants that suppressed three or more UAA markers were isolated in a haploid strain of yeast, Saccharomyces cerevisiae, containing the ψ+ cytoplasmic determinant which increases the efficiency of action of certain suppressors. These revertants were grouped into classes on the basis of suppression of four nutritional markers and the canavanine-resistant marker can1-100, and on the basis of the efficiency of suppression of the cyc1-72 marker which contains a defined UAA mutant codon corresponding to position 06 in iso-1-cytochrome c. Genetic analysis and other tests indicated that 40% of the suppressors were highly efficient and were allelic to one or another of the known tyrosine-inserting suppressors, that 59% of the suppressors were moderately efficient and were allelic to either the previously known serine-inserting suppressor SUP16 or to the newly discovered serine-inserting suppressor SUP17, and that 1% of the suppressors were inefficient and were allelic to the newly discovered SUP26 suppressor. The SUP16 suppressors were shown to be allelic to the previously characterized suppressor SUQ5 whose locus is on the right arm of chromosome XVI. This location and the pattern of suppression suggests that the SUP16 locus may be identical to the previously described SUP15 locus. Genetic analysis established that the newly discovered SUP17 locus is on the left arm of chromosome IX, between the his6 and lys11 markers. The examination of four different strains revealed that the SUP16 and SUP17 suppressors cause insertion of serine in iso-1-cytochrome c at the UAA site of the cyc1-72 mutant. It is suggested that the gene products of the SUP16 and SUP17 loci are redundant forms of the same serine transfer RNA. Because viable haploid strains containing both suppressors were obtainable, it was concluded that SUP16 and SUP17 could not be the sole genes coding for the only UCA-decoding species of serine tRNA. © 1979.