Frameshift suppressor mutations affecting the major glycine transfer RNAs of Saccharomyces cerevisiae

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Mutations have been identified in Saccharomyces cerevisiae glycine tRNA genes that result in suppression of +1 frameshift mutations in glycine codons. Wild-type and suppressor alleles of genes encoding the two major glycine tRNAs, tRNA GCC and tRNA UCC, were examined in this study. The genes were identified by genetic complementation and by hybridization to a yeast genomic library using purified tRNA probes. tRNA UCC is encoded by three genes, whereas approximately 15 genes encode tRNA GCC. The frameshift suppressor genes suf1 +, suf4 + and suf6 + were shown to encode the wild-type tRNA UCC tRNA. The suf1 + and suf4 + genes were identical in DNA sequence, whereas the suf6 + gene, whose DNA sequence was not determined, was shown by a hybridization experiment to encode tRNA UCC. The ultraviolet light-induced SUF1−1 and spontaneous SUF4−1 suppressor mutations were each shown to differ from wild-type at two positions in the anticodon, including a +1 base-pair insertion and a base-pair substitution. These changes resulted in a CCCC four-base anticodon rather than the CCU three-base anticodon found in wild-type. The RNA sequence of tRNA UCC was shown to contain a modified uridine in the wobble position. Mutant tRNA CCCC isolated from a SUF1−1 strain lacked this modification. Three unlinked genes that encode wild-type tRNA GCC, suf20 +, trn2, and suf17 +, were identical in DNA sequence to the previously described suf16 + frameshift suppressor gene. Spontaneous suppressor mutations at the SUF20 and SUF17 loci were analyzed. The SUF20−2 suppressor allele contained a CCCC anticodon. This allele was derived in two serial selections through two independent mutational events, a +1 base insertion and a base substitution in the anticodon. Presumably, the original suppressor allele, SUF20−1, contained the single base insertion. The SUF17−1 suppressor allele also contained a CCCC anticodon resulting from two mutations, a +1 insertion and a base substitution. However, this allele contained an additional base sibstitution at position 33 adjacent to the 5′ side of the four-base anticodon. The possible origin and significance of multiple mutations leading to frameshift suppression is discussed.