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

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.