Frameshift Suppressor Mutations Outside the Anticodon in Yeast Proline tRNAs Containing an Intervening Sequence

Abstract

Extragenic suppressors of +1 frameshift mutations in proline codons map in genes encoding two major proline tRNA isoacceptors. We have shown previously that one isoacceptor encoded by the SUF2 gene (chromosome 3) contains no intervening sequence. SUF2 suppressor mutations result from the base insertion of a G within a 3′-GGA-5′ anticodon, allowing the tRNA to read a 4-base code word. In this communication we describe suppressor mutations in genes encoding a second proline tRNA isoacceptor (wild-type anticodon 3′-GGU-5′) that result in a novel mechanism for translation of a 4-base genetic code word. The genes that encode this isoacceptor include SUF7 (chromosome 13), SUF8 (chromosome 8), trn1 (chromosome 1), and at least two additional unmapped genes, all of which contain an intervening sequence. We show that suppressor mutations in the SUF7 and SUF8 genes result in G-to-U base substitutions at position 39 that disrupted the normal G · C base pairing in the last base pair of the anticodon stem adjacent to the anticodon loop. These anticodon stem mutations might alter the size of the anticodon loop and permit the use of a 3′-GGGU-5′ sequence within the loop to read 4-base proline codons. Uncertainty regarding the exact structure of the mature suppressor tRNAs results from the possibility that anticodon stem mutations might affect sites of intervening sequence removal. The possible role of the intervening sequence in the generation of mature suppressor tRNA is discussed. Besides an analysis of suppressor tRNA genes, we have extended previous observations of the apparent relationship between tRNA genes and repetitive delta sequences found as solo elements or in association with the transposable element TY1. Hybridization studies and a computer analysis of the DNA sequence surrounding the SUF7 gene revealed two incomplete, inverted delta sequences that form a stem and loop structure located 165 base pairs from the 5′ end of the tRNA gene. In addition, sequences beginning 164 base pairs from the 5′ end of the trn1 gene also exhibit partial homology to delta. These observations provide further evidence for a nonrandom association between tRNA genes and delta sequences.