Multiple gene loci for a single species of glycine transfer ribonucleic acid

Abstract

The study of suppressors of tryptophan synthase A protein missense mutations in Escherichia coli has led to the establishment of two nonadjacent genetic loci (gly V and gly W) specifying identical nucleotide sequences for a single isoaccepting species of glycine transfer ribonucleic acid (tRNA (Gly 3) (GGU/C). In one case, suppression of the missense mutation trpA78 was due to a mutation in a structural gene (gly W) for tRNA (Gly 3) (GGU/C). This mutation resulted in a base change in the anticodon and modification of an A residue adjacent to the 3' side of the anticodon, leading to the production of a tRNA (Gly 3) (UGU/C) species. The resulting gly W51 (Su(UGU/C)) allele was mapped by interrupted mating and was located at approximately 37 min on the E. coli genetic map. Other suppressor mutations affecting the primary sequence of tRNA (Gly 3) (GGU/C) and giving rise to the Ins and Su+(A58) phenotypes were positioned at 86 min (gly V). Several independently arising missense suppressor mutations resulting in the Su+(A58) and Su+(A78) phenotypes were isolated and mapped at these two genetic loci (gly V and gly W). The ratio of appearance of suppressor mutations at gly V and gly W suggest that there are three or four tRNA (Gly 3) (GGU/C) structural gene copies at the gly V locus to one copy at the gly W locus. Structural genes for tRNA(ly) isoacceptors are now known at four distinct locations on the E. coli chromosome: gly T (77 min), tRNA(Gly 2)(GGA/G); gly U (55 min), tRNA(Gly 1-); and gly V (86 min) and gly W (37 min), tRNA(Gly 3)(GGU/C).