Abstract
Abstract Suppression ( su A78 + ) of the trpA78 missense mutation (Gly,GGU/C → Cys,UGU/C) in Escherichia coli involves a genetically altered tRNA Gly isoacceptor. Purification and sequence analysis of 32 P-labeled suppressor tRNA indicates that the su A78 + mutation results in the alteration of a small fraction of the tRNA GGU/C Gly3 of the cell. The resulting tRNA UGU/C Gly3 contains a C → A substitution at the 3′ end (C36) of the anticodon (GCC) and in addition, a modification of the adjacent A(A37) to form N 6 -( Δ 2 -isopentenyl)-2-thiomethyl-adenylic acid. Labeled glycyl-tRNA UGU/C Gly3 binds to E. coli ribosomes in the presence of the cysteine triplets, UpGpU and UpGpC, but not with the glycine triplets, GpGpU and GpGpC. The suppressor tRNA UGU/C Gly3 glycylates relatively slowly in the presence of the glycyl-tRNA synthetase, with a V max value 200-fold lower than that of wild-type tRNA GGU/C Gly3 . Multiple identical copies of genes specifying the sequence of tRNA GGU/C Gly3 apparently occur on the E. coli chromosome, since suppressor mutations alter the nucleotide sequence of only a fraction of the tRNA GGU/C Gly3 population.
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