TRNA imbalance promotes -1 frameshifting via near-cognate decoding

tRNA(1)(Gly) is the Escherichia coli glycine tRNA specific for GGG coons. A genetic selection for multicopy suppressors of a frameshift mutation has shown that increased levels of wild-type tRNA(1)(Gly) causes -1 frameshifting. Analysis of the suppression spectrum of this multicopy suppressor and peptide sequencing of the suppressed protein product showed that it promoted GG doublet decoding at the near-cognate GGA codons. It is proposed that increasing the concentration of the GGG-specific tRNA(1)(Gly) relative to the cognate GGA-decoding tRNA(2)(Gly) allows the near-cognate tRNA to read GGA codons. Near-cognate decoding of GGA codons by tRNA(1)(Gly) can occur by a two-out-of-three reading mechanism, in which only the first two bases of the GGA codon are paired with the anticodon, thus permitting doublet translocations. Ln mycoplasmas, a single tRNA typically decodes all four triplets of a codon family and introduction of a feature of the Mypoplasma mycoides tRNA(Gly) responsible for non-discriminate decoding, a C at position 32, into the anticodon E. coli tRNA(1)(Gly), enhanced the efficiency of doublet decoding. (C) 1998 Academic Press Limited.