Case for the genetic code as a triplet of triplets.

The efficiency of codon translation in vivo is controlled by many factors, including codon context. At a site early in the Salmonella flgM gene, the effects on translation of replacing codons Thr6 and Pro8 of flgM with synonymous alternates produced a 600-fold range in FlgM activity. Synonymous changes at Thr6 and Leu9 resulted in a twofold range in FlgM activity. The level of FlgM activity produced by any codon arrangement was directly proportional to the degree of in vivo ribosome stalling at synonymous codons. Synonymous codon suppressors that corrected the effect of a translation-defective synonymous flgM allele were restricted to two codons flanking the translation-defective codon. The various codon arrangements had no apparent effects on flgM mRNA stability or predicted mRNA secondary structures. Our data suggest that efficient mRNA translation is determined by a triplet-of-triplet genetic code. That is, the efficiency of translating a particular codon is influenced by the nature of the immediately adjacent flanking codons. A model explains these codon-context effects by suggesting that codon recognition by elongation factor-bound aminoacyl-tRNA is initiated by hydrogen bond interactions between the first two nucleotides of the codon and anticodon and then is stabilized by base-stacking energy over three successive codons.