Template activity of modified terminator codons.

Modified trinucleotides, 3MeU-A-G+, 3MeU-A-A, 5MeUA-G, 5MeU-A-A, BrU-A-G, hU-A-G, and U-A-I (see J. BioZ. Chem., 241, 527 (1966)), were synthesized by a combination of organic and enzymatic methods. The activity of each trinucleotide as a template for peptide chain termination was determined. The attachment of a methyl group to the NB position of uracil eliminates the template activity of terminator codons corresponding to release Factors 1 or 2. The attachment of methyl or bromo groups to the Cg position of uracil has little or no effect on the template activity of terminator codons. The trinucleotide U-A-I is an active template with release Factor 1 but not 2. hUA-G is inactive in the presence of release Factor 1. The data suggest that the NI proton and the Cq carboxyl moieties are essential for translation of terminator codons, whereas substituents attached to the Cs position are not essential.