Systematic alterations in the anticodon arm make tRNA(Glu)-Suoc a more efficient suppressor.

Using site-specific mutagenesis, we constructed five more efficient variants of tRNA(Glu)-Suoc, an extremely inefficient ochre suppressor. Each variant has an extended anticodon, or region of the anticodon arm, which is more similar to that found in normal tRNAs which translate codons Uxx. Suppressor efficiency invariably increases with similarity of the extended anticodon to that of a normal Uxx-translating tRNA. Altered nucleotides in both helix and loop strongly affect efficiency, with no position dependence and no significant interaction between substitutions. The variant with all substitutions is 230-fold more efficient (in one context) than the parental tRNA(Glu)-Suoc. Two other unexpected variants seem to be 'context mutants', having altered response to message context.