Coding properties of an ochre-suppressing derivative of Escherichia coli tRNA_I^Tyr

The ability of an ochre-suppressing derivative of Escherichia coli tRNA^Tyr_I to suppress amber and ochre mutations in the same codon (homotopic mutations) at several sites in three different genes (E. coli β-galactosidase, phage T4 rIIB cistron and T4 lysozyme) was tested. This suppressor transfer RNA nearly always suppresses the amber mutation better than the ochre mutation. The ratio of ochre codon to amber codon suppression efficiency in homotopic pairs is different at the different sites tested. Thus, reading context must play a role in suppression. However, the influence of context in the genes tested is different on suppressor tRNAs other than su⁺_oc-A2 indicating that parts of tRNAs other than the anticodon sequence UUA must also participate in the suppression process. At some sites nucleotides adjacent to the nonsense codon influenced suppression in a systematic fashion but no general mechanism to explain these effects is yet apparent.