A leucyl transfer RNA specified by the amber supressor gene Su6⁺

Abstract

The species of leucyl-tRNA responsible for the amber suppressor activity of the Su6⁺ gene was partially purified (60-fold increase in specific suppressor activity) by chromatography on benzoylated DEAE-cellulose. The homologous species of leucyl-tRNA (without suppressor activity) was similarly purified from an Su6^- strain. Each of the species of tRNA accounts for about 5% of the total leucine-acceptor activity. The binding to ribosomes of leucyl-suppressor tRNA from the Su6⁺ strain is stimulated by the amber codon UAG but not by the leucine codon UUG. The homologous leucyl-tRNA from the Su6^- strain shows the opposite codon specificity, binding to ribosomes with UUG but not with UAG, These results suggest that in the Su6⁺ strain, the anticodon CAA of one species of leucyl-tRNA has been changed to CUA as a result of the suppressor mutation in the Su6 gene. There is another species of leucyl-tRNA which also binds to ribosomes with UUG; this species is not affected by the suppressor mutation and therefore remains available in the Su6⁺ strain for the translation of UUG.