Does UGA suppressor tRNATrp from Escherichia coli have a unique CCA anticodon sequence?
The properties of the UGA-suppressor tRNATrp (anticodon CCA) from Escherichia coli has greatly influenced ideas about the specificity of codon-anticodon interactions showing that the anticodon sequence is not the sole determinant. However, a recent hypothesis for the mechanism of suppression by this tRNA proposes that the base change in position 24, in the dihydrouridine stem, leads to a change in translational specificity of the tRNA by increasing post-transcriptional modification of cytidine 34, in the anticodon wobble position [M. Yarus (1982) Science (Wash. DC) 218, 646-652]. The enzyme postulated to do this normally modifies C34 of a minor isoleucine isoacceptor specific for AUA codons. This modification should reduce reading of G in the third codon position: affinity chromatography on columns containing immobilised tRNAPro (anticodon VGG) has therefore been employed to isolate an enriched population of the putative suppressor species, if such a sub-population exists. The results obtained are difficult to reconcile with the presence of a subfraction, modified post-transcriptionally in C34, responsible for the suppression of UGA. This argues in favour of the previously advanced hypothesis for the mechanism of suppression, which depends on events outside the codon-anticodon interaction itself.