Translation of synonymous codons in family boxes by Mycoplasma capricolum tRNAs with unmodified uridine or adenosine at the first anticodon position.

In Myocoplasma capricolum, codon family boxes except for arginine and threonine (CUN leucine, GUN valine, UCN serine, CCN proline, GCN alanine, and GGN glycine) have only a single tRNA species with the anticodon sequence UNN (N is U, C, A or G), the first nucleoside U being unmodified. Incorporation of the [3H]amino acid into the peptide fraction was examined with the M. capricolum cell-free translation system. Synthetic mRNA containing each of the respective amino acid codons in the coding frame was subjected to translation. The tRNAUNN translated all the family box codons with similar, if not equal, efficiencies. In M. capricolum, there are two species of threonine tRNA, tRNA(UGUThr) and tRNA(AGUThr), the first nucleoside U or A being unmodified. The tRNA(UGUThr) species translates codons ACA, ACG and ACU efficiently, and ACC only poorly. In contrast, the tRNA(AGUThr) species translates codons ACU, ACC and ACG efficiently and ACA poorly.