Physiological levels of normal tRNA(CAGGln) can effect partial suppression of amber mutations in the yeast Saccharomyces cerevisiae.

A number of ciliated protozoa are known to read the stop codons UAA and UAG as sense codons that specify glutamine during protein synthesis. In considering evolutionary mechanisms for this curious divergence from the standard genetic code, we propose the existence of progenitor tRNAs for glutamine that can weakly suppress UAA and UAG codons. It has been previously shown that multicopy plasmids that overexpress normal tRNA(CAAGln) and tRNA(CAGGln) genes from the yeast Saccharomyces cerevisiae can partially suppress a number of yeast ochre and amber mutations, respectively. In the present study we show that the tRNA(CAGGln) gene can also function as a weak amber suppressor when expressed in cells at physiological levels. This observation is consistent with a role of tRNA(CAGGln) as an evolutionary progenitor of tRNAs that strongly decode UAG codons.