Normal yeast tRNA_CAG^Gln can suppress amber codons and is encoded by an essential gene

We have isolated a gene that can encode yeast tRNA_CAG^Gln. When present on a multicopy plasmid, this gene suppresses the phenotype of a number of amber mutants, but has no effect on the ocher mutants tested. We therefore conclude that the anticodon CUG in tRNA_CAG^Gln decode the amber codon UAG by G · U mispairing, possibly by wobble base-pairing in the first codon position. This represents the second example we have observed in this laboratory of nonsense suppression in yeast by natural tRNA^Gln, involving G · U mispairing in the first codon position. Replacing the genomic copy of the cloned gene with a disrupted tRNA gene results in recessive lethality in heterozygous diploids and is lethal to haploid cells. This lethality can be rescued by transformation of cells with a single copy plasmid containing the tRNA_CAG^Gln gene. Thus, the gene encoding tRNA_CAG^Gln is apparently essential for viability in yeast, suggesting that it is normally present as a single copy gene.