Serine tRNA complementary to the nonuniversal serine codon CUG in Candida cylindracea: evolutionary implications

In the asporogenic yeast Candida cylindracea, the codon CUG is read as serine instead of leucine. This is an unusual instance in which the amino acid assignment of a codon deviates from the universal code. To infer the evolutionary process of this change, the tRNA with the anticodon sequence CAG, which is complementary to and thus responsible for translation of the codon CUG, has been identified. Indeed, this tRNA translates an in-frame CUG codon in a synthetic mRNA as serine in an in vitro translation system. The gene for the tRNA is interrupted by an intron in the anticodon loop. Sequence comparisons of the tRNA and its gene suggest that a single cytidine was inserted into the anticodon loop of the gene for tRNA(Ser)IGA during evolution to produce tRNA(Ser)CAG. The tRNA(Ser)CAG may be produced from its precursor molecule containing the cytidine insertion by splicing.