Non-universal usage of the leucine CUG codon and the molecular phylogeny of the genus Candida.

CUG, a universal leucine codon, was reported to be read as serine in 10 species of the genus Candida. We used an in vitro cell-free translation system to identify the amino acid assignment of codon CUG in 78 species and 7 varieties of galactose-lacking Candida species equipped with Q9 as the major ubiquinone. Of these, only 11 species used codon CUG as a leucine codon. The remaining species decoded CUG as serine. Their small subunit ribosomal DNA sequences were also determined and analyzed using both Neighbor-Joining and Maximum Likelihood methods. The species decoding CUG as serine and leucine formed distinct clusters on both molecular phylogenetic trees. Our result suggests that non-universal decoding is not a rare event, and that it is widely distributed in the genus Candida.