The Candida albicans gene encoding the cytoplasmic leucyl-tRNA synthetase: implications for the evolution of CUG codon reassignment.

In a number of Candida species the 'universal' leucine codon CUG is decoded as serine. To help understand the evolution of such a codon reassignment we have analyzed the Candida albicans leucyl-tRNA synthetase (CaLeuRS) gene (CaCDC60). The predicted CaLeuRS sequence shows a significant level of amino acid identity to LeuRS from other organisms. A mitochondrial LeuRS (ScNAM2) homologue, which shared low identity with the CaLeuRS, was also identified in C. albicans. Antigenically-related LeuRSs were identified in a range of Candida species decoding the CUG codon as both serine and leucine, using an antibody raised against the N-terminal 15 amino acids of the CaLeuRS. Complementation experiments demonstrated that the CaLeuRS was able to functionally complement a Saccharomyces cerevisiae cdc60::kanMX null mutation. We conclude that there is no alteration in tRNA recognition and aminoacylation by the C. albicans LeuRS, which argues against it having a role in codon reassignment. The nucleotide sequences of the CaCDC60 and CaNAM2 genes were deposited at GenBank under Accession numbers AF293346 and AF352020, respectively.