Seryl-tRNA synthetase is not responsible for the evolution of CUG codon reassignment in Candida albicans

A number of Candida species translate the standard leucine-CUG codon as serine using a novel ser-tRNA(CAG). This tRNA, which has an unusual anticodon stem-loop structure, has been implicated in the evolution of this codon reassignment. However, such a sense codon reassignment might also require a change in the specificity of the cognate aminoacyl tRNA-synthetase, in this case the ser-tRNA synthetase. Here we describe the cloning and sequence analysis of the C. albicans seryl aminoacyl-tRNA synthetase (CaSerRS) gene (CaSES1). The predicted CaSerRS sequence shows a significant level of amino acid identity to SerRs from other organisms and fully complements a S. cerevisiae SerRS null strain without any apparent defect in growth rate. This suggests that the SerRS recognizes and charges S. cerevisiae ser-tRNAs with similar efficiency to that of the S. cerevisiae SerRS. Using an antibody raised against CaSerRS, we also demonstrate the presence of SerRS in a range of Candida spp. showing CUG codon reassignment. We conclude that the key element in CUG reassigment in Candida spp. is the tRNA that decodes the CUG codon rather than a SerRS structural change. The nucleotide sequence of the CaSES1 gene has been deposited at GenBank under Accession No. AF290915.