RESUMEN
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.
Asunto(s)
Candida albicans/genética , Leucina-ARNt Ligasa/genética , Secuencia de Aminoácidos , Candida albicans/enzimología , Clonación Molecular , Codón/genética , Citoplasma/enzimología , ADN de Hongos/química , ADN de Hongos/genética , Evolución Molecular , Genes Fúngicos/genética , Código Genético/genética , Prueba de Complementación Genética , Datos de Secuencia Molecular , Saccharomyces cerevisiae/genética , Análisis de Secuencia de ADN , Homología de Secuencia de AminoácidoRESUMEN
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.
