The majority of the CTG codons present in the Saccharomyces cerevisiae genome are encoded by TTG and TTA codons in theCandida albicans genome. (A) To quantify the relative conservation of amino acids and codons between C. albicans andS. cerevisiae, the two genomes were aligned using BLASTP at anE-value of 10−5. For each S. cerevisiaeamino acid, the corresponding codons present at homologous positions inC. albicans orthologs were identified, thus providing overall information about amino acid and codon conservation between the two species. To elucidate the mutational pattern of the CTG codon between the two species, its frequency of conversion at each position of the alignment was computed independently of the other leucine codons (thick red line in the graph). As would be expected, the major trend is residue conservation between the two species at each position for each respective codon family. For the CTG and the other leucine codons, two important trends are observed: first, their conversion into leucine TTG and TTA codons and also into conserved amino acids of leucine (Ile, Met, and Phe); second, the avoidance of nonconserved leucine, namely, serine codons. As observed in Figures 6 and 7, the preferential conversion of CTN codons into TTG does not follow the rules imposed by increased AT pressure in C. albicans as TTG is a G-ending codon, thus indicating that translational selection is a strong driving force in the evolution of the CTN codons in C. albicans. (B) Magnification of the previous graph in the regions of serine and leucine codons (boxes A and B, respectively).
