The evolution of a non universal codon as detected in Candida rugosa lipase

In several Candicia species belonging to the same monophyletic group, CUG - one of the six triplets coding leucine in the universal genetic code - is read as an additional codon for serine. CUG for serine is rarely employed but in the lipase genes of Candida rugosa, where it is the major serine codon. This yeast secretes multiple lipase isoenzymes (CRLs) tightly related in their amino acid sequence. CRL proteins contain 16 to 19 CUG-serines comprehensive of Ser 209 in the catalytic center and other serines having an obvious structural role. In this paper, results obtained from sequence analysis and mutagenesis are discussed and shown to be consistent with an evolutionary pathway in which the codon was reassigned via ambiguous decoding by a novel seryl-tRNA whose abundance conferred positive selection pressure for the extensive use of CUG as a serine codon.