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 ra rely employed but in the lipase genes of Candida rugosa, where it is t he major serine codon. This yeast secretes multiple lipase isoenzymes (CRLs) tightly related in their amino acid sequence. CRL proteins cont ain 16 to 19 CUG-serines comprehensive of Ser 209 in the catalytic cen ter and other serines having an obvious structural role. In this paper , results obtained from sequence analysis and mutagenesis are discusse d and shown to be consistent with an evolutionary pathway in which the codon was reassigned via ambiguous decoding by a novel seryl-tRNA who se abundance conferred positive selection pressure for the extensive u se of CUG as a serine codon.