A DEVIATION FRONT THE UNIVERSAL GENETIC-CODE IN CANDIDA-MALTOSA AND CONSEQUENCES FOR HETEROLOGOUS EXPRESSION OF CYTOCHROMES P450 52A4 AND 52A5 IN SACCHAROMYCES-CEREVISIAE

We demonstrate that serine instead of leucine is specified by the CUG codon in the yeast Cannida maltosa. Evidence for this deviation from t he universal genetic code was obtained by means of in vitro translatio n experiments. Depending on the cell-free system used, either serine, in the C. maltosa system, or leucine, in the control with the conventi onal wheat germ system, was found to be incorporated into the translat ion products of articia CUG-containing mRNAs. Moreover, we were able t o transfer the non-universal decoding of CUG to the wheat germ system by adding a tRNA fraction isolated from C. maltosa. This finding indic ates the presence in C. maltosa of an unusual serine tRNA that recogni zes CUG. As a consequence of the altered genetic code, expression in S accharomyces cerevisiae of C. maltosa cytochrome P450 genes required a n exchange of their CTG triplets by TCT encoding serine in order to pr oduce the authentic proteins. In contrast, heterologous expression of the original C. maltosa genes resulted in the formation of still activ e but unstable enzymes probably subject to selective proteolysis in th e host cells.