POLYPRENOL FORMATION IN THE YEAST SACCHAROMYCES-CEREVISIAE - EFFECT OF FARNESYL DIPHOSPHATE SYNTHASE OVEREXPRESSION

Biosynthesis of polyprenols was followed in the erg mutants of Sacchar omyces cerevisiae impaired in various steps of the mevalonate pathway. The end products of the enzymatic reaction carried out in vitro, in t he wild type yeast and all mutants tested, were identified as dehydrod olichols (alpha-unsaturated polyprenols) whereas in vivo, yeast synthe size dolichols (a-saturated polyprenols) (Biochimie, 1996.78:111-112.) The strain defective in the farnesyl diphosphate (FPP) synthase, (cod ed by the erg20-2 gene) required the presence of exogenous FPP for syn thesis of dehydrodolichols to occur in vitro. Overexpression of the ER G20 gene restored synthesis of polyprenols in vitro indicating that FP P is the allylic ''starter'' for cis-prenyltransferase in yeast. Overe xpression of the ERG20 gene in the erg 9 mutant, defective in squalene synthase activity, not only restored synthesis of dehydrodolichols in vitro, but also increased the synthesis of dolichols in vivo, almost 10-fold in comparison with wild type yeast. On the other hand overexpr ession of the mutated FPP synthase, coded by the gene erg20-2 in the s ame genetic background, resulted in a 100-fold increase of the amount of dehydrodolichols. Interestingly, in addition to the family of typic al for yeast C-60-C-80 compounds, dehydrodolichols of chain length up to C-135 were synthesized both in vitro and in vivo.