EFFECTS OF OVERPRODUCTION OF THE CATALYTIC DOMAIN OF 3-HYDROXY-3-METHYLGLUTARYL COENZYME-A REDUCTASE ON SQUALENE SYNTHESIS IN SACCHAROMYCES-CEREVISIAE

The enzyme 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase ( HMG-R) is the major rate-limiting enzyme of the mevalonate pathway in many organisms, including yeasts. In the yeast Saccharomyces cerevisia e, there are two isoenzymes of HMG-R (Hmg1p and Hmg2p). Both consist o f an anchoring transmembrane domain and a catalytic domain. We have re moved the known controlling features of HMG-R by overproducing the cat alytic domain of Hmg1p. This overproduction leads to an enhancement of squalene production, implying that HMG-R has been deregulated. The en hancement is apparent under semianaerobic and aerobic conditions. Desp ite the increase in squalene production, the amount of ergosterol prod uced by the HMG-R-overproducing yeast was not increased. This result s uggests the presence of another regulatory step between squalene and e rgosterol formation. Squalene levels generated by cells overproducing the catalytic domain of HMG-R were estimated to be up to 10 times thos e produced by wild-type cells. The enhancement in squalene production coincided with a reduction in growth rate. This reduction may be a dir ect consequence of the buildup of high concentrations of squalene and presqualene intermediates of the pathway.