2 RADISH GENES FOR 3-HYDROXY-3-METHYLGLUTARYL-COA REDUCTASE ISOZYMES COMPLEMENT MEVALONATE AUXOTROPHY IN A YEAST MUTANT AND YIELD MEMBRANE-BOUND ACTIVE ENZYME

Expression of two full-length cDNAs encoding two related isozymes of r adish (Raphanus sativus L. var. Saxa Knacker) 3-hydroxy-3-methylglutar yl-CoA reductase (HMGR, EC 1.1.1.34) can complement mevalonate auxotro phy in a yeast mutant having both HMGR genes (HMG1, HMG2) disrupted [s ee Basson et al., 1988]. HMGR activity in transformants, placed under the control of the GAL10 promoter, can be induced in the presence of g alactose and is exclusively localized to membranes. Yeast HMGRs contai n seven or more membrane-spanning hydrophobic regions, but functional complementation and evidently targeting to yeast membranes shows that the two membrane spans present in plant HMGRs are already sufficient f or membrane insertion.