Lovastatin, a cyclic nonaketide from Aspergillus terreus, is a hypercholesterolemic agent and a precursor to simvastatin, a semi-synthetic cholesterol-lowering drug. The biosynthesis of the lovastatin backbone (dihydromonacolin L) and the final 2-methylbutyryl decoration have been fully characterized. However, it remains unclear how two central reactions are catalyzed, namely, introduction of the 4a,5-double bond and hydroxylation at C-8. A cytochrome P450 gene, lovA, clustered with polyketide synthase lovB, has been a prime candidate for these reactions, but inability to obtain LovA recombinant enzyme has impeded detailed biochemical analyses. The synthetic codon optimization and/or N-terminal peptide replacement of lovA allowed the lovA expression in yeast (Saccharomyces cerevisiae). Both in vivo feeding and in vitro enzyme assays showed that LovA catalyzed the conversion of dihydromonacolin L add to monacolin L add and monacolin J add, two proposed pathway intermediates in the biosynthesis of lovastatin. LovA was demonstrated to catalyze the regio- and stereospecific hydroxylation of monacolin L add to yield monacolin J acid. These results demonstrate that LovA is the single enzyme that performs both of the two elusive oxidative reactions in the lovastatin biosynthesis.