Aromatase, an enzyme complex localized in the endoplasmic reticulum of estrogen-producing cells, is composed of NADPH-cytochrome P-450 reductase, and aromatase cytochrome P-450 (cytochrome P-450AROM). To define the molecular mechanisms involved in the multifactorial regulation of cytochrome P-450AROM in estrogen-producing cells, we have isolated a cDNA specific for human cytochrome P-450AROM and have used this cDNA to isolate the human cytochrome P-450AROM gene. The cDNA sequence encodes a polypeptide of 503 amino acids and contains--near the carboxy-terminus, a region of high homology with the putative heme-binding regions of other P-450 cytochromes. COS1 cells transfected with an expression plasmid containing the cytochrome P-450AROM cDNA had the capacity to aromatize testosterone, androstenedione and 16 alpha-hydroxyandrostenedione, suggesting that a single polypeptide catalyzes all steps of the aromatization reaction using either of the three major C19-substrates. The human cytochrome P-450AROM gene is greater than 52 kb in size and consists of 10 exons and 9 introns. Hormonally induced changes in aromatase activity of human ovarian granulosa and adipose stromal cells are associated with comparable changes in cytochrome P-450AROM gene expression and synthesis, whereas the reductase component is only modestly affected. Studies are in progress to define the molecular mechanisms involved in the regulation of cytochrome P-450AROM gene expression in estrogen-producing cells.