The CYP11A1 gene encodes P450scc (cholesterol side-chain cleavage enzyme), which catalyzes the first step for the synthesis of steroids. Expression of CYP11A1 is controlled by transcription factor SF-1 (steroidogenic factor 1). Two functional SF-1-binding sites, P and U, located at -40 and -1,600 regions of the CYP11A1 gene, have been identified, but their exact functions with respect to basal activation vs. cAMP response have not been dissected. We have addressed this question by examining the ability of the mutated human CYP11A1 promoter to drive LacZ reporter gene expression in transgenic mouse lines. The activity of the mtP mutant promoter was greatly reduced, indicating the importance of the P site. Mutation of the upstream U site also resulted in reduced reporter gene expression, but some residual activity remained. This residual reporter gene activity was detected in the adrenal and gonad in a tissue-specific manner. ACTH and hCG can stimulate LacZ gene expression in the adrenals and testes of transgenic mice driven by the wild-type but not the mtU promoter. These results indicate that the upstream SF-1-binding site is required for hormonal stimulation. Our experiments demonstrate the participation of both the proximal and the upstream SF-1-binding sites in hormone-responsive transcription.