RPE65 is essential for all-trans-to 11-cis-retinoid isomerization, the hallmark reaction of the retinal pigment epithelium (RPE). Here, we identify regulatory elements in the Rpe65 gene and demonstrate their functional relevance to Rpe65 gene expression. We show that the 5 flanking region of the mouse Rpe65 gene, like the human gene, lacks a canonical TATA box and consensus GC and CAAT boxes. The mouse and human genes do share several cis-acting elements, including an octamer, a nuclear factor one (NFI) site, and two E-box sites, suggesting a conserved mode of regulation. A mouse Rpe65 promoter/-galactosidase transgene containing bases -655 to ؉52 (TR4) of the mouse 5 flanking region was sufficient to direct high RPE-specific expression in transgenic mice, whereas shorter fragments (-297 to ؉52 or -188 to ؉52) generated only background activity. Furthermore, transient transfection of analogous TR4/luciferase constructs also directed high reporter activity in the human RPE cell line D407 but weak activity in the non-RPE cell lines HeLa, HepG2, and HS27. Functional binding of potential transcription factors to the octamer sequence, AP-4, and NFI sites was demonstrated by directed mutagenesis, electrophoretic mobility shift assay, and cross-linking. Mutations of these sites abolished binding and corresponding transcriptional activity and indicated that octamer and E-box transcription factors synergistically regulate the RPE65 promoter function. Thus, we have identified the regulatory region in the Rpe65 gene that accounts for tissue-specific expression in the RPE and found that octamer and E-box transcription factors play a critical role in the transcriptional regulation of the Rpe65 gene.All-trans-to 11-cis-isomerization of vitamin A is an obligate and tissue-specific enzymatic step in the renewal of 11-cisretinal, the universal chromophore of rhodopsin and other visual pigment proteins, in the visual cycle (1) of the retinal pigment epithelium (RPE).1 Several components, including 11-cis-retinol dehydrogenase (2), cellular 11-cis-retinaldehydebinding protein (CRALBP) (3, 4) and lecithin:retinol acyltransferase (5, 6), all essential to the visual cycle activity, are found highly expressed, but not exclusively expressed, in the RPE. However, the retinol isomerase activity (7-9), central to 11-cischromophore synthesis, is expected, mechanistically, to be highly tissue-specific. A tissue-specific component of the RPE, RPE65 (10 -12), which copurifies with 11-cis-retinol dehydrogenase (2), appears to play a crucial role in retinoid isomerization. Thus, in the Rpe65-deficient mouse (13), rod photoreceptor function is abolished due to lack of the 11-cis-retinal chromophore. Furthermore, mutations in the human RPE65 gene cause several forms of severe early onset blindness (14 -17). Clearly, RPE65 is essential to the visual cycle in general and to all-trans-to 11-cis-retinoid isomerization in particular. RPE65 is the major protein of the RPE microsomal membrane fraction. The bovine (10), human (18), dog (19), rat (20), a...