The amyloid-beta (A beta) peptide, the proteolytic fragment of A beta precursor protein (APP), aggregates and forms neuritic plaques, a major hallmark of Alzheimer's disease (AD). The limiting step in generating the A beta peptide from APP is cleavage by the P-secretase enzyme, BACE1 Regulation of the BACE1 gene is likely to play an important role in AD etiology and treatment. We therefore studied the activity of a 4.1-kb F-flanking region (-3765/+364, +1 being the transcription start site) of the BACE1 gene, both in 5'- and T-deletion series and through Northern blotting. We show that the BACE1 promoter has regulatory activity throughout the 4.1-kb length, both positive and negative, and that this activity can be quantitatively modeled according to promoter sequence length, with the specific model depending on the presence of negative regulatory elements as the 5'-most portion of the sequence. We also examined a previously identified 141-bp proximal fragment (+224/+364) of the BACE1 promoter and two constituent (91- and 50-bp) subfragments. We report that the 91-bp fragment (+224/+314) is the most likely seat of neuronal expression of the BACE1 gene and that it is the portion of the 141-bp fragment that accounts for observed DNA-protein interactions in brain extracts. The 50-bp fragment (+315/+364), which showed significant reporter gene activity from the empty vector, binds nuclear proteins in a cell type-specific manner and contains the AP2 site as shown by the electrophoretic mobility shift assay. Overall, the 141-bp fragment had no strong matches within GenBank, and the 91-bp fragment is predicted to have several potential stem-loop sites. Taken together, BACE1 gene promoter activity is differentially regulated, and the 91-bp fragment represents a novel promoter region for cell type-specific regulation. This fragment might be a useful target to regulate BACE1 expression leading to A beta production and to understand the neuropathogenesis of AD.