Analysis of Basal Regulatory Elements in the 25-Hydroxyvitamin D3 1α-Hydroxylase Gene Promoter

5′-Deletion analysis of the 1.7-kb mouse 1α-hydroxylase gene promoter reveals that the minimal promoter region for basal activity is −85/+22 and requires a functional CCAAT element. Mutational analysis also demonstrates that deletion of the internal promoter region from nucleotides −1125 to −86 leads to a 25- to 30-fold increase in basal promoter activity. The increased activity is not the result of positional effects, but is caused in part by the removal of an AC repeat. Further analysis of the promoter revealed an enhancer element localizes to an upstream region −1385 to −1125, which contains three consensus AP-1 sites. Deletion of the most proximal AP-1 site causes a 60% loss of enhancer activity. The data suggest the presence of the AC repeat prevents the full potential activation of the 1α-hydroxylase promoter by factors binding to AP-1 sites.