Truncation and mutagenesis analysis of the human X-arrestin gene promoter

X-arrestin (arrestin-3) is an arrestin present specifically in the outer segments of red-, green-, and blue-cone photoreceptors. The X-arrestin gene is on Xcen-q22, and consists of 17 exons with a promoter containing a TATA box and elements important for photoreceptor expression, including three CRX and one PCE-1-like element. In order to delineate the promoter structure necessary for the pan-cone-specific expression of X-arrestin, the expression of the gene in retinoblastoma cell lines was investigated, and a structure-function analysis of the promoter was conducted in the appropriate cellular substrate. Expression of X-arrestin was detected at a low level in the Y79 retinoblastoma cell line but not in the WERI retinoblastoma cell line. Truncation and expression analysis of the X-arrestin promoter in Y79 showed maximal activity in the proximal 378-bp region containing the CRX and PCE-1-like elements upstream of the TATA and CAAT boxes and a negative regulator in the distal 1-2-kbp region. Mutagenesis of the three CRX and PCE-1-like elements and expression analysis demonstrated complete elimination of the promoter activity. Mutagenesis of the TATA box and PCE-1-like element individually resulted in similar decrease in promoter activity, but the decrease in the promoter activity was greater when the CRX elements were mutagenized with a 5' to 3' spatial gradient in the negative effect, suggesting a cooperative effect of the three CRX elements. The regulation of expression from this promoter may involve the binding of a multi-protein enhanceosome complex at the CRX triplet and the PCE-1-like element, resulting in the recruitment and activation of the RNA polymerase II complex at the downstream TATA box.