The gonadotropin alpha-gene contains multiple protein binding domains that interact to modulate basal and cAMP-responsive transcription

Abstract

DNA sequences that modulate basal and cAMP-stimulated transcription are located within the initial 169 base pairs of the alpha-gene 5'-flanking sequence. Using DNase I protection analyses and gel-mobility shift assays, we examined in vitro the domains in the human alpha-gene 5'-flanking sequence that bind nuclear proteins extracted from JEG-3 choriocarcinoma cells. DNase I protection studies of the sequences between -236 and -100 demonstrate two major protected regions: -178 to -156 corresponding to an upstream regulatory element (URE) and -146 to -112 corresponding to 18-base pair repeated sequences that contain cAMP-responsive enhancers (CREs). Nuclear proteins extracted from JEG-3 choriocarcinoma cells bind specifically to oligonucleotides corresponding to both the URE and CRE domains as well as to a downstream domain (-99 to -72) that contains consensus CCAAT motifs on both the sense and antisense strands. Binding to a DNA fragment (-236 to -100) that contains both the URE and CRE domains was 10-fold more effective than that using either fragment alone. Binding to this multisite DNA fragment is readily disrupted using the URE sequence, but not the CRE sequence as a competitor, suggesting that the URE binding factor may stabilize DNA-protein interactions in these adjacent complexes. The amount of protein binding to each of the alpha-gene 5'-flanking domains was unaffected by treatment with 8-bromo-cAMP. These studies indicate that there are multiple adjacent protein binding domains in human alpha-gene 5'-flanking sequence that correspond to cis-acting regulatory elements including an upstream element that activates basal expression, repeated cAMP-response elements, and a downstream sequence containing consensus CCAAT box elements. Interactions between regulatory domains facilitate protein binding and synergistically stimulate alpha-gene transcription.