Functional analysis of the placenta-specific enhancer of the human glycoprotein hormone alpha subunit gene. Emergence of a new element

Placental expression of the alpha subunit gene of the human glycoprotein hormones requires a multicomponent enhancer composed of tandem cAMP-response elements and an adjacent upstream regulatory element. Based on recent studies indicating that the upstream regulatory element includes binding sites for more than one protein, we investigated how functional activity correlated with these binding sites. Through extensive replacement mutagenesis of the native promoter regulatory region, we provide the first functional map of the upstream regulatory element. Within this region, we find that distinct proteins interact with three overlapping binding sites. While each site is functionally significant, no single site is essential or displays clear dominance. This is surprising since one of the sites binds a placenta-specific protein that heretofore has been regarded as essential for activity of the human alpha subunit placenta-specific enhancer. Consequently, our refined functional map of the upstream regulatory element reveals a complex combinatorial code that directs expression of the human alpha subunit gene to placenta.