Human chorionic somatomammotropin gene enhancer activity is dependent on the blockade of a repressor mechanism.

The human chorionic somatomammotropin genes (hCS-A and -B) are expressed at high levels in the syncytiotrophoblast during pregnancy. A 22-base-pair (bp) transcriptional enhancer factor-1 (TEF-1) element in a 1022-bp fragment of the hCS-B 3'-flanking DNA (nucleotides 1-1022) was shown to be important for efficient promoter activity in placental cells. However, the TEF-1 site used alone does not contain all of the information required for the complete enhancer activity seen with the 1022-bp fragment. A 241-bp region of the 1022-bp fragment (nucleotides 1-241) maintains full enhancer activity in placental cells. Interactions between placental nuclear factors and sequences distinct from the TEF-1 element (nucleotides 117-139) were identified by gel mobility shift assay using the up-stream region corresponding to nucleotides 1-80. Interaction between these factors and the TEF-1 element was indicated by competition of the 1-80 bp region for complex formation by a TEF-1 site. We mutated sequences within the 1-80 bp region of the 241-bp enhancer fragment and assessed the enhancer function of wild-type and modified 241-bp fragments. We identified a sequence (DF-1 site) upstream of the TEF-1 site which is required for hCS-B enhancer function. DF-1 derepresses a repressor mechanism present in the 241-bp fragment that inhibits TEF-1 activity. A component of this repressor mechanism (RF-1 site) is present in the 1-80 bp region adjacent to the DF-1 site. Gel mobility shift competition analysis shows that the RF-1 and DF-1 sites participate in the formation of a common complex or compete for common protein factors in a tissue-specific manner.