HUMAN CHORIONIC SOMATOMAMMOTROPIN GENE ENHANCER ACTIVITY IS DEPENDENTAN THE BLOCKADE OF A REPRESSOR MECHANISM

The human chorionic somatomammotropin genes (hCS-A and -B) are express ed at high levels in the syncytiotrophoblast during pregnancy. A 22-ba se-pair (bp) transcriptional enhancer factor-1 (TEF-1) element in a 10 22-bp fragment of the hCS-B 3'-flanking DNA (nucleotides 1-1022) was s hown to be important for efficient promoter activity in placental cell s. However, the TEF-1 site used alone does not contain all of the info rmation required for the complete enhancer activity seen with the 1022 -bp fragment. A 241-bp region of the 1022-bp fragment (nucleotides 1-2 41) 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. In teraction 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 si te. We mutated sequences within the 1-80 bp region of the 241-bp enhan cer fragment and assessed the enhancer function of wild-type and modif ied 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 de represses a repressor mechanism present in the 241-bp fragment that in hibits TEF-1 activity. A component of this repressor mechanism (RF-1 s ite) is present in the 1-80 bp region adjacent to the DF-1 site. Gel m obility 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.