‘Repair’ of the chorionic somatomammotropin-A ‘enhancer’ region reveals a novel functional element in the chorionic somatomammotropin-B enhancer

Human chorionic somatomammotropin (CS) synthesis results from the independent expression of two homologous genes, CS-A and CS-B. A transcription enhancer factor-1 (TEF-1) element and an upstream 81 bp modulatory domain, containing repressor (RF-1) and derepressor (DF-1) activities, are important for efficient CS-B enhancer function in transfected placental JEG-3 cells. The equivalent region of the CS-A gene is not active. Although the TEF-1 element is conserved between the CS-A and CS-B genes, a single base substitution is present in the DF-1 element and two more are located between the RF-1 and DF-1 sites in a region we term AF-1. Repair of the DF-1 site increased CS-A enhancer function ∼70-fold, but repair of previously uncharacterized AF-1 sequences was also required for full (CS-B like) enhancer activity. A 5 bp disruption of AF-1 sequences in the CS-B enhancer region, resulted in a 97% loss of stimulatory activity. The AF-1 sequences showed no intrinsic enhancer activity, however, they were able to significantly repress heterologous promoter activity stimulated by a TEF-1 enhancer element. A high affinity/specificity interaction between JEG-3 nuclear protein and AF-1 sequences was confirmed by gel mobility shift assay. By comparison to ‘wild type’ AF-1 sequences, this interaction was competed to a lesser extent by both RF-1 and DF-1 elements, but not by mutated AF-1 sequences. The major protein binding to AF-1 sequences was estimated to be 23 kDa by UV crosslinking. These data indicate that enhancer activity can be generated by modulating binding events proximal to the TEF-1 element in the CS-A ‘enhancer’ region and that coordinated binding of AF-1 and DF-1 are required for efficient (CS-B) enhancer activity.