The enhancer element of hepatitis B virus (HBV) regulates the transcription of all HBV mRNA, including the pregenomic mRNA used during replication. This pregenomic mRNA is transcribed from the core gene promoter which is located 500 bp downstream from the HBV enhancer element. To examine the effect of the HBV enhancer on the activity of the core gene promoter, we constructed various plasmids containing different combinations of HBV enhancer and core gene promoter sequences regulating the expression of the chloramphenicol acetyltransferase gene. When the HBV enhancer was positioned immediately adjacent to the core gene promoter, the enhancer increased the activity of the core gene promoter nearly 30-fold. In contrast, the HBV enhancer only modestly stimulated the core gene promoter (less than threefold) at its native position in the HBV genome. This weak HBV enhancer activity was due to a DNA sequence located between the enhancer and the core gene promoter and not due to the increased distance between the enhancer and the core gene promoter. Competition experiments demonstrated that a trans-acting factor(s) bound this sequence and repressed the enhancer. This DNA sequence contains the C/EBP, AP-1 and NF-1 regulatory sites. No inhibition of enhancer activity was observed when only the AP-1 and C/EBP sites were present. Repression of the HBV enhancer was not detected when the NF-1 site was disrupted, indicating that the NF-1 site was involved in the suppression of the HBV enhancer.
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