Interactions of the transcription factors MIBP1 and RFX1 with the EP element of the hepatitis B virus enhancer

We previously demonstrated that MIBP1 and RFX1 polypeptides associate in vivo to form a complex that binds to the MIF-1 element in the c-myc gene and the major histocompatibility complex class II X-box recognition sequence. We now show that the EP element, a key regulatory sequence within hepatitis B virus enhancer I, also associates with MIBP1 and RFX1. Using polyclonal antisera directed against either oligonucleotide-purified MIBP1 or a peptide derived from the major histocompatibility complex class II promoter-binding protein RFX1, we showed that MIBP1 and RFX1 are both present in the DNA-protein complexes at the EP site. In addition, while the EP element can act cooperatively with several adjacent elements to transactivate hepatitis B virus expression, we demonstrated that the EP site alone can repress transcription of simian virus 40 promoter in a position- and orientation-independent manner, suggesting a silencer function in hepatocarcinoma cells.