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 recognit ion sequence. We now show that the EP element, a key regulatory sequen ce within hepatitis B virus enhancer I, also associates with MIBP1 and RFX1. Using polyclonal antisera directed against either oligonucleoti de-purified MIBP1 or a peptide derived from the major histocompatibili ty complex class II promoter-binding protein RFX1, we showed that MIBP 1 and RFX1 are both present in the DNA-protein complexes at the EP sit e. In addition, while the EP element can act cooperatively with severa l adjacent elements to transactivate hepatitis B virus expression, we demonstrated that the EP site alone can repress transcription of simia n virus 40 promoter in a position- and orientation-independent manner, suggesting a silencer function in hepatocarcinoma cells.