THE HEPATITIS-B VIRUS X-PROTEIN IS A COACTIVATOR OF ACTIVATED TRANSCRIPTION THAT MODULATES THE TRANSCRIPTION MACHINERY AND DISTAL BINDING ACTIVATORS

Hepatitis B virus X protein (HBx) transactivates viral and cellular ge nes through a wide variety of cis-elements, but the mechanism has not been well elucidated. Evidence for nuclear events in HBx transactivati on has been reported. Here we examine the role of HBx in modulation of transcription with a transient transfection system and an in vitro tr anscription assay. Reporters bearing Gal4-binding sites were applied t o avoid the effects of endogenous transcription factors with or withou t signaling processes. The Gal4-DNA binding domain fused form of HBx e xhibited no effect on Gal4-responsive reporters. However, HBx augmente d activated transcription by transcriptional activators, suggesting HB x retains a co-activator but not a transcriptional activator function. The functional domain for co-activation was the same as that for HBx transactivation, and the transcription factor IIB and RNA polymerase I I subunit B-interacting sites of HBx, which were critical for HBx tran sactivation, were shown to be crucial for the co-activation function. Importantly, HBx stimulated transcription on templates bearing the X r esponsive elements in vitro with endogenous activators. These results imply that HBx acts as a co-activator that modulates transcriptional m achinery and distal-binding activators, which may explain one of the m echanisms of transactivation by HBx when localized in nuclei.