THE 17 KDA HBX PROTEIN ENCODED BY HEPATITIS-B VIRUS INTERACTS WITH THE ACTIVATION DOMAINS OF OCT-1, AND FUNCTIONS AS A COACTIVATOR IN THE ACTIVATION AND REPRESSION OF A HUMAN U6 PROMOTER

The hepatitis B virus 17 kDa X gene product expressed in bacteria tran sactivated a human U6 promoter three- to eightfold in an ATP-independe nt manner in HeLa cell NTP-depleted extracts containing preassembled t ranscription preinitiation complexes. However, if added prior to assem bly, HBx squelches the promoter. Both the HBx dependent ''squelching'' of U6 transcription observed in transient transfection analysis, and the transactivation observed in vitro is dependent on the presence of an upstream octamer element. HBx is incorporated via protein-protein i nteractions into DNA complexes containing the activation domains of Oc t-1, and into a stable U6 preinitiation complex. This is consistent wi th a role as a coactivator interacting with the basal transcription ma chinery. We propose that the HBx dependent transactivation and repress ion of U6 transcription occurs by changes in the transcription factor limiting initiation, and propose that HBx may have a dual role in the regulation of transcription in vivo.