The retinoblastoma gene product activates transcription of the hepatitis B virus pregenomic promoter through the Sp1 binding sites

The hepatitis B virus (HBV) pregenomic promoter is regulated by two enhancers and cis-elements. We have studied whether the retinoblastoma susceptibility gene product (Rb) modulates the activity of the HBV pregenomic promoter. Cotransfection of the Rb expression vector, phRB, with pCENCAT (containing the pregenomic promoter region: nt 248-1874) increased transcription from the HBV pregenomic promoter in HepG2 cells. Deletion analysis of the pregenomic promoter indicated that the region between nt -96 and -66, which contains two Sp1 binding sites, is responsible for activation by Rb. Mutation of the Sp1 binding sites abolished activation of the pregenomic promoter by Rb in the heterologous and natural promoter context. Therefore, our results suggest that Rb can activate the HBV pregenomic promoter through the Sp1 binding sites.