Multiple nuclear proteins in liver cells are bound to hepatitis B virus enhancer element and its upstream sequences

The transcriptional enhancer element in the hepatitis B virus (HBV) genome displays tissue-specific activity, suggesting that this element interacts with cellular specific factors. Using a nitrocellulose filter binding assay and DNase I footprinting, we have found that liver cell-specific nuclear proteins are bound to the HBV enhancer element (the E site) and its adjacent sequences. Four DNase I-protected sites were revealed, all contain a sequence motif resembling the sequence of the SV40 enhancer core element. Evidence is provided to show that: (i) these sites are protected by at least three distinct nuclear proteins and (ii) the presence of some of these proteins is dependent on the differentiation stage of the liver cells. Interestingly an octamer sequence found in the E site appears also in the promoter region of several liver-specific genes, which suggests that the E site and its corresponding binding protein(s) determine the tissue-specific expression of the HBV enhancer element.