Identification of protein-binding sites in the hepatitis B virus enhancer and core promoter domains.

We have investigated the role of liver-specific trans-acting factor(s) in the regulation of hepatitis B virus (HBV) gene expression. A recorder plasmid (pEcoAluCAT; HBV nucleotides 1 through 1878) was constructed containing the HBV enhancer and the promoter region of the pregenomic RNA, which was ligated to the bacterial chloramphenicol acetyltransferase (CAT) gene. Upon transfecting this plasmid into various cell lines, the CAT gene was expressed only in cells of liver origin. Moreover, competition cotransfections with pEcoAluCAT and plasmids containing HBV enhancer sequences in human hepatoblastoma-derived HepG2 cells indicated the presence of titratable trans-acting factor(s) in these cells. Gel mobility shift assays using HBV enhancer and core promoter domains confirmed the existence of sequence-specific DNA-binding proteins in liver cell nuclear extract which bound to these regions. These binding sites encompass 17- and 12-nucleotide palindromes in the HBV enhancer and core promoter domains, respectively, when mapped by the methylation interference assay.