Transcriptional repression of human hepatitis B virus genes by a bZIP family member, E4BP4

Box alpha is an essential element of both the upstream regulatory sequence of the core promoter and the second enhancer, which positively regulate the transcription of human hepatitis B virus (HBV) genes. In this paper, we de scribe the cloning and characterization of a box alpha binding protein, E4B P4. E4BP4 is a bZIP type of transcription factor. Overexpression of E4BP4 r epresses the stimulating activity of box alpha in the upstream regulatory s equence of the core promoter and the second enhancer in differentiated huma n hepatoma cell lines. E4BP4 can also suppress the transcription of HBV gen es and the production of HBV virions in a transient-transfection system tha t mimics the viral infection in vivo. Expression of an E4BP4 antisense tran script can, instead, elevate the transcription of the core promoter. A low abundance of E4BP4 protein and mRNA in differentiated human hepatoma cell l ines is detected, and E4BP4 is not a major component of box alpha binding p roteins in untransfected differentiated human hepatoma cell lines. C/EBP al pha and C/EBP beta, in contrast, are major components of the box alpha bind ing activity present in nuclear extracts. E4BP4 has a stronger binding affi nity towards box alpha than the endogenous box alpha binding activity prese nt in nuclear extracts. Structure and function analysis of E4BP4 reveals th at DNA binding activity is sufficient to confer the negative regulatory fun ction of E4BP4. These results indicate that binding site occlusion is the m echanism whereby E4BP4 suppresses transcription in HBV.