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Journal of Virology, April 1999, p. 3197-3209, Vol. 73, No. 4
0022-538X/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Transcriptional Repression of Human Hepatitis B Virus Genes by a bZIP Family Member, E4BP4

Chao-Kuen Lai and Ling-Pai Ting*

Institute of Microbiology and Immunology, School of Life Science, National Yang-Ming University, Shih-Pai, Taipei 11221, Taiwan, Republic of China

Received 22 September 1998/Accepted 17 December 1998

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 describe the cloning and characterization of a box alpha  binding protein, E4BP4. E4BP4 is a bZIP type of transcription factor. Overexpression of E4BP4 represses the stimulating activity of box alpha  in the upstream regulatory sequence of the core promoter and the second enhancer in differentiated human hepatoma cell lines. E4BP4 can also suppress the transcription of HBV genes and the production of HBV virions in a transient-transfection system that mimics the viral infection in vivo. Expression of an E4BP4 antisense transcript can, instead, elevate the transcription of the core promoter. A low abundance of E4BP4 protein and mRNA in differentiated human hepatoma cell lines is detected, and E4BP4 is not a major component of box alpha  binding proteins in untransfected differentiated human hepatoma cell lines. C/EBPalpha and C/EBPbeta , in contrast, are major components of the box alpha  binding activity present in nuclear extracts. E4BP4 has a stronger binding affinity towards box alpha  than the endogenous box alpha  binding activity present in nuclear extracts. Structure and function analysis of E4BP4 reveals that DNA binding activity is sufficient to confer the negative regulatory function of E4BP4. These results indicate that binding site occlusion is the mechanism whereby E4BP4 suppresses transcription in HBV.

* Corresponding author. Mailing address: Institute of Microbiology and Immunology, School of Life Science, National Yang-Ming University, Shih-Pai, Taipei 11221, Taiwan. Phone: 886-2-28222400. Fax: 886-2-28212880. E-mail: lpting{at}ym.edu.tw.

Journal of Virology, April 1999, p. 3197-3209, Vol. 73, No. 4
0022-538X/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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