The X protein of the hepatitis B virus acts as a transcription factor when targeted to its responsive element.
The X protein of hepatitis B virus (HBV) stimulates transcription of a large number of viral enhancers. This protein augments the activity of the HBV enhancer through a specific cis element, termed X responsive element (XRE). Multimers of XRE exhibit enhancer activity which is further stimulated by X. XRE binds multiple cellular transcription factors one of which is the C/EBP. We have constructed the DB gene containing the DNA-binding domain of the C/EBP. This gene efficiently represses the enhancer activity of the XRE by competitive displacement of the XRE-binding factors. Under these conditions, X was found to have only a partially stimulatory effect on transcription, suggesting that the XRE-binding proteins are required for the activity of X. In contrast, an X-DB hybrid protein that binds to the XRE is a strong transcription factor and acts without additional XRE-binding proteins. Furthermore, studies of X mutants revealed that the carboxy-terminus of the protein is required for this activation. These data show that X directly stimulates the cellular transcription machinery, possibly by protein-protein interaction with the XRE-binding factors.
Unger
T
T
Department of Molecular Genetics and Virology, Weizmann Institute of Science, Rehovot, Israel.
Shaul
Y
Y
D000595:Amino Acid Sequence; D001483:Base Sequence; D002678:Chimera; D004279:DNA, Viral / Q000378:metabolism*; D005814:Genes, Viral; D006515:Hepatitis B virus / Q000235:genetics*; D008969:Molecular Sequence Data; D012191:Retroviridae Proteins / Q000235:genetics*; D015534:Trans-Activators / Q000235:genetics*; D014157:Transcription Factors / Q000235:genetics*; D014158:Transcription, Genetic; D015533:Transcriptional Activation; D054334:Viral Regulatory and Accessory Proteins
D004279:DNA, Viral; D012191:Retroviridae Proteins; D015534:Trans-Activators; D014157:Transcription Factors; D054334:Viral Regulatory and Accessory Proteins; C057628:hepatitis B virus X protein