Abstract
Human hepatitis B virus X protein, HBx, is widely acknowledged as a transcriptional transactivator. While HBx has been shown to increase gene expression in trans, it is generally believed that it does not bind double-stranded DNA. Using several experimental approaches, we show that HBx interacts with single-stranded DNA in a manner that is not sequence-specific. Various heterologous single-stranded DNA (ssDNA) oligonucleotides were able to complete in HBx-ssDNA interactions in gel shift assays. Escherichia coli non-sequence-specific, single-stranded DNA binding protein, E. coli SSB, displaced the HBx-ssDNA interactions, confirming the ability of HBx to interact with single-stranded DNA in a non-sequence-specific manner. We have further characterized the HBx-ssDNA interactions under various biochemical conditions. These include the effects of mono- and divalent cations, the effect of cardiolipin and heparin, pH and temperature dependence and variations in the incubation time. HBx bound more tightly to d(pyrimidines) sub(25) than to d(purines) sub(25), a property that is characteristic of other single-stranded DNA-binding proteins (SSBs). Collectively the results presented here provide the first evidence of HBx's interaction with ssDNA. The biochemical parameters of these interactions were similar to those of known viral and cellular SSBs.