Human hepatitis virus X gene encodes a regulatory domain that represses transactivation of X protein.

The human hepatitis B virus (HBV) X gene seems to be essential for establishment of viral infection, and the X gene product, HBx, transactivates virus and host genes through a wide variety of cis-elements, whereas regulation of HBx has not been fully understood. We found that transactivation-negative HBx mutants truncated at the C-terminal portion specifically repressed the HBx transactivation in trans. The ability to trans-repress the HBx transactivation is confined to the N-terminal third of HBx. Transactivation-positive constructs of HBx were divided into two groups by their sensitivity to trans-repression due to the presence of the N-terminal third. Thus the regulatory domain, the N-terminal third, is separated from the transacting domain and responsible for the negative regulations, the trans-repression and sensitivity to X trans-repression. A possible direct association between the HBx regulatory domains was tested by far-Western blotting using purified fused forms of HBx proteins. The regulatory domain was found to associate preferentially with the full HBx or the regulatory domain, but not with the transacting domain. Taken together, it is possible that HBx has a self-regulatory mechanism that avoids excessive HBx transactivation and is important for regulation of X gene expression.