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 est ablishment of viral infection, and the X gene product, HBx, transactiv ates virus and host genes through a wide variety of cis elements, wher eas regulation of HBx has not been fully understood. We found that tra nsactivation-negative HBx mutants truncated at the C-terminal portion specifically repressed the HBx transactivation in trans. The ability t o 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 transrepression due to the pre sence of the N-terminal third. Thus the regulatory domain, the N-termi nal third, is separated from the transacting domain and responsible fo r the negative regulations, the trans-repression and sensitivity to X trans-repression. A possible direct association between the HBx regula tory domains was tested by far-Western blotting using purified fused f orms of HBx proteins. The regulatory domain was found to associate pre ferentially with the full HBx or the regulatory domain, but not with t he transacting domain. Taken together, it is possible that HBx has a s elf-regulatory mechanism that avoids excessive HBx transactivation and is important for regulation of X gene expression.