HEPATITIS-B VIRUS PX TARGETS TFIIB IN TRANSCRIPTION COACTIVATION

pX, the hepatitis B virus (HBV)-encoded regulator, coactivates transcr iption through an unknown mechanism, pX interacts with several compone nts of the transcription machinery, including certain activators, TFII B, TFIIH, and the RNA polymerase TT (POLII) enzyme, We show that pX lo calizes in the nucleus and coimmunoprecipitates with TFIIB from nuclea r extracts, We used TFIIB mutants inactive In binding either POLII or TATA binding protein to study the role of TFIIB-pX interaction in tran scription coactivation, pX was able to bind the former type of TFIIB m utant and not the latter, Neither of these sets of TFIIB mutants suppo rts transcription. Remarkably, the latter TFIIB mutants fully block pX activity, suggesting the role of TFIIB in pX-mediated coactivation, B y contrast, in the presence of pX, TFIIB mutants with disrupted POLII binding acquire the wild-type phenotype, both in vivo and in vitro, Th ese results suggest that pX may establish the otherwise inefficient TF IIB mutant-POLII interaction, by acting as a molecular bridge, Collect ively, our results demonstrate that TFIIB is the in vivo target of pX.