HEPATITIS-B VIRUS X-PROTEIN IS A TRANSCRIPTIONAL MODULATOR THAT COMMUNICATES WITH TRANSCRIPTION FACTOR IIB AND THE RNA-POLYMERASE-II SUBUNIT-5

Hepatitis B virus X protein (HBx) transactivates viral and cellular ge nes through a wide variety of cis-elements, However, the mechanism is still obscure, Our finding that HBx directly interacts with RNA polyme rase II subunit 5 (RPB5), a common subunit of RNA polymerases, implies that HBx directly modulates the function of RNA polymerase (Cheong, J , H., Yi, M., Lin, Y,, and Murakami, S. (1995) EMBO J. 14, 142-150), I n this context, we examined the possibility that HBx and RPB5 interact with other general transcription factors, HBx and RPB5 specifically b ound to transcription factor LIE (TFIIB) in vitro, both of which were detected by either far Western blotting or the glutathione S-transfera se-resin pull-down assay, Delineation of the binding regions of these three proteins revealed that HBx, RPB5, and TFIIB each has two binding regions for the other two proteins. Co-immunoprecipitation using HepG 2 cell lysates that express HBx demonstrated trimeric interaction in v ivo, Some HBx substitution mutants, which had severely impaired transa cting activity, exhibited reduced binding affinity with either TFIIB o r RPB5 in a mutually exclusive manner, suggesting that HBx transactiva tion requires the interactions of both RPB5 and TFIIB, These results i ndicated that HBx is a novel virus modulator that facilitates transcri ptional initiation by stabilizing the association between RNA polymera se and TFIIB through communication with RPB5 and TFIIB.