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Mol. Cell. Biol., 12 1997, 6838-6846, Vol 17, No. 12
HD Wang, A Trivedi and DL Johnson
Our previous studies have shown that the hepatitis B virus protein, X,
activates all three classes of RNA polymerase III (pol III)-dependent
promoters by increasing the cellular level of TATA-binding protein (TBP)
(H.-D. Wang et al., Mol. Cell. Biol. 15:6720-6728, 1995), a limiting
transcription component (A. Trivedi et al., Mol. Cell. Biol. 16:6909-6916,
1996). We have investigated whether these X-mediated events are dependent
on the activation of the Ras/Raf-1 signaling pathway. Transient expression
of a dominant-negative mutant Ras gene (Ras-ala15) in a Drosophila S-2
stable cell line expressing X (X-S2), or incubation of the cells with a Ras
farnesylation inhibitor, specifically blocked both the X-dependent
activation of a cotransfected tRNA gene and the increase in cellular TBP
levels. Transient expression of a constitutively activated form of Ras
(Ras-val12) in control S2 cells produced both an increase in tRNA gene
transcription and an increase in cellular TBP levels. These events are not
cell type specific since X-mediated gene induction was also shown to be
dependent on Ras activation in a stable rat 1A cell line expressing X.
Furthermore, increases in RNA pol III-dependent gene activity and TBP
levels could be restored in X-S2 cells expressing Ras-ala15 by coexpressing
a constitutively activated form of Raf-1. These events are serum dependent,
and when the cells are serum deprived, the X-mediated effects are
augmented. Together, these results demonstrate that the X- mediated
induction of RNA pol III-dependent genes and increase in TBP are both
dependent on the activation of the Ras/Raf-1 signaling cascade. In
addition, these studies define two new and important consequences mediated
by the activation of the Ras signal transduction pathway: an increase in
the central transcription factor, TBP, and the induction of RNA pol
III-dependent gene activity.
Copyright © 1997, American Society for Microbiology
Hepatitis B virus X protein induces RNA polymerase III-dependent gene transcription and increases cellular TATA-binding protein by activating the Ras signaling pathway
Department of Molecular Pharmacology, School of Pharmacy, University of Southern California, Los Angeles 90033, USA.
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