Deborah L. Johnson, Ph.D.

Molecular Pharmacology and Toxicology
Biochemistry & Molecular Biology

RNA polymerase (pol) III Transcription

My laboratory's overall interest is to define specific mechanisms by which eucaryotic genes synthesize RNA products, and to ultimately understand how this transcription process is regulated within cells. We have focussed our studies on analyzing the transcription process carried out by RNA polymerase (pol) III. This enzyme is responsible for synthesizing many small untranslated cellular and viral RNAs. In addition to this enzyme, other transcription factor proteins are required to specifically transcribe RNA pol III-dependent genes in vitro. Our aim is to delineate the various ordered interactions that occur between the transcription factor proteins and the DNA , and between the proteins themselves, which define the transcription process. Since we have discovered that certain viral proteins can modulate the activity of RNA pol III genes, we are also examining the mechanism for how these proteins regulate gene activity.

We have shown that the hepatitis B virus protein product, X, is a potent activator of RNA pol III genes. Since evidence supports that X plays a key role in the development and progression of liver disease in individuals chronically infected with the virus, we are interested in defining how X activates specific cellular genes. Examining potential alterations in the activities of the RNA pol III transcription factors, we find that X specifically modulates an increase in transcription factor B (TFIIIB) activity by increasing the cellular levels of one of its subunits, the TATA-binding protein (TBP). We have also discovered that TBP is the limiting component for the expression of RNA pol III genes. Since TBP is required for the transcription of all cellular genes, the X-mediated increase in TBP may be the single most important event that allows it to activate so many different types of cellular genes. Thus, we are further studying how X mediates the increase in cellular TBP, and we are identifying other cellular genes that are limiting for TBP and responsive to X. We have also shown that both the X-mediated increases in RNA pol III gene activity and cellular TBP require the activation of cellular protein kinases, most specifically the proto-oncogene, ras. We are currently determining which proteins within the ras cellular signaling cascade are altered in response to X. These and other studies are aimed at elucidating cellular targets of the X protein and the biochemical consequences of its activity on gene expression that lead to cellular transformation.

Selected Publications

1. Vilalta, A., Kickhoefer, V.A., Rome, L.H., and Johnson, D.L. The rat vault RNA gene contains a unique RNA polymerase III promoter composed of both external and internal promoters that function synergistically. J. Biol. Chem. 269: 29752-29759 (1994).
2. Wang, H.D., Yuh, C.H., Dang, C.V., and Johnson, D.L. "The hepatitis B virus X protein increases cellular TATA-binding protein and transactivates Class III genes." Mol. Cell. Biol. 15: 6720-6728 (1995).
3. Gottesfeld, J.M., Johnson, D.L., and Nyborg, J.K. "Transcriptional activation of RNA polymerase III-dependent genes by the human T-cell leukemia virus Type 1 Tax protein." Mol. Cell. Biol. 16: 1777-1785 (1996).
4. Trivedi, A., Vilalta, A., Gopalan, S. and Johnson, D.L. "TATA-binding protein is limiting for both TATA-containing and TATA-lacking RNA polymerase III promoters in Drosophila cells." Mol. Cell. Biol. , in press (1996).