Human TFIIA in the
TFIIA-TBP-DNA Ternary Complex
Allison Hector '21 and Callie Gompf-Phillips '21
Contents:
I. Introduction
The formation of a multi-subunit preinitiation complex (PIC) is necessary for the initiation of RNA polymerase II found in eukaryotes. This complex, composed of general transcription factors, binds the promoter sequence and melts the DNA, poising the polymerase for the transition to the elongation complex. Transcription factor (TF) IIA is the first of the general transcription factors to join the PIC assembly following TBP recognition and distortion of the TATA sequence.
(TBP is a subunit of TFIID responsible for binding the core promoter). TFIIA performs functions necessary during RNA polymerase II transcription: stabilizing TFIID binding to the core promoter and mediating the effects of upstream activators through interactions with the TBP. TFIIA is required for basal and activated transcription for many protein-encoding genes.
II. General Structure
The ternary is composed of human TFIIA, human TBP, and a 17bpcontaining DNA fragment. TFIIA is constructed from twopolypeptide chains aB and y which altogether results in a structure comprised of aperpendicular to aextending away from TBP. Once translation has been completed in differentiated cells, the aB polypeptide is proteolyzed to generate the subunits TFIIAa and TFIIAB. TFIIA joins the TBP/DNA complex by adhering to the N-terminal stirrup region of TBP and generally contacting the DNA backbone near the TATA-box.
III. TFIIA/TBP Binding Interface
TFIIA contacts TBP through a parallel B-sheetjoining the edge of TFIIA�s B-barrel (TFIIAy S2, TFIIAB C-terminal, TFIIAB S2-S3 loop) to the downstream stirrup region of the TBP anti-parallel B-sheet by interacting with TBP H1, S2, S3 and the S4 loop.TFIIAy S2 Y65 and F67 to TBP S2 N189, A190, and Y192 are necessary to stabilize the binding interface between TFIIA and TBP.to carbonyl groups A184, L185, and A187 in TBP H1 accommodate TFIIAy�s R66 guanidine group. The TBP N189 side chain additionally hydrogen bonds directly to the TFIIAy Y65 amide group extending the B-arrangement.
The TBP R205 side chain forms asurrounding the W376 indol ring by making hydrogen bonds to the TFIIAB terminal carboxyl, TFIIAy L62 and N63 main-chain carbonyl groups. TBP R208 further contributes to pocket formation by making a hydrogen bond to the TFIIAB terminal carboxyl group.
IV. DNA Binding
The TBP covers the minor groove of the TATA box of the DNA while TFIIA is connected to the N-terminal that is downstream and extends to contact the DNA backbone both within and upstream of the TATA box. The interaction between the DNA backbone and TFIIA does not alter the DNA path within the TATA box. TFIIABmake hydrogen bonds to the phosphate groups of bases -11 and -13 in the TATA box.make hydrogen bonds to the phosphate groups of bases 6 and 7 in the upstream coding strand. All these interactions need DNA distortion.
V. Future Studies
Here, the formation of the promoter-bound TFIIA/TBP complex is characterized. This contributes to our understanding of the regulation of the transcription assembly complex. In the future, we can study the interactions between TFIIA with other TBP-associated factors (TAFs) as well as other general transcription factors such as TFIIB. By understanding how transcription regulation operates in humans, we can more easily identify errors in the process and begin to study how to fix them.
VI. References
Bleichenbacher, M., Tan, S., & Richmond, T. J. Novel interactions between the components of human and yeast TFIIA/TBP/DNA complexes. Journal of Molecular Biology, 332 (2003): 783-793.
Hori, R., & Carey, M. Protease footprinting analysis of ternary complex formation by human TFIIA. J. Biol. Chem. 272 (1997):1180�1187.
Ozer, J., Moore, P. A., Bolden, A. H., Lee, A., Rosen, C. A., & Lieberman, P. M. Molecular cloning of the small (gamma) subunit of human TFIIA reveals functions critical for activated transcription. Genes Dev. 8 (1994), 2324�2335.
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