Nucleic Acids Research, Vol 24, Issue 8 1472-1480, Copyright © 1996 by Oxford University Press
KJ Neil, RA Ridsdale, B Rutherford, L Taylor, DE Larson, M Glibetic, LI Rothblum and G Harauz
We have studied the structure of recombinant rat UBF (rrUBF), an RNA
polymerase I transcription factor, by electron microscopy and image
analysis of single particles contrasted with methylamine tungstate.
Recombinant rat UBF appeared to be a flat, U-shaped protein with a central
region of low density. In the dominant projections, 2-fold mirror symmetry
was seen, consistent with the dimerization properties of this molecule, and
of dimensions in agreement with the length of DNA that rat UBF protects in
footprinting studies. Electron microscopy of various rrUBF-DNA complexes
confirmed that our recombinant protein was fully able to bind the 45S rDNA
promoter, and that it caused substantial bends in the DNA. Upon extended
incubation in a droplet covered by a lipid monolayer at the liquid-air
interface, rrUBF formed long filamentous arrays with a railway track
appearance. This structure was interpreted to consist of overlapping rrUBF
dimers 3.5 nm apart, which value would represent the thickness of the
protein. Our results show rrUBF to interact with and bend the promoter DNA
into a roughly 10 nm diameter superhelix. Based on all these electron
microscopical results, an atomic structure was predicted by homology
modelling of the HMG fingers, and connected by energy minimized intervening
segments.
ARTICLES
Structure of recombinant rat UBF by electron image analysis and homology modelling
Department of Molecular Biology and Genetics, University of Guelph, Ontario, Canada.
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