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Single particles of the mediator of transcriptional regulation (Mediator) and of RNA polymerase II holoenzyme were revealed by electron microscopy and image processing. Mediator alone appeared compact, but at high pH or in the presence of RNA polymerase II it displayed an extended conformation. Holoenzyme contained Mediator in a fully extended state, partially enveloping the globular polymerase, with points of apparent contact in the vicinity of the polymerase carboxyl-terminal domain and the DNA-binding channel. A similarity in appearance and conformational behavior of yeast and murine complexes indicates a conservation of Mediator structure among eukaryotes.
↵* Present address: Department of Cell Biology MB 25, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, CA 92037, USA.
↵† Present address: Institute of Medical Biochemistry, Göteborg University, Medicinaregatan 13, 405 30 Göteborg, Sweden.
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