The structure of E. coli core RNA polymerase (RNAP) has been determined to ∼23 Å resolution by three-dimensional reconstruction from electron micrographs of flattened helical crystals. The structure reveals extensive conformational changes when compared with the previously determined E. coli RNAP holoenzyme structure, but resembles the yeast RNAPII structure. While each of these structures contains a thumb-like projection surrounding a channel 25 Å in diameter, the E. coli RNAP holoenzyme thumb defines a deep but open groove on the molecule, whereas the thumb of E. coli core and yeast RNAPII form part of a ring that surrounds the channel. This may define promoter-binding and elongation conformations of RNAP, as E. coli holoenzyme recognizes promoter sites on double-stranded DNA, while both E. coli core and yeast RNAPII are elongating forms of the polymerase and are incapable of promoter recognition. © 1995.
Polyakov, A., Severinova, E., & Darst, S. A. (1995). Three-dimensional structure of E. coil core RNA polymerase: Promoter binding and elongation conformations of the enzyme. Cell, 83(3), 365–373. https://doi.org/10.1016/0092-8674(95)90114-0