Two conformations of RNA polymerase II revealed by electron crystallography.

A new two-dimensional crystal form of yeast RNA polymerase II was obtained in which the conformation of the enzyme appears "open", allowing entry of DNA, as required for the initiation of transcription. By contrast, a previous crystal form contained the enzyme in a "closed" conformation, appropriate for retention of DNA during RNA chain elongation. Interaction with two polymerase subunits, Rpb4 and Rpb7, favors the closed conformation, and binding of general transcription factor TFIIE may do so as well. The effect of Rpb4 and Rpb7, together with previous biochemical evidence, leads to the conclusion that the open to closed transition is a crucial step in the transcription initiation process.