2 CONFORMATIONS OF RNA-POLYMERASE-II REVEALED BY ELECTRON CRYSTALLOGRAPHY

A new two-dimensional crystal form of yeast RNA polymerase II was obta ined in which the conformation of the enzyme appears ''open'', allowin g entry of DNA, as required for the initiation of transcription. By co ntrast, a previous crystal form contained the enzyme in a ''closed'' c onformation, appropriate for retention of DNA during RNA chain elongat ion. interaction with two polymerase subunits, Rpb4 and Rpb7, favors t he closed conformation, and binding of general transcription factor TF IIE may do so as well. The effect of Rpb4 and Rpb7, together with prev ious biochemical evidence, leads to the conclusion that the open to cl osed transition is a crucial step in the transcription initiation proc ess. (C) 1997 Academic Press Limited.