A mechanism for the action of chromatin remodeling complexes. (a) When a nucleosome covers a promoter, a chromatin remodeling complex can expose the promoter by pulling the DNA out of the nucleosome and into a loop. The loop can be propagated around the nucleosome, with the net effect of moving the nucleosome away from the promoter. (b) The shape of the chromatin remodeling complex RSC of yeast, based on three-dimensional image reconstruction from numerous images taken in the electron microscope with different views and tilts. By grouping the microscopically observed shapes into distinctive categories, different conformations can be deduced, as shown here.