Although structural characterizations of chromatin remodeling complexes would help to understand how this remodelers work, to date their electron microscopy reconstructions have been limited, because of its large macromolecular size and other impediments. In this paper, authors used a novel approach in cryo- electron microscopy, the orthogonal tilt reconstruction (OTR) method, to obtain a structure of the yeast RSC (remodel the structure of chromatin) complex, a remodeler that belongs to the SWI/SNF family. OTR consists on collecting data with a complete rectangle angle of difference, resulting on orthogonal views. With this new approach, they identified a conformational variability in the RSC structure (a region that opens and closes much like a turntable needle) that might have functional repercussions, because it could function as a entry point and it could promote the translocations of the nucleosomes. They also showed, using EMSA assays, that RSC has no preference for mono- or dinucleosomes. Perhaps what I found more interesting is that they employed optical tweezers to demonstrate, in single molecule experiments, that these translocations occur in mononucleosomes. All these results support their proposed model that fits only a mononucleosome, in both open and close conformations.
Taken together, the striking complementarity between the RSC central pocket and a nucleosome, the similar affinity of RSC for mono- and dinucleosomes, and the fact that the translocation observed with mononucleosomal substrates in single molecule assays is identical to that observed on nucleosomal arrays, strongly suggest that remodeling by RSC only requires the binding of a single nucleosome.
Besides of their conclusions respect to the RSC structure, as I am a big fan of single molecule experiments, I look forward to see more applications of optical tweezers and other biophysics tools in Epigenetics research.
Leschziner AE et al. 2007. Proc Natl Acad Sci USA 104(12):4913-4918
epigenetics
