| Description |
The eukaryotic genome is packaged into conserved protein-DNA structures called chromatin, which play crucial roles in various cellular processes like transcription, replication, repair and recombination. The basic repeating unit of chromatin is a nucleosome, which consists of 146 bp of DNA wrapped around a cylindrical histone octamer. Nucleosomes repress transcription by blocking the access of various factors to nucleosomal DNA. However, there are chromatin remodeling complexes (remodelers) the couple ATP hydrolysis to mobilize nucleosomes to increase factor access to nucleosomal DNA. We have utilized RSC (remodels the structure of chromatin), an essential and abundant remodeler for S. cerevisiae to study the mechanistic basis of chromatin remodeling. Using triple-helix strand displacement activity, and DNA length-dependent ATPase activity we established that RSC, and its isolated ATPase subunit Sth1, are DNA translocases. We found that a DNA, template, RSC/Sth1 tracked along one strand of the DNA duplex with a 3'?5' translocation polarity and a tracking requirement of one base. We further investigated how the property of directional ATP-dependent DNA translocation is utilized to access nucleosomal DNA. RSC, as well as the translocase domain of Sth1, bind to the nucleosome resulting in the generation of hypersensitive sites about two turns from either side of the nucleosomal dyad. Using nucleosome substrates bearing the different linker lengths, or bearing gaps of nicks in the DNA, we found that the restriction enzyme accessibility of the nucleosomal DNA involved RSC drawing DNA from one side of the nucleosome and pumping it out of the octamer from the other rend, to make the underlying DNA assessable. Based on these results, we propose that nucleosome mobilization involves directional DNA translocation along the surface of the histone octamer from a fixed internal site about two turns from the nucleosomal dyad. |
