Nucleosome Dynamics Studied by Single Molecule FRET

We studied the mechanism of nucleosome opening to allow DNA access, using single molecule FRET. Here we show evidence for a previously uncharacterized intermediate structural state that occurs before H2A-H2B dimer release, and is characterized by an increased distance between H2B and the nucleosomal dyad. Our data suggest that the first step in nucleosome disassembly is the opening of the (H3-H4)2 tetramer/(H2A-H2B) dimer interface, followed by H2A-H2B dimer release from the DNA and, lastly, (H3-H4)2 tetramer removal (see Figure). We estimate that the open intermediate state is populated at 0.2 - 3 % under physiological conditions, and could have significant in vivo implications for factor-mediated histone removal and exchange, as well as for regulating DNA accessibility to the transcription and replication machinery.Additionally, histone variants and histone modifications could substantially change the stability of the nucleosome. Our most recent spFRET data indicate that such effects may occur both at the level of DNA-histone and histone-histone interactions.View Large Image | View Hi-Res Image | Download PowerPoint Slide