Single-pair FRET experiments on nucleosome conformational dynamics

Nucleosomes, the basic units of DNA compaction in eukaryotes, play a crucial role in regulating all processes involving DNA, including transcription, replication and repair. Nucleosomes modulate DNA accessibility through conformational dynamics like DNA breathing – the transient unwrapping of DNA from the nucleosome, repositioning of nucleosomes along the DNA, or partial dissociation. Single molecule techniques, in particular single-pair Fluorescence Resonance Energy Transfer (spFRET), have resolved such conformational dynamics in individual nucleosomes. Here, we review the results of FRET experiments on single nucleosomes, including fluorescence correlation spectroscopy (FCS), confocal single molecule microscopy on freely diffusing nucleosomes and widefield total internal reflection fluorescence (TIRF) microscopy on immobilized nucleosomes. The combined spFRET studies on single nucleosomes reveal a very dynamic organization of the nucleosome, that has been shown to be modulated by post-translational modifications of the histones and by DNA sequence.