Dynamic unwrapping of nucleosomes by HsRAD51 that includes sliding and rotational motion of histone octamers

Wrapping of genomic DNA into nucleosomes poses thermodynamic and kinetic barriers to biological processes such as replication, transcription, repair and recombination. Previous biochemical studies have demonstrated that in the presence of adenosine triphosphate (ATP) the human RAD51 (HsRAD51) recombinase can form a nucleoprotein filament (NPF) on double-stranded DNA (dsDNA) that is capable of unwrapping the nucleosomal DNA from the histone octamer (HO). Here, we have used single molecule Förster Resonance Energy Transfer (smFRET) to examine the real time nucleosome dynamics in the presence of the HsRAD51 NPF. We show that oligomerization of HsRAD51 leads to stepwise, but stochastic unwrapping of the DNA from the HO in the presence of ATP. The highly reversible dynamics observed in single-molecule trajectories suggests an antagonistic mechanism between HsRAD51 binding and rewrapping of the DNA around the HO. These stochastic dynamics were independent of the nucleosomal DNA sequence or the asymmetry created by the presence of a linker DNA. We also observed sliding and rotational oscillations of the HO with respect to the nucleosomal DNA. These studies underline the dynamic nature of even tightly associated protein-DNA complexes such as nucleosomes.