Yeast Chd1p Unwraps the Exit Side DNA upon ATP Binding to Facilitate the Nucleosome Translocation Occurring upon ATP Hydrolysis.

Chromodomain-helicase-DNA-binding protein 1 (CHD1) remodels chromatin by translocating nucleosomes along DNA, but its mechanism remains poorly understood. We use single-molecule fluorescence experiments to clarify the mechanism by which yeast CHD1 (Chd1p) remodels nucleosomes. We find that binding of ATP to Chd1p induces transient unwrapping of the DNA on the exit side of the nucleosome, facilitating nucleosome translocation. ATP hydrolysis is required to induce nucleosome translocation. The unwrapped DNA after translocation is then rewrapped after the release of the hydrolyzed nucleotide and phosphate, revealing that each step of the ATP hydrolysis cycle is responsible for a distinct step of nucleosome remodeling. These results show that Chd1p remodels nucleosomes via a mechanism that is unique among the other ATP-dependent chromatin remodelers.

Multiple RPAs make WRN syndrome protein a superhelicase.

RPA is known to stimulate the helicase activity of Werner syndrome protein (WRN), but the exact stimulation mechanism is not understood. We use single-molecule FRET and magnetic tweezers to investigate the helicase activity of WRN and its stimulation by RPA. We show that WRN alone is a weak helicase which repetitively unwind just a few tens of base pairs, but that binding of multiple RPAs to the enzyme converts WRN into a superhelicase that unidirectionally unwinds double-stranded DNA more than 1 kb. Our study provides a good case in which the activity and biological functions of the enzyme may be fundamentally altered by the binding of cofactors.

NAP1L1 accelerates activation and decreases pausing to enhance nucleosome remodeling by CSB.

Cockayne syndrome protein B (CSB) belongs to the SWI2/SNF2 ATP-dependent chromatin remodeler family, and CSB is the only ATP-dependent chromatin remodeler essential for transcription-coupled nucleotide excision DNA repair. CSB alone remodels nucleosomes ∼10-fold slower than the ACF remodeling complex. Strikingly, NAP1-like histone chaperones interact with CSB and greatly enhance CSB-mediated chromatin remodeling. While chromatin remodeling by CSB and NAP1-like proteins is crucial for efficient transcription-coupled DNA repair, the mechanism by which NAP1-like proteins enhance chromatin remodeling by CSB remains unknown. Here we studied CSB's DNA-binding and nucleosome-remodeling activities at the single molecule level in real time. We also determined how the NAP1L1 chaperone modulates these activities. We found that CSB interacts with DNA in two principle ways: by simple binding and a more complex association that involves gross DNA distortion. Remarkably, NAP1L1 suppresses both these interactions. Additionally, we demonstrate that nucleosome remodeling by CSB consists of three distinct phases: activation, translocation and pausing, similar to ACF. Importantly, we found that NAP1L1 promotes CSB-mediated remodeling by accelerating both activation and translocation. Additionally, NAP1L1 increases CSB processivity by decreasing the pausing probability during translocation. Our study, therefore, uncovers the different steps of CSB-mediated chromatin remodeling that can be regulated by NAP1L1.