Nucleosome Dynamics during Transcription Elongation

The nucleosome is the basic packing unit of the eukaryotic genome. Dynamic interactions between DNA and histones in the nucleosome are the molecular basis of gene accessibility regulation that governs the kinetics of various DNA-templated processes such as transcription elongation by RNA Polymerase II (Pol II). On the basis of single-molecule FRET measurements with chemically modified histones, we investigated the nucleosome dynamics during transcription elongation and how it is affected by histone acetylation at H3 K56 and the histone chaperone Nap1, both of which can affect DNA-histone interactions. We observed that H3K56 acetylation dramatically shortens the pause duration of Pol II near the entry region of the nucleosome, while Nap1 induces no noticeable difference. We also found that the elongation rate of Pol II through the nucleosome is unaffected by the acetylation or Nap1. These results indicate that H3K56 acetylation facilitates Pol II translocation through the nucleosome by assisting paused Pol II to resume and that Nap1 does not affect Pol II progression. Following transcription, only a small fraction of nucleosomes remain intact, which is unaffected by H3K56 acetylation or Nap1. These results suggest that (i) spontaneous nucleosome opening enables Pol II progression, (ii) Pol II mediates nucleosome reassembly very inefficiently, and (iii) Nap1 in the absence of other factors does not promote nucleosome disassembly or reassembly during transcription.