Nucleosome assembly and disassembly pathways.

Nucleosome assembly and disassembly play a central role in the regulation of gene expression. Here we use PhAST (Photochemical Analysis of Structural Transitions) to monitor at the base pair level, structural alterations induced all along DNA upon histone binding or release. By offering the first consistent, detailed comparison of nucleosome assembly and disassembly in vitro, we are able to reveal similarities and differences between the two processes. We identify multiple intermediate states characterised by specific PhAST signatures; revealing a complexity that goes beyond the known sequential events involving (H3-H4)2 tetramer and H2A-H2B heterodimers. Such signatures localise and quantify the extent of the asymmetry of DNA/histone interactions with respect to the nucleosome dyad. This asymmetry is therefore defined by the localisation and amplitude of the signals. The localisation of the signal is consistent between assembly and disassembly and dictated by the DNA sequence. However, the amplitude component of this asymmetry not only evolves during the assembly and disassembly but does so differently between the two processes. Understanding the regulation of gene expression requires a complete knowledge of nucleosome dynamics. Our unexpected observation of differences between assembly and disassembly opens up new avenues to define the role of the DNA sequence in these processes. Overall, we provide new insights into how the intrinsic properties of DNA are integrated into a holistic mechanism that controls chromatin structure. Statement of SignificanceThis manuscript addresses the question of nucleosome dissociation compares with association. We used PhAST which is a non-intrusive photochemical technique to follow nucleosome dynamics at base pair resolution. We observed structural asymmetry during nucleosome turnover. We also showed for the first time that the process of nucleosome dissociation is not a reversal of association. This asymmetry favours intermediate states involved in chromatin organisation suggesting novel models for the role of nucleosome turnover in the epigenetic regulation of gene expression.