The Role of H2A-H2B Dimers in the Mechanical Response of Single Nucleosomes

Abstract

During transcription RNA polymerase frequently evicts the H2A-H2B dimers from nucleosomes. This might be necessary to access the DNA but is also believed to mediate a feedback mechanism by which transcription of a gene facilitates the exchange of epigenetic marks and histone variants. This in turn can then immediately influence gene expression by consecutive RNA polymerases. Little is known about the role that H2A-H2B dimers play in the response of chromatin to mechanical stresses during transcription.In order to elucidate the fate of dimers during transcription, we studied DNA unwrapping from single nucleosomes in the presence of an external force using magnetic tweezers. We subjected reconstituted nucleosomal arrays, containing up to 48 nucleosomes, to tension and monitored the DNA length in relation to force. We find significant differences in the DNA unwrapping forces between nucleosomes containing canonical histones only, the H2AvD histone variant and H3-H4 tetramers. This reveals an important role of the H2A-H2B dimer during mechanical DNA disruption from the nucleosome, as might occur during transcription.