Sequence-dependent nucleosome structural and dynamic polymorphism. Potential involvement of histone H2B N-terminal tail proximal domain.

Relaxation of nucleosomes on an homologous series (pBR) of ca 350-370 bp DNA minicircles originating from plasmid pBR322 was recently used as a tool to study their structure and dynamics. These nucleosomes thermally fluctuated between three distinct DNA conformations within a histone N-terminal tail-modulated equilibrium: one conformation was canonical, with 1.75 turn wrapping and negatively crossed entering and exiting DNAs; another was also "closed", but with these DNAs positively crossed; and the third was "open", with a lower than 1.5 turn wrapping and uncrossed DNAs. In this work, a new minicircle series (5S) of similar size was used, which contained the 5S nucleosome positioning sequence. Results showed that DNA in pBR nucleosomes was untwisted by approximately 0.2 turn relative to 5S nucleosomes, which DNase I footprinting confirmed in revealing a approximately 1 bp untwisting at each of the two dyad-distal sites where H2B N-terminal tails pass between the two gyres. In contrast, both nucleosomes showed untwistings at the dyad-proximal sites, i.e. on the other gyre, which were also observed in the high-resolution crystal structure. 5S nucleosomes also differ with respect to their dynamics: they hardly accessed the positively crossed conformation, but had an easier access to the negatively crossed conformation. Simulation showed that such reverse effects on the conformational free energies could be simply achieved by slightly altering the trajectories of entering and exiting DNAs. We propose that this is accomplished by H2B tail untwisting at the distal sites through action at a distance ( approximately 20 bp) on H3-tail interactions with the small groove at the nucleosome entry-exit. These results may help to gain a first glimpse into the two perhaps most intriguing features of the high-resolution structure: the alignment of the grooves on the two gyres and the passage of H2B and H3 N-terminal tails between them.