Article,
A Theory of DNA Dissociation from the Nucleosome
J. Mol. Biol., (1995)
Abstract
Previous analysis of an elastic model of the nucleosome indicated
that 10 bp end segments of DNA can exist in a continuum of mechanically
stable trajectories ranging from complete winding on the histone
octamer to complete unwinding. Stable states of 20 bp and 40 bp end
segments, however, are grouped in bands separated by gaps where DNA
trajectories are unstable. We extend these results to cover the entire
range up to a complete nucleosomal turn, 80 bp. We find that 10 to
60 bp segments have states intermediate between fully wound and fully
unwound that are mechanically stable. In striking contrast, there
is no stable intermediate trajectory for 70 bp or 80 bp segments.
Segments of these lengths constitute a two-state system. A 70 or
80 bp segment is either fully wound or fully unwound, and the population
of these states is governed by Boltzmann's thermal distribution We
have found a plausible dissociation pathway from the fully wound
to the fully unwound state for the 80 bp segment. In a ponderous
breathing motion that breaks all contacts with the histone surface,
the segment climbs to an activation peak of about 12 kcal/mol, then
rapidly straightens away from the histone core to complete dissociation.
(C) 1995 Academic Press Limited
