Nucleosome
Structure
David Marcey
© 2006
I.
Introduction
II. The Histone Octamer
III. The DNA Superhelix
IV. References
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I.
Introduction
The nucleosome
core particle contains two copies of each histone
protein (H2A, H2B, H3 and H4) and 146 basepairs (bp) of superhelical
DNA wrapped around this histone octamer. It represents the first order
of DNA packaging in the nucleus and as such is the principal structure
that determines DNA accessibility.
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II. The Histone
Octamer
Two copies
of each histone protein, H2A,
H2B, H3, and
H4, are assembled into an octameric disc.
Although each monomer has an N-terminal tail that projects from the
histone core, the structure at left shows only a single tail from
one H3 monomer.
All four
histone proteins share a highly similar structural motif, the histone
fold, comprising three alpha helices connected by two loops (shown
here for H2A):
alpha1-loop1-alpha2-loop2-
alpha3.
The histone
octomer consists of four histone heterodimers: two each of H3-H4
and H2A-H2B.
The histone
fold motifs of the heterodimers are arranged with the loop1
of one monomer closely juxtaposed to the loop2
of the second monomer, shown here for a
H3-H4
heterodimer. An axis of symmetry passes between the two long alpha2-helices
of the two monomers.
The H3-H4 heterodimers pair to form a tetramer through interactions of a four-helix
bundle (alpha2 and alpha3
of H3 from each dimer). The association
of this (H3-H4)2
tetramer with DNA is the first step in nucleosome assembly.
Each H2A-H2B
heterodimer binds to the (H3-H4)2
tetramer via another, homologous, four-helix bundle (alpha2
and alpha3 from both H2B
and H4), joining the H2B
and H4 histone folds.
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III. The DNA
Superhelix
146 bp of
double helical DNA are wrapped around
the histone octamer in a superhelix.
A two-fold symmetry axis falls between
the H3-H4
heterodimers and the
H2A-H2B
heterodimers.
This
axis of symmetry intersects a single bp
in the center of the superhelix, dividing the 146 bp DNA into 72
bp and 73 bp halves defined by
the central bp.
The
left-handed DNA superhelix wraps around the histone core in 1.65 turns.
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IV. References
Luger, K., Mader, A.W., Richmond, R.K., Sargent, D.F., Richmond,
T.J. Crystal structure of the nucleosome core particle at 2.8 A resolution.
Nature v389 pp.251-260 , 1997
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