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Figure 2.
Figure 2 H3-H4 histone-fold pair. The  1-L1-
2-L2-
3
structural elements are shown. A pseudodyad axis of symmetry
runs vertically through SHL1.5. Side chains that make hydrogen
bonds or hydrophobic interactions with the DNA backbones, and
arginines inserted in the minor groove are shown.
Main-chain-to-DNA-phosphate hydrogen bonds are shown in magenta.
Leucine 65 is in contact with a thymidine methyl group (yellow
bond). Two of three arginine-threonine pairs are hydrogen bonded
(cyan bond). b, H2A-H2B histone-fold pair. As in a, except that
the pseudodyad axis runs through SHL4.5. c, H3-H4 L1L2
DNA-binding site. The H3 L1 and H4 L2 loops make 3 hydrogen
bonds (cyan) with each other in a parallel  -structure.
The L2 loop contains buried hydrogen bonds (cyan) between H4-R78
and H4-D85. The hydrogen-bonding interactions (magenta) between
protein and DNA-phosphate groups involve both main-chain amide
atoms and side-chain atoms. d, H2A-H2B L1L2 DNA-binding site. In
contrast to the H3 L1 and H4 L2 loops, the other three L1L2
sites have only one well formed -structure
hydrogen bond, as for H2A L1 and H2B L2 shown here (cyan). An
arginine (H2B-R83) hydrogen bonds to a phosphate group instead
of an acidic side chain (H2B-E90), as occurs at the homologous
sites in the other L1L2 loops. e, H3'-H3 4-helix bundle. The
H3-H4 histone pairs form the tetramer through the interaction of
the C-terminal halves of the 2
helices and the 3
helix of H3' and H3 across the molecular dyad axis. Histidine
113' makes a buried hydrogen bond to link the H3 molecules. In
addition, several hydrophobic interactions occur. f, H2B-H4
4-helix bundle. The H2A-H2B dimer assembles into the histone
octamer through the interaction of H2B with H4. The hydrogen
bond of H4-H75 to H2B-E90 is analogous to that in H3'-H3, but is
not buried here because of the orientation of H2B-R83. Across
the pseudo-twofold axis, however, H2B-Y80 replaces the H4
histidine and, together with H4-Y72 and H4-Y88, forms a
hydrophobic cluster.
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