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The secondary structure of the calf thymus core histone octamer,
(H2A-H2B-H3-H4)2, and its two physiological subunits, the H2A-H2B dimer and
(H3-H4)2 tetramer, was analyzed by ORD spectropolarimetry as a function of
temperature and solvent ionic strength within the ranges of these experimental
parameters where assembly of the core histone octamer exhibits pronounced
sensitivity. While the secondary structure of the dimer is relatively stable
from 0.1 to 2.0 M NaCl, the secondary structure of the tetramer exhibits complex
changes over this range of NaCl concentrations. Both complexes exhibit only
modest responses to temperature changes. ORD spectra of very high and very low
concentrations of stoichiometric mixtures of the core histones revealed no
evidence of changes in the ordered structure of the histones as a result of the
octamer assembly process at NaCl concentrations above 0.67 M, nor were
time-dependent changes detected in the secondary structure of tetramer dissolved
in low ionic strength solvent. The secondary structure of the chicken
erythrocyte octamer dissolved in high concentrations of ammonium sulfate,
including those of our crystallization conditions, was found to be essentially
unchanged from that in 2 M NaCl when examined by both ORD and CD
spectropolarimetry. The two well-defined cleaved products of the H2A-H2B dimer,
cH2A-H2B and cH2A-cH2B, exhibited reduced amounts of ordered structure; in the
case of the doubly cleaved moiety cH2A-cH2B, the reductions were so pronounced
as to suggest marked structural rearrangements.
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