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Ionic strength‐dependent conformational transitions of chromatin. Circular dichroism and thermal denaturation studies
Author(s) -
Fulmer Andrew W.,
Fasman Gerald D.
Publication year - 1979
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.1979.360181115
Subject(s) - hyperchromicity , chromatin , ionic strength , chemistry , dna , circular dichroism , micrococcal nuclease , crystallography , histone , biophysics , denaturation (fissile materials) , ionic bonding , nucleosome , biochemistry , aqueous solution , biology , organic chemistry , ion , nuclear chemistry
High‐molecular‐weight chicken erythrocyte chromatin was prepared by mild digestion of nuclei with micrococcal nuclease. Samples of chromatin containing both core (H3, H4, H2A, H2B) and lysine‐rich (H1, H5) histone proteins (whole chromatin) or only core histone proteins (core chromatin) were examined by CD and thermal denaturation as a function of ionic strength between 0.75 and 7.0 × 10 −3 M Na + . CD studies at 21°C revealed a conformational transition over this range of ionic strengths in core chromatin, which indicated a partial unfolding of a segment of the core particle DNA at the lowest ionic strength studied. This transition is prevented by the presence of the lysine‐rich histones in whole chromatin. Thermal‐denaturation profiles of both whole and core chromatins, recorded by hyperchromicity at 260 nm, reproducibly and systematically varied with the ionic strength of the medium. Both materials displayed three resolvable thermal transitions, which represented the total DNA hyperchromicity on denaturation. The fractions of the total DNA which melted in each of these transitions were extremely sensitive to ionic strength. These effects are considered to result from intra‐ and/or internucleosomal electrostatic repulsions in chromatin studied at very low ionic strengths. Comparison of the whole and core chromatin melting profiles indicated substantial stabilization of the core‐particle DNA by binding sites between the H1/H5 histones and the 140‐base‐pair core particle.