Histone-histone interaction mediates chromatin unfolding at physiological ionic strength

High-resolution thermal denaturation data on chicken erythrocyte chromatin are reported over 4 orders of magnitude in NaCl concentration which includes the physiological region. A novel technique using critical-point polyacrylamide sols instead of ordinary solvents effectively stabilizes chromatin against precipitation at high salt concentrations. These sols are optically transparent from 260 to 320 nm and are thermally stable over the temperature ranges studied. At Na+ ion concentrations below 10 mM, the polyacrylamide slightly destabilizes chromatin at the nucleosome level, possibly through interactions of histones H1 and H5 with the carboxylic acid residues. At the same low salts, polyacrylamide stabilizes pure DNA against denaturation, presumably by mechanically stabilizing it against helix-distorting thermal fluctuations. In both cases, however, the polyacrylamide sols are entirely noninvasive at higher salts. Prominent low-temperature thermal transitions are observed in chromatin at and above 100 mM NaCl which evidently are associated with conformational changes in DNA. Our results are in accord with the idea that histone-histone interactions at physiological ionic strengths (approximately 100 mM Na+) may be comparable to histone-DNA interactions and hence may be sufficient to promote the destabilization of the DNA helix in chromatin under these conditions. The biological implications of this are discussed, and a possible model for the local decondensation of chromatin under physiological conditions is proposed.