Effects of pH on low-salt transition of chromatin core particles

The low-salt transition of chicken erythrocyte core particles containing uniform 145 base pair DNA was studied as a function of pH and of salt concentration. Intrinsic tyrosine fluorescence was used to follow the changes. Potassium salts of the anions C1-, H2PO4-, and SO4(2-) were indistinguishable in their ability to affect the transition. Divalent cations (Mg2+, Mn2+, Ca2+) were effective at 36-fold lower total concentration than monovalent cations (Li+, Na+, K+, Tris+), but no significant differences were observed within the two classes of cations. These results indicate that cation binding to the core particle is involved in the transition. At pH 9 the transition was broadened and shifted to higher monovalent cation concentration as compared to that at pH 6. At both pHs the fluorescence changes could be resolved into two steps by numerical least-squares analysis. On the basis of what is known about histone--histone interactions, a two-step mechanism is suggested, involving changes in the interactions between dimers of histones 2a and 2b with a tetramer of histones 3 and 4. The pH-induced changes appear to be correlated with a structural transition, which was detected as a function of pH at near physiological ionic strength (0.1 M). This structural change was accompanied by a small decrease in the tyrosine fluorescence anisotropy. An apparent pKa value near 7 is indicated, suggesting that the structural changes involved may be of physiological significance.