A conformational study of the binding of a high mobility group protein with chromatin

The nature of the binding of a high mobility group protein (HMG 17) to native and H1-H5-depleted chicken erythrocyte chromatin was studied, as a function of ionic strength, using circular dichroism and thermal denaturation techniques. The circular dichroism properties of the HMG 17-reconstituted whole chromatin and H1-H5-depleted chromatin demonstrated that a condensation of chromatin structure occurred upon HMG 17 binding at low ionic strength (1 mM Na phosphate, 0.25 mM EDTA, pH 7.0). Thermal denaturation profiles confirmed this change in the structure of chromatin induced by HMG 17. Thermal denaturation profiles were resolved into three-component transitions. In general, a shift in the temperature of maximum dh/dT for each transition (Tm) was observed for all transitions upon HMG 17 binding. DNA melting in the first transition, originating from linker regions of whole chromatin, was nearly totally depleted and was distributed mainly into the highest melting transition. The same trends were also observed in H1-H5-depleted chromatin. These results indicate that the binding sites of HMG 17 are situated in the linker regions immediately adjacent to the core. The nature of the interaction of HMG 17 at higher ionic strength (50 mM NaCl, 1 mM Na phosphate, 0.25 mM EDTA, pH 7.0) with whole chromatin and H1-H5-depleted chromatin was found to be different but a decrease in [theta] values was found in both chromatins. These observations suggest that HMG 17 does not loosen chromatin structure but produces an overall stabilization and condensation of structure. The implications of these results to the currently accepted models of transcriptionally active chromatin are discussed.