Salt-induced structural changes in nucleosomes.

Ehrlich ascites tumor (EAT) nucleosomes treated with increasing NaCl concentrations were analyzed by sucrose density gradient centrifugation. Two events were found to take place in the course of the salt treatment: a) increasing amounts of nucleosomes dissociated into free DNA and protein in the interval 0.6M-1.5M NaCl, and b) the sedimentation coefficient of the nucleosomes decreased from 11S to 8S in the interval 0.6M - 1M NaCl. This decrease was not caused by loss of protein and was fully reversible upon slow and gradual lowering of the ionic strength. This shows that before dissociation of the protein core from DNA, nucleosomes undergo a structural transition. The electron microscopic observations revealed that it consisted in detachment of the ends of nucleosomal DNA from the protein core. It is suggested that an arginine-rich domain in the protein core exists, which holds more tightly the central part of the nucleosomal DNA, while its ends are relatively loosely bound to lysine-rish domains.