COMPACTIZATION OF INDIVIDUAL DNA-MOLECULES DURING INTERACTION WITH HISTONE H1

DNA complexes with histone H1 were investigated by the protein-free film method, in which compact intramolecular structures arising from interaction of individual DNA molecules with histone H1 are obtained. When histone H1 was added to the hypophase at concentrations of 0.2-5-mu-g/ml, three types of structures were seen on electron microscopic preparations: noncompacted DNA strands; compact fibrils with twisted DNA strands; and compact, rod-like and circular structures in which separate duplex DNA strands could not be distinguished. The morphology of the compact structures suggests that they are formed as a result of side-by-side association of DNA fibrils, as in the formation of triple rings during compactization of circular DNA in the complex with trivaline. At higher ionic strengths there is a tendency for transition from structures of the second type to structures of the third type, which may be explained by a transition from noncooperative to cooperative binding of histone H1 to DNA.