By using direct competition experiments, the binding of histone H1AB (a mixture of H1A and H1B) and H1° to superhelical and linear DNA forms was studied. Mouse liver H1 isohistones and plasmid pαGD containing part of the 5′ flanking and part of the coding sequence of the mouse α-globin gene in pUC18 were used as partners in the binding reaction. The competition experiments were performed by direct mixing of the histone with labelled supercoiled DNA (at 125 mM NaCI and at a histone/DNA ratio of 1.0 and addition to the mixture of increasing amounts of cold competitor DNA, either supercoiled or linear. The radioactivity of the complex formed was determined by filter binding. The results show that both histones H1 and H1° possess a strong binding preference for supercoiled DNA forms. Thus, histone H1° resembles the regular somatic set of histone H1 and not the other differentiation-specific histone H5 studied thus far.
