Transition from noncooperative to cooperative and selective binding of histone H1 to DNA

A transition from noncooperative to cooperative binding of DNA and histone h1 occurs between 20 and 40mMNaCI in 5 mM Tris-HCI, pH 7.5. Below 20mM NaCI in mixtures in H1 and excess DNA, H1 binds to all of the DNA molecules, causing them to sediment faster, and does not distinguish between DNA molecules that differ in size or base composition. However, at NaCI concentrations above the narrow transition range, H1 binds to only some of the DNA molecules and leaves the rest free. If the DNA molecules in a mixture are the same size, H1 selectively binds those that have the highest content of adenosine (A)+thymidine (T). By means of competion experiments at salt concentrations spanning the transition range, it is demonstrated that H1 selectivity requires cooperativity. A high degree of selectivity based on A + T content can be produced by cooperative binding: for average DNA sizes of 2 X10(6) daltons, more than ten molecules of calf lymphocyte DNA (57% a+ t) are chosen per molecule of Escherichia coli DNA (50% A+T).