Distribution of histone F1 on calf thymus nucleohistone DNA.

A method of large-scale preparation of the histone F1-DNA complex by removing all other proteins from calf thymus nucleohistone was established. This involved gel filtration of nucleohistone through a column containing a band of sodium dodecyl sulfate. The F1-DNA complex obtained had the original amount of F1 and no other. The F1-DNA complex exhibited distinct two-step melting on thermal denaturation. The first step was apparently attributable to naked DNA regions and the second step, about 30 deg. C higher than the first step, to the regions covered with F1. Buoyant density experiments with the complex after fixation with formalin revealed that F1 was distributed fairly evenly over DNA fragments of an average molecular weight of about 4 × 10⁶. Electron microscopic examination of the complex after various degrees of denaturation with formalin indicated that the longest stretch of unbound DNA was about 0·3 μm.