The Modification of Deoxyribonucleohistone by Trypsin and Chymotrypsin

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Abstract
Trypsin and chymotrypsin completely degraded whole histone in less than 15 min as judged by changes in the peptide dichroic band at 220 nm and gel electrophoresis patterns. By contrast, the action of trypsin in deoxyribonucleohistone under conditions of low ionic strength and relatively low concentrations of substrate (200—600 μg/ml) produced partial degradation of f1 and f3 only after 3‐h incubation with enzyme. Finally, all fractions were degraded.Chymotrypsin caused rapid and almost complete degradation of f1 in nucleohistone in less than 1 h and f3 in less than 3 h. F2b was degraded after a lag of about 30 min. All molecules of f1, f3 and f2b were attacked inside 6 h. The degradation of f2b gave rise to a large peptide fragment, not observed in digests of whole histone, which remained bound to the DNA in 0.7 M NaCl. F2al and f2a2 were not significantly degraded.Degradation of f3 was accompanied by an increase in viscosity and the nucleohistone lost its rigid, rod‐like properties, becoming more flexible. Thus f3 is involved in maintaining the supercoil in its rigid conformation.The sites of trypsin attack are not initially accessible in bound histone fractions f2a1, f2a2 and f2b, whereas these sites appear to be partially accessible in f1 and f3. On the other hand the chymotryptic cleavage points are accessible in bound f1, f3 and f2b but not in f2a1 and f2a2. The all‐or‐nothing character of the degradation suggests that f3 and f2b are regularly arranged along the axis of the supercoiled nucleohistone.