A nuclear protein-modifying enzyme is responsive to ordered chromatin structure.

Poly (ADP-ribose) polymerase, a nuclear protein-modifying enzyme, binds to the internucleosomal linker region of chromatin, although it modifies certain core nucleosomal histones in addition to histone H1. The activity per unit of DNA chromatin changes with the nucleosome repeat number. It reaches a maximum on chromatin of 8-10 nucleosomes in length. As the complexity of chromatin with respect to nucleosome repeat number and compactness increases, a decline and stabilization of specific activity is noted. The difference in specific activity is maintained through resedimentation and dialysis of particles. It does not appear due to differences in polymer chain length or differential degradation of poly (ADP-ribose). The data suggest a relationship between ADP-ribosylation and chromatin organization and vice versa.