Acceptors for the poly ADP-ribosylation modification of chromatin structure are altered by carcinogen-induced DNA...

Poly(ADP-Rib) polymerase is activated by strand breaks in DNA and appears to play an important role in DNA repair. The enzyme catalyses the poly ADP-ribosylation of histones and non-histone proteins, yet the contribution of these major alterations in chromatin composition have, as yet, not been critically evaluated with regard to DNA strand breaks. In the present study, the effects of N-methyl-N-nitrosourea (MNU) upon the poly ADP-ribosylation of nuclear protein acceptors have been identified and quantified at the oligonucleosomal level of chromatin. Treatment of HeLa cells with MNU (4.5 mM) for 1 h resulted in a reduction in the cellular NAD pool (30%), a 2-3 fold stimulation of poly ADP-ribosylation in isolated nuclei and in isolated oligonucleosomes. Of acceptors modified, the automodification of the polymerase was stimulated at least 3-fold. Analysis of the acid-soluble acceptors showed a stimulation in the modification of the core histones and a 2-fold increase in histone H1 poly ADP-ribosylation. This modification causes a novel crosslinking of the latter histone, and this has been studied as it relates to DNA strand breaks in the present work. In vivo treatment with MNU resulted in the synthesis of longer chain or more complex polymer species at the expense of the shorter chained ADP-ribose moieties.