Alterations in the polynucleosomal structure of chromatin by poly ADP-ribosylation of nuclear proteins

Abstract

The chromatin-associated enzyme, poly (ADP-Rib) polymerase causes an NAD-dependent aggregation of modified oligonucleosomes as demonstrated by electrophoretic and sedimentation analysis. It has been speculated that poly ADP-ribosylation of histone H1 and its subsequent crosslinking may be crucial in this aggregation. The present work describes some approaches we have taken to investigate this hypothesis. We have characterized the synthesis of poly (ADP-Rib)-H1 dimer in isolated nucleosomes, and using a new immunological method have demonstrated the occurrence of this H1 adduct in living cells. The faithful reassociation of histone H1 with chromatin previously depleted of this histone, has been a major aim of our investigations in order to study the above phenomenon. Our approach has been to monitor this reconstitution by the accuracy of poly ADP-ribosylation of this histone. This criterion was assumed to be a highly sensitive indicator of H1-H1 alignment along oligonucleosome fibers, since a crosslinked dimer of H1 is one product of this reaction. Of special interest was the observation that the removal of H1 from nucleosomes abolished the NAD-promoted aggregation of poly ADP-ribosylated chromatin. This activity could be totally restored with the reconstitution of histone H1. The results suggest a major role for histone H1 and poly ADP-ribosylation in chromatin organization.