Poly-ADP-ribosylated histones: potent DNA suppressors

When rat liver nuclear chromatin was sonicated in buffer containing 0.35 M (NH4)2SO4 to release the engaged RNA polymerases, a potent inhibitor was also released. This inhibitor elicited dramatic inhibition of RNA synthesis regardless of whether the free or engaged RNA polymerase was used. On further analysis, it became apparent that the site of inhibition was on the DNA template, not on the enzyme. This inhibitor could be extracted into 0.25 N HCl by the standard procedure for the isolation of histones. This acid-soluble inhibitor, showing typical histone band on gel, was RNase A and DNase I resistant, but was sensitive to both pronase and snake venom phosphodiesterase digestion, as well as to 0.1 N KOH hydrolysis. Furthermore, when [14C]adenine labeled poly-ADP-ribosylated histones were digested by snake venom phosphodiesterase, the release of radioactivity was in parallel to the loss of inhibitor activity. We conclude that the inhibitor substances are poly-ADP-ribosylated histones and propose that the poly-ADP-ribosylated histones rather than the histones are the natural suppressors of the gene.