Polynucleosomal Synthesis of Poly(ADP‐ribose) Causes Chromatin Unfolding as Determined by Micrococcal Nuclease Digestion

We have evaluated the influence of protein poly(ADP-ribosyl)ation in the relaxation of chromatin by exposing a rat liver polynucleosomal extract to micrococcal nuclease (MNase) digestion. The kinetic susceptibility of polynucleosomes to endonuclease digestion was determined as a function of the time of incubation as well as endonuclease concentration. To validate our assay, we also ran control experiments with protein-free calf thymus DNA as the opposite of polynucleosomal DNA. Rat liver chromatin was also incubated in the absence or presence of exogenously added 200 microM betaNAD(+), the poly(ADP-ribosyl)ation substrate, before MNase digestion. For incubations in the presence of betaNAD(+), the synthesis of polynucleosomal poly(ADP-ribose) was stopped with 1 mM benzamide. After addition of MNase, endonuclease digestion was blocked with EDTA to chelate the Mg(2+) ions needed for enzymatic activation, and the samples were subjected to electrophoresis through 1.5% agarose gels. As expected, a faster degradation of chromatin into oligonucleosomal DNA ladders was observed upon protein poly(ADP-ribosyl)ation when the chromatin extract was preincubated with 200 microM betaNAD(+). Thus, our results are consistent with the conclusion that the covalent poly(ADP-ribosyl)ation of polynucleosomal proteins favors a more "relaxed" or "open" structure, which renders chromatin more susceptible to MNase digestion.