Poly(ADP-Ribose) Polymerase: DNA Complexes Visualized as Looped Structures by Electron Microscopy

Poly(ADP-ribose) polymerase is a chromatin bound enzyme which catalyzes the covalent attachment of ADP-ribose units from the coenzyme NAD to various nuclear proteins (1–3). Poly(ADP-ribosyl)ation is a posttranslational modification which appears to be involved in DNA excision repair, cellular proliferation and differentiation (1–3) and in modulation of chromatin structure (4, 5). This DNA dependent enzyme is inactive unless stimulated by DNA strand breaks. Although the DNA structures which activate the enzyme have been identified (6, 7) the basis for the DNA requirement as well as the stimulation of the enzyme activity are not yet understood. We have studied the interaction between poly(ADP- ribose) polymerase and different DNAs using electron microscopy and gel retardation electrophoresis.