Poly(ADP-ribose)polymerase: a perplexing participant in cellular responses to DNA breakage

Poly(ADP-ribose) polymerase is a major nuclear protein of 116 kd, coded by a gene on chromosome 1, that plays a role in cellular responses to DNA breakage. The polymerase binds to DNA at single- and double-strand breaks and synthesizes long branched chains of poly(ADP-ribose), which covalently, but transiently, modifies itself and numerous other cellular proteins and depletes cells of NAD+. This much is known, but the physiological role of the polymerization-degradation cycle is still unclear. Poly(ADP-ribosyl)ation of proteins generally inhibits their function and can dissociated chromatin proteins from DNA. Inhibition of poly(ADP-ribose) polymerase increases to toxicity of alkylating agents and some other DNA-damaging agents and increases sister-chromatid exchange frequencies. During repair of alkylation damage, inhibition of poly(ADP-ribose) polymerase makes no change in excision of damaged products. increases the total number of repair patches, accelerates the rejoining of DNA breaks, and makes variable increases or decreases in net break frequencies. The polymerization cycle consequently is a major player in the response of cells to DNA breakage, but the game it plays is yet to be explained.