Study On The Mechanism Of Polymeric ADP - Ribosyl Glycosyl Hydrolytic Enzyme Against

Poly(ADP-ribosyl)ation is a post-translational modification of proteins. Upon DNA damage, poly(ADP-ribose) polymerases(PARPs) catalyzes the transfer of ADP-ribose moieties from NAD+ onto acceptor proteins to form long and branched polymers. Poly(ADP-ribose) glycohydrolase (PARG) hydrolyzes covalent glycosidic linkages of poly(ADP-ribose)(PAR) synthesized by PARP and liberates ADP-ribose residues. Homeostasis of PAR is thought to play an important role in cellular processes. The metabolism of PAR is critical for genomic stability in multicellular eukaryotes. Here, we report that failure to degrade PAR in pargl mutants causes enhanced sensitivity to genotoxin stress, reduced root growth, retardation in true leaf development and cell death. This phenotype results from failure to hydrolyze PAR, because PARG1 deficient Arabidopsis seedlings accumulate a higher level of PAR and undergo cell death under genotoxin treatment. Moreover, poly(ADP-ribose) polymerase inhibitor 3-Aminobenzamide (3-AB) is partially able to rescue the phenotype. The expression of PARG1 is induced by genotoxin. Arabidopsis is a rare organism that has two PARG genes. In Vitro enzyme assay demonstrates that both PARG1 and PARG2 have PAR-degradation activity. The expression of PARG2 is also induced by genotoxin. PARG1 is localized into the nucleus while PARG2 is localized into the cytoplasm and nucleus. Although the morphology and growth of parg2 mutants are indistinguishable from those of wild-type seedlings, double mutants generated by silencing PARG2 in pargl background display a more sensitive phenotype than pargl mutants. These results provide evidence for an involvement of both PARG1 and PARG2 in maintaining genomic integrity and progression of cell cycle under genotoxin stress. In addition, accumulation of excessive PAR has deleterious consequences in Arabidopsis. Taken together, AtPARGl and AtPARG2 play synergistic roles in maintaining PAR homeostasis under genotoxin stress.