| Summary: | Post-translational modifications (PTMs) are receiving more and more attention, since it has been found that the majority of the proteome is altered in some way. The best known PTMs are phosporylation, which activates or deactivates several important enzyme pathways, ubiqiutination, which targets proteins for degradation and glycosylation, which is a requirement of plasma membrane and secretory proteins for localisation. One of the lesser known protein modifications is poly(ADP-ribosyl)ation, which is the attachment of polymers of ADP-ribose moieties onto glutamate residues of target proteins. The reaction is catalysed by poly(ADP-ribose) polymerase (PARP), which uses NAD+ as a substrate. The removal of the polymer is catalysed by poly(ADP-ribose) glycohydrolase (PARG). There are many PARPs in every organism, but only one gene encoding for PARG. The main functions of poly(ADP-ribosyl)ation are thought to be in DNA repair, cell division and genome maintenance. While this PTM is well characterized in mammals, there is limited information on its role in plants. Plants contain 3 PARPs. So far AtPARP1 and AtPARP2 transcripts have been demonstrated to increase after exposure to DNA damaging stress, and links to a response to abiotic stress have been made, while there is virtually no information available about AtPARP3. One of the only organisms found to contain two PARGs is Arabidopsis thaliana. The discovery of a mutant of AtPARG1 highlighted a possible connection to circadian rhythm. This study aimed to expand the current knowledge of poly(ADP-ribosyl)ation in plants. Insertional mutant lines were obtained for each of the plant family members, and characterisation of these revealed a hypersentivity of parg1 to DNA damaging agents, as well as hypersensitivity of both parg1 and parg2 to salt. The expression of recombinant AtPARGs, revealed that AtPARG1 had significantly higher activity than AtPARG2. Site directed mutagenesis of highly conserved residues confirmed their importance in plant PARGs. In addition, this study provided a comprehensive comparison of transcript levels of all the plant poly(ADP-ribosyl)ation family members, and thereby provided novel information on the regulation of the two virtually uncharacterised genes, AtPARP3 and AtPARG2.
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