DNA base excision repair (BER) constitutes a major mechanism to restore the integrity of the genome following modifications of nucleobases. Although it is well established that poly-(ADP-ribosylation) facilitates BER, the mechanism of this stimulation has remained unknown. Previous observations suggested that poly (ADP-ribose), which is synthesised from NAD+, could serve as a unique source of ATP required for the ligation step in BER [1]. This pathway of ATP generation relies on the release of pyrophosphate during DNA repair synthesis. Now we present evidence that, in situations of cellular energy depletion, the synthesis of poly (ADP-ribose) is indeed stimulated. Simultaneously, the number of nucleotides incorporated by Polb during DNA repair synthesis is increased suggesting a shift from single-nucleotide to long-patch BER. Using a reconstituted system with recombinant BER proteins including Polb, XRCC1, Lig III, FEN 1, and APE-1, it is demonstrated that the absence of ATP enhances both, poly (ADP-ribosylation) catalised by PARP-1 and long-patch DNA synthesis by Polb. It is proposed that this significant stimulation serves to compensate acute ATP shortage during DNA repair and may be regulated by the adenylation state of Lig III. Therefore, this mechanism is likely to be of importance in cellular stress situations and in a number of pathological states which are accompanied by a diminished energy supply. References: 1.Oei SL, Ziegler M: ATP for the DNA ligation step in base excision repair is generated from poly(ADP-ribose). J Biol Chem, 2000; 275: 23234-23239

Keywords: Base excision repair, PARP, DNA Pol b, ligase III