ROLE OF THE DNA-LIGASE-III ZINC-FINGER IN POLYNUCLEOTIDE BINDING AND LIGATION

Mammalian DNA ligase III exists as two distinct isoforms denoted alpha and beta. Both forms possess a motif that is homologous to the putati ve zinc finger present in poly(ADP-ribose) polymerase, Here, the role of this motif in the binding and ligation of nicked DNA and RNA substr ates in vitro has been examined in both isoforms, Disruption of the pu tative zinc finger did not affect DNA ligase III activity on nicked DN A duplex, nor did it abolish DNA ligase Ill-alpha activity during DNA base excision repair in a cell-free assay. In contrast, disruption of this motif reduced 3-fold the activity of both DNA ligase III isoforms on nicked RNA present in RNA/DNA homopolymers, Furthermore, whereas d isruption of the motif did not prevent binding of DNA ligase III to ni cked DNA duplex, binding to nicked RNA homopolymers was reduced simila r to 10-fold. These results suggest that the putative zinc finger does not stimulate DNA ligase III activity on simple nicked DNA substrates , but indicate that this motif can target the binding and activity of DNA ligase III to nicked RNA homopolymer. The implications of these re sults to the cellular role of the putative zinc finger are discussed.