A bacteriophage model system for studying topoisomerase inhibitors.

The bacteriophage T4 provides a unique and informative system in which to study the mechanism of action of antitumor agents that inhibit type II DNA topoisomerases. The evolutionary conservation of inhibitor sensitivity provides a strong argument for a conserved inhibitor binding site at or very near the active site of the enzyme-DNA complex. Studies of the wild-type and drug-resistant T4 topoisomerases have provided several important arguments that the drug binding site is located very near the phosphodiester bonds that are cleaved by the topoisomerase. One reasonable model is that the inhibitors intercalate between the two bases on each side of the cleaved phosphodiester bond and physically block the resealing reaction. Finally, genetic analyses using the T4 system have provided some of the most detailed information concerning the role of type II topoisomerase in various aspects of DNA metabolism. The topoisomerase is involved in two distinct pathways of mutagenesis, one that generates frameshift mutations and the other involving gross DNA rearrangements. Both pathways operate precisely at the DNA sites that are cleaved by the enzyme in the presence of inhibitors. Furthermore, recombinational repair can apparently correct lesions that are generated upon inhibition of the T4 topoisomerase, and these inhibitors correspondingly stimulate homologous recombination in phage-infected cells. A complete description of the action of antitumor agents that inhibit type II topoisomerases clearly involves many diverse aspects of nucleic acid metabolism.