Topoisomerase IV catalysis and the mechanism of quinolone action

Mechanism of fluoroquinolone action

When fluoroquinolones bind to gyrase or topoisomerase IV in the presence of DNA, they alter protein conformation. DNA cleavage results with diminished religation, so the enzymes are trapped in ternary complexes with drug and cleaved DNA. Preferential localization of gyrase ahead of replication forks and topoisomerase IV behind them causes fluoroquinolone-mediated complexes with the two enzymes to have different physiological consequences.

Interactions between DNA helicases and frozen topoisomerase IV-quinolone-DNA ternary complexes

Collisions between replication forks and topoisomerase-drug-DNA ternary complexes result in the inhibition of DNA replication and the conversion of the normally reversible ternary complex to a nonreversible form. Ultimately, this can lead to the double strand break formation and subsequent cell death. To understand the molecular mechanisms of replication fork arrest by the ternary complexes, we have investigated molecular events during collisions between DNA helicases and topoisomerase-DNA complexes. A strand displacement assay was employed to assess the effect of topoisomerase IV (Topo IV)-norfloxacin-DNA ternary complexes on the DnaB, T7 gene 4 protein, SV40 T-antigen, and UvrD DNA helicases. The ternary complexes inhibited the strand displacement activities of these DNA helicases. Unlike replication fork arrest, however, this general inhibition of DNA helicases by Topo IV-norfloxacin-DNA ternary complexes did not require the cleavage and reunion activity of Topo IV. We also examined the reversibility of the ternary complexes after collisions with these DNA helicases. UvrD converted the ternary complex to a nonreversible form, whereas DnaB, T7 gene 4 protein, and SV40 T-antigen did not. These results suggest that the inhibition of DnaB translocation may be sufficient to arrest the replication fork progression but it is not sufficient to generate cytotoxic DNA lesion.