Mechanism of inhibition of DNA gyrase by cyclothialidine, a novel DNA gyrase inhibitor

We investigated how cyclothialidine (Ro 09-1437), a novel DNA gyrase inhibitor belonging to a new chemical class of compounds, acts to inhibit Escherichia coli DNA gyrase. Cyclothialidine up to 100 micrograms/ml showed no effect on DNA gyrase when linear DNA was used as a substrate. Under the same conditions, quinolones, which inhibit the resealing reaction of DNA gyrase, caused a decrease in the amount of linear DNA used. No effect of cyclothialidine was observed on the accumulation of the covalent complex of DNA and the A subunit of DNA gyrase induced by ofloxacin in the absence of ATP. The effect of cyclothialidine on the DNA supercoiling reaction was antagonized by ATP, reducing the inhibitory activity 11-fold as the ATP concentration was increased from 0.5 to 5 mM. Cyclothialidine competitively inhibited the ATPase activity of DNA gyrase (Ki = 6 nM). The binding of [14C]benzoyl-cyclothialidine to E. coli gyrase was inhibited by ATP and novobiocin, but not by ofloxacin. These results suggest that cyclothialidine acts by interfering with the ATPase activity of the B subunit of DNA gyrase. Cyclothialidine was active against a DNA gyrase resistant to novobiocin, suggesting that its precise site of action might be different from that of novobiocin.