Selective targeting of topoisomerase IV and DNA gyrase in Staphylococcus aureus: different patterns of quinolone-induced inhibition of DNA synthesis.

The effect of quinolones on the inhibition of DNA synthesis in Staphylococcus aureus was examined by using single resistance mutations in parC or gyrA to distinguish action against gyrase or topoisomerase IV, respectively. Norfloxacin preferentially attacked topoisomerase IV and blocked DNA synthesis slowly, while nalidixic acid targeted gyrase and inhibited replication rapidly. Ciprofloxacin exhibited an intermediate response, consistent with both enzymes being targeted. The absence of RecA had little influence on target choice by this assay, indicating that differences in rebound (repair) DNA synthesis were not responsible for the results. At saturating drug concentrations, norfloxacin and a gyrA mutant were used to show that topoisomerase IV-norfloxacin-cleaved DNA complexes are distributed on the S. aureus chromosome at intervals of about 30 kbp. If cleaved complexes block DNA replication, as indicated by previous work, such close spacing of topoisomerase-quinolone-DNA complexes should block replication rapidly (replication forks are likely to encounter a cleaved complex within a minute). Thus, the slow inhibition of DNA synthesis at growth-inhibitory concentrations suggests that a subset of more distantly distributed complexes is physiologically relevant for drug action and is unlikely to be located immediately in front of the DNA replication fork.