Quinolone resistance locus nfxD of Escherichia coli is a mutant allele of the parE gene encoding a subunit of topoisomerase IV.

The locus nfxD, which contributes to high-level quinolone resistance in Escherichia coli KF111b (gyrAr nfxB nfxD), is only expressed in the presence of a gyrA mutation, and maps to the region of the parC and parE genes, was outcrossed into strain KF130, creating strain DH161 (gyrAr nfxD). DNA sequence analysis of DH161 revealed no changes in the topoisomerase IV parC quinolone resistance-determining region but did identify a single T-to-A mutation in parE at codon 445, leading to a change from Leu to His. Full-length cloned parE+ partially complemented the resistance phenotype in KF111b and DH161, but did not complement the resistance phenotype in strain KF130 (gyrAr). No complementation was seen with cloned, truncated parE+. To confirm these findings, gyrAr was first outcrossed from KF130 into E. coli W3110parE10 [parE temperature sensitive(Ts)] and KL16. The transduced strains KL16 and W3110parE10 were subsequently transformed with plasmids containing cloned parE from DH161 or KL16. Cloned parE from DH161 increased norfloxacin resistance in the parE(Ts) background twofold at 30 degrees C and fourfold at 42 degrees C compared to those for cloned parE from KL16. The same experiment with a non-Ts background revealed a twofold increase in the norfloxacin MIC at both 30 and 42 degrees C. These data identify the nfxD conditional resistance locus as a mutant allele of parE. This report is the first of a quinolone-resistant parE mutant and confirms the role of topoisomerase IV as a secondary target of norfloxacin in E. coli.