Interactions between QnrB, QnrB mutants, and DNA gyrase

Area of Science:

• Microbiology
• Molecular Biology
• Drug Resistance

Background:

• Plasmid-encoded protein QnrB1 confers resistance to fluoroquinolone antibiotics by protecting DNA gyrase.
• Understanding the interaction between QnrB1 and DNA gyrase is crucial for developing new antibacterial strategies.
• Fluoroquinolones like ciprofloxacin inhibit bacterial DNA gyrase, a vital enzyme for DNA replication.

Purpose of the Study:

• To investigate the physical interactions between QnrB1 and the subunits of DNA gyrase (GyrA and GyrB).
• To determine the role of different QnrB1 domains and quinolone antibiotics in these interactions.
• To elucidate the mechanism by which QnrB1 confers quinolone resistance.

Main Methods:

• Bacterial two-hybrid system was employed to assess protein-protein interactions.
• Wild-type and mutant QnrB1 proteins were tested against GyrA, GyrB, and GyrBA fusion proteins.
• Interactions were evaluated in the presence and absence of sub-inhibitory concentrations of ciprofloxacin and nalidixic acid.

Main Results:

• QnrB1 exhibited a 10-fold higher interaction with GyrB and GyrBA compared to GyrA, indicating GyrB's importance.
• Ciprofloxacin and nalidixic acid reduced QnrB1 interaction with GyrA and GyrBA, but not GyrB, suggesting competitive binding.
• Mutations in QnrB1's C/N termini and specific loops (A and B) affected interactions with gyrase subunits differently, impacting quinolone resistance.

Conclusions:

• QnrB1 and quinolones compete for binding to DNA gyrase, with GyrB being a primary interaction site.
• Specific loops (A and B) in QnrB1 are critical for its proper positioning on gyrase, rather than direct binding stabilization.
• These findings provide insights into quinolone resistance mechanisms and potential targets for new drugs.