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Antimicrobial Agents and Chemotherapy, February 2006, p. 600-606, Vol. 50, No. 2
0066-4804/06/$08.00+0     doi:10.1128/AAC.50.2.600-606.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Role of the Extended {alpha}4 Domain of Staphylococcus aureus Gyrase A Protein in Determining Low Sensitivity to Quinolones

Jacob Strahilevitz, Ari Robicsek, and David C. Hooper*

Division of Infectious Diseases, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts 02114

Received 1 August 2005/ Returned for modification 18 October 2005/ Accepted 8 November 2005

Fluoroquinolones target two bacterial type II topoisomerases, DNA gyrase and topoisomerase IV. Acquired resistance to quinolones occurs stepwise, with the first mutation occurring in the more sensitive target enzyme. To limit the emergence of resistance, quinolones should ideally possess dual activities against the two enzymes. For reasons that are as yet unclear, Staphylococcus aureus gyrase is less sensitive to quinolones than topoisomerase IV, counter to its greater sensitivity in Escherichia coli, thereby limiting the use of quinolones for the treatment of staphylococcal infections. Mutations in the {alpha}4-helix domain of the GyrA subunit of gyrase are important in determining quinolone resistance. We replaced an extended region encompassing the {alpha}4 domain in the E. coli GyrA protein with its homolog in S. aureus and tested for its ability to complement a thermosensitive gyrase and its catalytic and noncatalytic properties. Purified gyrase reconstituted with chimeric GyrA was more resistant to ciprofloxacin than wild-type gyrase at both inhibition of catalytic activity and stimulation of cleavage complexes, and this difference was more apparent in the presence of K+-glutamate. The chimeric GyrA subunit was able to complement thermosensitive gyrase, similar to wild-type GyrA. Without supplemental K+-glutamate the MICs of ciprofloxacin for thermosensitive E. coli complemented with chimeric DNA gyrase were equal to those for E. coli complemented with wild-type gyrase but were twofold higher in the presence of K+-glutamate. Our findings suggest that the extended {alpha}4 domain of S. aureus GyrA is responsible, at least in part, for the increased resistance of S. aureus gyrase to quinolones and that this effect is modulated by K+-glutamate.

* Corresponding author. Mailing address: Division of Infectious Diseases, Massachusetts General Hospital, 55 Fruit Street, Boston, MA 02114-2696. Phone: (617) 726-3812. Fax: (617) 726-7416. E-mail: dhooper{at}partners.org.

Antimicrobial Agents and Chemotherapy, February 2006, p. 600-606, Vol. 50, No. 2
0066-4804/06/$08.00+0     doi:10.1128/AAC.50.2.600-606.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.



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