Mutational Analysis of Quinolone Resistance Protein QnrB1

Abstract

Alanine substitutions and selected deletions have been used to localize amino acids in QnrB essential for its protective activity. Essential amino acids are found at positions i and i−2in the pentapeptide repeat module and in the larger of two loops, where deletion of only a single amino acid compromises activity. Deletion of 10 amino acids at the N terminus is tolerated, but removal of 3 amino acids in the C-terminal dimerization unit destroys activity.

qnrE1 , a Member of a New Family of Plasmid-Located Quinolone Resistance Genes, Originated from the Chromosome of Enterobacter Species

“…1A). In addition, the sequence of this putative new protein displayed the typical pentapeptide repeat structure of the Qnr proteins and contained the loops A and B, which have proven to be essential for quinoloneprotective activity (11)(12)(13) (Fig. 1B).…”

“…Interestingly, an amino acid change was observed in each loop of the putative new Qnr protein regarding all of the previously described QnrB proteins: K52Q and S113C (QnrB amino acid numbering) for loops A and B, respectively. Only two reports analyzed the effect on quinolone susceptibility driven by substitutions in these positions of QnrB1: the CIP MIC was reduced 16 times in the presence of S113D or S113A (11, 13) while no effect was observed for K52A (13).…”

Mutational Analysis of Quinolone Resistance Protein QnrVC7 Provides Novel Insights into the Structure-Activity Relationship of Qnr Proteins

“…However, substitution with Gly was not tolerated either, suggesting that the small side chain is required for forming the stable stack between coils. The best-fitting residues in these well-organized structure were also observable in the i Ϫ2 positions of other faces in QnrVC and other Qnr proteins, such as QnrB1 (3,9,12). It was shown in our previous study that QnrVC7 differed from QnrVC5, QnrVC6, and other Qnr proteins by having a threonine at the 152 site (12).…”

“…Mutational analyses of QnrA, QnrB, QnrC, and QnrS have identified several conserved residues that play an important role in the stabilization of the Qnr protein structure (1, 3, 9-11), most of which being located at the i and i Ϫ2 positions, as defined by Vetting et al (1). In addition, the C terminus of Qnr proteins, which has been suggested to be responsible for the formation of dimers, was found to be critical for Qnr function (3). Two conservative extra loops of QnrB1 were also shown to be critical for Qnr protective activity, as the deletion of these loops dramatically affected its protective function (1).…”

“…This class of proteins exhibits structural similarity to double-stranded DNA, with 5 amino acids forming a tandemrepeat unit, which in turn folds into a right-handed quadrilateral ␤-helix. The Qnr proteins confer reduced susceptibility to quinolones through competition with DNA in binding to DNA gyrase and topoisomerase IV, thereby preventing DNA breakage due to quinolone binding (2,3). To date, a total of 6 classes of Qnr proteins, namely, QnrA, QnrB, QnrC, QnrD, QnrS, and QnrVC, have been discovered, among which QnrB is the most prevalent subtype and can be further subdivided into 80 variants (4)(5)(6)(7)(8).…”