J Korean Med Sci.  2006 Oct;21(5):827-832. 10.3346/jkms.2006.21.5.827.

Molecular Analysis of Isoleucyl-tRNA Synthetase Mutations in Clinical Isolates of Methicillin-Resistant Staphylococcus aureus with Low-Level Mupirocin Resistance

• Yang JA
• Park DW
• Sohn JW
• Yang IS
• Kim KH
• Kim MJ

• KMID: 1781900
• DOI: http://doi.org/10.3346/jkms.2006.21.5.827

Abstract

Emergence and spread of low-level mupirocin resistance in staphylococci have been increasingly reported in recent years. The aim of this study was to characterize missense mutations within the chromosomal isoleucyl-tRNA synthetase gene (ileS) among clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA) with low-level mupirocin resistance. A total of 20 isolates of MRSA with low-level mupirocin resistance (minimal inhibitory concentration, 16-64 microgram/mL) were collected from 79 patients in intensive care units for six months. The isolates were analyzed for isoleucyl-tRNA synthetase (IleS) mutations that might affect the binding of mupirocin to the three-dimensional structure of the S. aureus IleS enzyme. All isolates with low-level mupirocin resistance contained the known V588F mutation affecting the Rossman fold, and some of them additionally had previously unidentified mutations such as P187F, K226T, F227L, Q612H, or V767D. Interestingly, Q612H was a novel mutation that was involved in stabilizing the conformation of the catalytic loop containing the KMSKS motif. In conclusion, this study confirms that molecular heterogeneity in ileS gene is common among clinical MRSA isolates with low-level mupirocin resistance, and further study on clinical mutants is needed to understand the structural basis of low-level mupirocin resistance.

Keyword

Staphylococcus aureus; Mupirocin; Drug Resistance; Isoleucine-tRNA Ligase; Mutation, Missense

MeSH Terms

Staphylococcus aureus/drug effects/*genetics
*Mutation, Missense
Mupirocin/*pharmacology
Methicillin Resistance
Isoleucine-tRNA Ligase/*genetics
Intensive Care Units
Humans
Electrophoresis, Gel, Pulsed-Field
Drug Resistance, Bacterial

Figure

• 

  Fig. 1 Structure of IleS from S. aureus Oxford. Distribution of mutated residues identified from MRSA isolates with low-level mupirocin resistance. (A) Overall IleS structure showing bound tRNAIle (violet) and mupirocin (slate/red) (22). The backbone fold of IleS is shown as a ribbon, with individual sites of mutation highlighted displaying 6 residues' side chains (red) and sequence numbers, and yellow-colored structure with the Q612H mutation. (B) Enlarged view of the beta-helix region adjacent to the Rossman fold and residues 588 and 612. Note that the Q612H mutation has potential to interact with mupirocin. Q612 sits close to the core of IleS Rossman fold motif (yellow/blue), where it forms a water-bridged hydrogen bond with the side chain of D635 (arrow).

• 

  Fig. 2 Representative PFGE patterns of Sma I macrorestriction fragments of genomic DNA of MRSA strains with low-level mupirocin resistance isolated from patients in ICUs. Lane 1, λ ladder marker; lane 2 to lane 7 show PFGE patterns A0, A1, B, C, D, and E, respectively.

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