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Antimicrobial Agents and Chemotherapy, May 2002, p. 1246-1252, Vol. 46, No. 5
0066-4804/02/$04.00+0     DOI: 10.1128/AAC.46.5.1246-1252.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

A Missense Mutation in the fabB (ß-Ketoacyl-Acyl Carrier Protein Synthase I) Gene Confers Thiolactomycin Resistance to Escherichia coli

Protein Science Division, Department of Infectious Diseases,¹ Department of Structural Biology, St. Jude Children's Research Hospital, Memphis, Tennessee 38105,³ Department of Molecular Sciences, University of Tennessee Health Science Center, Memphis, Tennessee 38163²

Received 30 July 2001/ Returned for modification 29 October 2001/ Accepted 24 January 2002

Thiolactomycin (TLM) is an antibiotic that inhibits bacterial type II fatty acid synthesis at the condensing enzyme step, and ß-ketoacyl-acyl carrier protein synthase I (FabB) is the relevant target in Escherichia coli. TLM resistance is associated with the upregulation of efflux pumps. Therefore, a tolC knockout mutant (strain ANS1) was constructed to eliminate the contribution of type I secretion systems to TLM resistance. Six independent TLM-resistant clones of strain ANS1 were isolated, and all possessed the same missense mutation in the fabB gene (T1168G) that directed the expression of a mutant protein, FabB(F390V). FabB(F390V) was resistant to TLM in vitro. Leucine is the only other amino acid found at position 390 in nature, and the Staphylococcus aureus FabF protein, which contains this substitution, was sensitive to TLM. Structural modeling predicted that the CG2 methyl group of the valine side chain interfered with the positioning of the C11 methyl on the isoprenoid side chain of TLM in the binary complex, whereas the absence of a bulky methyl group on the leucine side chain permitted TLM binding. These data illustrate that missense mutations that introduce valine at position 390 confer TLM resistance while maintaining the vital catalytic properties of FabB.

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