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Identification of LpxL, a Late Acyltransferase of Francisella tularensis ^,

Molly K. McLendon,^1, Birgit Schilling,^2, Jason R. Hunt,¹ Michael A. Apicella,¹ and Bradford W. Gibson^2,3*

Department of Microbiology and Inflammation Program, University of Iowa, Iowa City, Iowa,¹ The Buck Institute for Age Research, Novato, California,² Department of Pharmaceutical Chemistry, The University of California, San Francisco, California³

Received 10 August 2006/ Returned for modification 16 October 2006/ Accepted 31 July 2007

Lipopolysaccharide (LPS) is a major component of the outer membrane of gram-negative bacteria, and the lipid A region of LPS mediates stimulation of the immune system in a structure-dependent manner. Unlike the LPS of many other gram-negative bacteria, the LPS of Francisella tularensis isolated from in vitro cultures is not proinflammatory. This observed lack of proinflammatory prowess may reflect structural features of the lipid A, such as the number and length of the acyl chains and the single-phosphate group. To better understand this phenotype, we have begun to elucidate LPS biosynthesis in F. tularensis. We present complementation, mutational, and chemical data demonstrating that F. tularensis FTT0232c encodes a functional late acyltransferase enzyme with specificity similar to that of the Escherichia coli LpxL ortholog. Expression of this late acyltransferase complemented the temperature-sensitive and hypoacylated lipid A phenotypes of an E. coli lpxL mutant, expression of FTT0232c is increased during intracellular growth relative to that during in vitro growth, and finally, LPS obtained from a mutant of F. tularensis lacking FTT0232c showed an abundant triacyl lipid A species after mass spectrometric analysis, consistent with the loss of an LpxL late acyltransferase.

Published ahead of print on 27 August 2007.

Supplemental material for this article may be found at http://iai.asm.org/.

Editor: F. C. Fang

These authors contributed equally to this work.