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Journal of Bacteriology, July 2008, p. 4549-4558, Vol. 190, No. 13
0021-9193/08/$08.00+0     doi:10.1128/JB.00234-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Acyl Chain Specificity of the Acyltransferases LpxA and LpxD and Substrate Availability Contribute to Lipid A Fatty Acid Heterogeneity in Porphyromonas gingivalis

Brian W. Bainbridge, Lisa Karimi-Naser, Robert Reife, Fleur Blethen, Robert K. Ernst, and Richard P. Darveau^*

Department of Periodontics and Oral Biology and Medicine, University of Washington, D-652 Health Sciences Building, 1959 NE Pacific St., Seattle, Washington 98195-7444

Received 15 February 2008/ Accepted 22 April 2008

Porphyromonas gingivalis lipid A is heterogeneous with regard to the number, type, and placement of fatty acids. Analysis of lipid A by matrix-assisted laser desorption ionization-time of flight mass spectrometry reveals clusters of peaks differing by 14 mass units indicative of an altered distribution of the fatty acids generating different lipid A structures. To examine whether the transfer of hydroxy fatty acids with different chain lengths could account for the clustering of lipid A structures, P. gingivalis lpxA (lpxA_Pg) and lpxD_Pg were cloned and expressed in Escherichia coli strains in which the homologous gene was mutated. Lipid A from strains expressing either of the P. gingivalis transferases was found to contain 16-carbon hydroxy fatty acids in addition to the normal E. coli 14-carbon hydroxy fatty acids, demonstrating that these acyltransferases display a relaxed acyl chain length specificity. Both LpxA and LpxD, from either E. coli or P. gingivalis, were also able to incorporate odd-chain fatty acids into lipid A when grown in the presence of 1% propionic acid. This indicates that E. coli lipid A acyltransferases do not have an absolute specificity for 14-carbon hydroxy fatty acids but can transfer fatty acids differing by one carbon unit if the fatty acid substrates are available. We conclude that the relaxed specificity of the P. gingivalis lipid A acyltransferases and the substrate availability account for the lipid A structural clusters that differ by 14 mass units observed in P. gingivalis lipopolysaccharide preparations.

Published ahead of print on 2 May 2008.

Present address: Department of Oral Biology, University of Florida, Gainesville, FL 32608.

Present address: VLST Corp., 201 Elliot Ave. W, Ste. 450, Seattle, WA 98119.

Present address: Astarte Biologics, LLC, PO Box 2308, Redmond, WA 98073.