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Structure and biosynthesis of free lipid A molecules that replace lipopolysaccharide in Francisella tularensis subsp. novicida.

Wang X, Ribeiro AA, Guan Z, McGrath SC, Cotter RJ, Raetz CR

Department of Biochemistry, Duke University Medical Center, Durham, North Carolina 27710, USA.

Francisella tularensis subsp. novicida U112 phospholipids, extracted without hydrolysis, consist mainly of phosphatidylethanolamine, phosphatidylglycerol, phosphatidylcholine, and two lipid A species, designated A1 and A2. These lipid A species, present in a ratio of 7:1, comprise 15% of the total phospholipids, as judged by 32Pi labeling. Although lipopolysaccharide is detectable in F. tularensis subsp. novicida U112, less than 5% of the total lipid A is covalently linked to it. A1 and A2 were analyzed by electrospray ionization and matrix-assisted laser desorption ionization mass spectrometry, gas chromatography/mass spectrometry, and NMR spectroscopy. Both compounds are disaccharides of glucosamine, acylated with primary 3-hydroxystearoyl chains at positions 2, 3, and 2' and a secondary palmitoyl residue at position 2'. Minor isobaric species and some lipid A molecules containing a 3-hydroxypalmitoyl chain in place of 3-hydroxystearate are also present. The 4'- and 3'-positions of A1 and A2 are not derivatized, and 3-deoxy-d-manno-octulosonic acid (Kdo) is not detectable. The 1-phosphate groups of both A1 and A2 are modified with an alpha-linked galactosamine residue, as shown by NMR spectroscopy and gas chromatography/mass spectrometry. An alpha-linked glucose moiety is attached to the 6'-position of A2. The lipid A released by mild acid hydrolysis of F. tularensis subsp. novicida lipopolysaccharide consists solely of component A1. F. tularensis subsp. novicida mutants lacking the arnT gene do not contain a galactosamine residue on their lipid A. Formation of free lipid A in F. tularensis subsp. novicida might be initiated by an unusual Kdo hydrolase present in the membranes of this organism.

Published 28 November 2006 in Biochemistry, 45(48): 14427-40.
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