Sinorhizobium meliloti acpXL Mutant Lacks the C28 Hydroxylated Fatty Acid Moiety of Lipid A and Does Not Express a Slow Migrating Form of Lipopolysaccharide

• Published in: The Journal of biological chemistry 2003-04, Vol.278 (15), p.12946-12954
• Main Authors: Sharypova, Larissa A, Niehaus, Karsten, Scheidle, Heiko, Holst, Otto, Becker, Anke
• Format: Article
• Language: English
• Subjects: Acyl Carrier Protein - genetics; Bacterial Proteins; Carbohydrate Sequence; Escherichia coli - genetics; Hydroxylation; Lipid A - genetics; Lipid A - metabolism; Lipopolysaccharides - biosynthesis; Lipopolysaccharides - chemistry; Medicago sativa - microbiology; Molecular Sequence Data; Multigene Family; Mutation; Sinorhizobium meliloti - genetics
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M209389200

Lipid A is the hydrophobic anchor of lipopolysaccharide (LPS) in the outer membrane of Gram-negative bacteria. Lipid A of all Rhizobiaceae is acylated with a long fatty acid chain, 27-hydroxyoctacosanoic acid. Biosynthesis of this long acyl substitution requires a special acyl carrier protein, AcpXL, which serves as a donor of C28 (ω-1)-hydroxylated fatty acid for acylation of rhizobial lipid A (Brozek, K.A., Carlson, R.W., and Raetz, C. R. (1996) J. Biol. Chem. 271, 32126–32136). To determine the biological function of the C28 acylation of lipid A, we constructed an acpXL mutant of Sinorhizobium meliloti strain 1021. Gas-liquid chromatography and mass spectrometry analysis of the fatty acid composition showed that the acpXL mutation indeed blocked C28 acylation of lipid A. SDS-PAGE analysis of acpXL mutant LPS revealed only a fast migrating band, rough LPS, whereas the parental strain 1021 manifested both rough and smooth LPS. Regardless of this, the LPS of parental and mutant strains had a similar sugar composition and exposed the same antigenic epitopes, implying that different electrophoretic profiles might account for different aggregation properties of LPS molecules with and without a long acyl chain. The acpXL mutant of strain 1021 displayed sensitivity to deoxycholate, delayed nodulation of Medicago sativa , and a reduced competitive ability. However, nodules elicited by this mutant on roots of M. sativa and Medicago truncatula had a normal morphology and fixed nitrogen. Thus, the C28 fatty acid moiety of lipid A is not crucial, but it is beneficial for establishing an effective symbiosis with host plants. acpXL lies upstream from a cluster of five genes, including msbB ( lpxXL ), which might be also involved in biosynthesis and transfer of the C28 fatty acid to the lipid A precursor.