title: Sinorhizobium meliloti acpXL mutant lacks the C28 hydroxylated fatty acid moiety of lipid A and does not express a slow migrating form of lipopolysaccharide.
abstract: 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 (omega-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.
authors.affiliation: Institute of Genetics, Biology VI, University of Bielefeld, Postfach 100131, Bielefeld D-33501, Germany. Larissa.Sharpova@Genetik.Uni-Bielefeld.DE
authors.forename: Larissa A
authors.initials: LA
authors.lastname: Sharypova
authors.forename: Karsten
authors.initials: K
authors.lastname: Niehaus
authors.forename: Heiko
authors.initials: H
authors.lastname: Scheidle
authors.forename: Otto
authors.initials: O
authors.lastname: Holst
authors.forename: Anke
authors.initials: A
authors.lastname: Becker
mesh_terms: D000213:Acyl Carrier Protein / Q000235:genetics*; D001426:Bacterial Proteins; D002240:Carbohydrate Sequence; D004926:Escherichia coli / Q000235:genetics; D006900:Hydroxylation; D008050:Lipid A / Q000235:genetics / Q000378:metabolism*; D008070:Lipopolysaccharides / Q000096:biosynthesis / Q000737:chemistry*; D000455:Medicago sativa / Q000382:microbiology; D008969:Molecular Sequence Data; D005810:Multigene Family; D009154:Mutation; D016962:Sinorhizobium meliloti / Q000235:genetics*
chemical_list: C102838:AcpXL protein, Rhizobium; D000213:Acyl Carrier Protein; D001426:Bacterial Proteins; D008050:Lipid A; D008070:Lipopolysaccharides