Rhizobium leguminosarum is a Gram-negative bacterium that forms
nitrogen-fixing symbioses with compatible leguminous plants via intracellular invasion and establishes a
persistent infection within host membrane-derived subcellular compartments. Notably, an unusual very-long-chain
fatty acid (VLCFA) is found in the
lipid A of R. leguminosarum as well as in the
lipid A of the medically relevant pathogens Brucella abortus, Brucella melitensis, Bartonella henselae, and Legionella pneumophila, which are also able to persist within intracellular host-derived membranes. These bacterial symbionts and pathogens each contain a homologous gene region necessary for the synthesis and transfer of the VLCFA to the
lipid A. Within this region lies a gene that encodes the specialized
acyl carrier protein AcpXL, on which the VLCFA is built. This study describes the biochemical and
infection phenotypes of an acpXL mutant which lacks the VLCFA. The mutation was created in R. leguminosarum bv. phaseoli strain 8002, which forms symbiosis with Phaseolus vulgaris, a determinate nodulating legume. Structural analysis using gas chromatography and mass spectrometry revealed that the mutant
lipid A lacked the VLCFA. Compared to the parent strain, the mutant was more sensitive to the
detergents deoxycholate and
dodecyl sulfate and the
antimicrobial peptide polymyxin B, suggesting a compromise to membrane stability. In addition, the mutant was more sensitive to higher
salt concentrations. Passage through the plant restored salt tolerance. Electron microscopic examination showed that the mutant was developmentally delayed during symbiotic
infection of the host plant Phaseolus vulgaris and produced abnormal symbiosome structures.