Membrane Lipids in Plant-Associated Bacteria: Their Biosyntheses and Possible Functions

Abstract

Membrane lipids in most bacteria generally consist of the glycerophospholipids phosphatidylglycerol, cardiolipin, and phosphatidylethanolamine (PE). A subset of bacteria also possesses the methylated derivatives of PE, monomethylphosphatidylethanolamine, dimethylphosphatidylethanolamine, and phosphatidylcholine (PC). In Sinorhizobium meliloti, which can form a nitrogen-fixing root nodule symbiosis with Medicago spp., PC can be formed by two entirely different biosynthetic pathways, either the PE methylation pathway or the recently discovered PC synthase pathway. In the latter pathway, one of the building blocks for PC formation, choline, is obtained from the eukaryotic host. Under phosphorus-limiting conditions of growth, S. meliloti replaces its membrane phospholipids by membrane-forming lipids that do not contain phosphorus; namely, the sulfolipid sulfoquinovosyl diacylglycerol, ornithine-derived lipids, and diacylglyceryl-N,N,N-trimethylhomoserine. Although none of these phosphorus-free lipids is essential for growth in culture media rich in phosphorus or for the symbiotic interaction with the legume host, they are expected to have major roles under free-living conditions in environments poor in accessible phosphorus. In contrast, sinorhizobial mutants deficient in PC show severe growth defects and are completely unable to form nodules on their host plants. Even bradyrhizobial mutants with reduced PC biosynthesis can form only root nodules displaying reduced rates of nitrogen fixation. Therefore, in the cases of these microsymbionts, the ability to form sufficient bacterial PC is crucial for a successful interplay with their host plants.

A Bifunctional Glycosyltransferase from Agrobacterium tumefaciens Synthesizes Monoglucosyl and Glucuronosyl Diacylglycerol under Phosphate Deprivation

“…It is generally accepted that SQD especially replaces the acidic phospholipid phosphatidylglycerol under phosphate-limiting conditions (1,6,36,37). Different species of Rhizobiales contain either SQD (Sinorhizobium and Rhizobium) or GlcAD (Blastochloris), but there are no reports on the presence of the two lipids in one species of Rhizobiales (1, 3).…”

A shotgun lipidomics approach in Sinorhizobium meliloti as a tool in functional genomics

“…Lipids with FA composition 36:2 contained two fatty acyl chains of cis-11-octadecenoic acid, which is the most abundant FA in S. meliloti membranes (29). The similarity in the FA distributions in these lipid classes is not surprising, because they are closely related biosynthetically (45). SLs on the other hand, showed different FA distributions given by the presence of abundant species that contained hexadecanoic and hexadecenoic acid (32:1 and 32:0).…”

Pathways for phosphatidylcholine biosynthesis in bacteria

“…Based on sequence comparison with known Pmt or Pcs enzymes, ORFs from numerous organisms were suggested to encode Pmt or Pcs activities (Ló pez-Lara Sohlenkamp et al, 2003;Ló pez-Lara et al, 2003). To evaluate some of these predictions, selected ORFs were cloned and expressed in E. coli, and cell-free extracts were assayed for Pmt or Pcs activity.…”

“…In this second pathway, choline is condensed directly with CDP-diacylglycerol to form PC and CMP in a reaction catalysed by PCS. The existence of a PCS pathway has been proposed for a number of bacteria (Sohlenkamp et al, 2000Ló pez-Lara & Geiger, 2001;Ló pez-Lara et al, 2003) and now there is evidence that this pathway exists in Pseudomonas aeruginosa (Wilderman et al, 2002) and at least indirect evidence that it occurs in Agrobacterium ( Karnezis et al, 2002).…”