Phosphorus-Free Membrane Lipids of Sinorhizobium meliloti Are Not Required for the Symbiosis with Alfalfa but Contribute to Increased Cell Yields Under Phosphorus-Limiting Conditions of Growth

Abstract

The microsymbiont of alfalfa, Sinorhizobium meliloti, possesses phosphatidylglycerol, cardiolipin, phosphatidylethanolamine, and phosphatidylcholine as major membrane phospholipids, when grown in the presence of sufficient accessible phosphorus sources. Under phosphate-limiting conditions of growth, S. meliloti replaces its phospholipids by membrane lipids that do not contain any phosphorus in their molecular structure and, in S. meliloti, these phosphorus-free membrane lipids are sulphoquinovosyl diacylglycerols (SL), ornithine-containing lipids (OL), and diacylglyceryl-N,N,N-trimethylhomoserines (DGTS). In earlier work, we demonstrated that neither SL nor OL are required for establishing a nitrogen-fixing root nodule symbiosis with alfalfa. We now report the identification of the two structural genes btaA and btaB from S. meliloti required for DGTS biosynthesis. When the sinorhizobial btaA and btaB genes are expressed in Escherichia coli, they cause the formation of DGTS in this latter organism. A btaA-deficient mutant of S. meliloti is unable to form DGTS but can form nitrogen-fixing root nodules on alfalfa, demonstrating that sinorhizobial DGTS is not required for establishing a successful symbiosis with the host plant. Even a triple mutant of S. meliloti, unable to form any of the phosphorus-free membrane lipids SL, OL, or DGTS is equally competitive for nodule occupancy as the wild type. Only under growth-limiting concentrations of phosphate in culture media did mutants that could form neither OL nor DGTS grow to lesser cell densities.

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

“…This may be explained by a mutual replacement of glycolipids and DGTS. A triple mutant of Sinorhizobium lacking SQD, OL, and DGTS was not impaired in its ability to form nodules on its host alfalfa (38). However, the loss of OL and DGTS or of all non-phosphorus lipids (OL, DGTS, and SQD) resulted in a decreased growth under phosphate starvation, suggesting that these lipids serve as bulk lipids.…”

“…Surprisingly, PC was strongly reduced in cells lacking MGlcD/GlcAD, although the amount of DGTS in ⌬agt was not changed. In contrast to Agrobacterium, the content of PC is negatively correlated to that of DGTS in Sinorhizobium (38,39). The reduction of PC in ⌬agt or ⌬agt ⌬pgt may have different reasons as follows.…”

OlsG (Sinac_1600) Is an Ornithine Lipid N-Methyltransferase from the Planctomycete Singulisphaera acidiphila

“…Two families of bacterial phospholipid N-methyltransferases (Pmts) have been described (21,28,29). A third bacterial lipid N-methyltransferase is BtaB, involved in the synthesis of the betaine lipid diacylglyceryl-N,N,N-trimethylhomoserine (DGTS) (30,31). The S. acidiphila genome was searched for genes encoding homologs of the bacterial N-methyltransferases PmtA from S. meliloti, PmtA from R. sphaeroides, and BtaB from S. meliloti.…”

Cyclopropane fatty acyl synthase in Sinorhizobium meliloti

“…Minor lipid classes (5 %) include ornithine lipids (OLs) and sulfoquinovosyldiacylglycerols (SLs). Under P i -limiting conditions, S. meliloti partially substitutes membrane phospholipids with non-phosphorus-containing lipids, which can represent up to 70 % of total lipids; these lipids include 1,2-diacylglyceryl-3-O-49-(N,N,N-trimethyl)-homoserine lipids (TMHSs), SLs and OLs (Lopez-Lara et al, 2005). Recently, we showed for the first time that the percentage composition of Abbreviations: CFA, cyclopropane fatty acid/acyl; DMPE, dimethylphosphatidylethanolamine; FAME, fatty acid methyl ester; MMPE, monomethylphosphatidylethanolamine; PC, phosphatidylcholine; PCA, principal components analysis; PE, phosphatidylethanolamine; PG, phosphatidylglycerol; SL, sulfoquinovosyldiacylglycerol; TMHS, 1,2-diacylglyceryl-3-O-49-(N,N,N-trimethyl)-homoserine.…”