Biosynthesis of phosphatidylcholine in bacteria

Synergy between extracellular group IIA phospholipase A2 and phagocyte NADPH oxidase in digestion of phospholipids of Staphylococcus aureus ingested by human neutrophils

Acute inflammatory responses to invading bacteria such as Staphylococcus aureus include mobilization of polymorphonuclear leukocytes (PMN) and extracellular group IIA phospholipase A2 (gIIA-PLA2). Although accumulating coincidentally, the in vitro anti-staphylococcal activities of PMN and gIIA-PLA2 have thus far been studied separately. We now show that degradation of S. aureus phospholipids during and after phagocytosis by human PMN requires the presence of extracellular gIIA-PLA2. The concentration of extracellular gIIA-PLA2 required to produce bacterial digestion was reduced 10-fold by PMN. The effects of added gIIA-PLA2 were greater when present before phagocytosis but even apparent when added after S. aureus were ingested by PMN. Related group V and X PLA2, which are present within PMN granules, do not contribute to bacterial phospholipid degradation during and after phagocytosis even when added at concentrations 30-fold higher than that needed for action of the gIIA-PLA2. The action of added gIIA-PLA2 required catalytically active gIIA-PLA2 and, in PMN, a functional NADPH oxidase but not myeloperoxidase. These findings reveal a novel collaboration between cellular oxygen-dependent and extracellular oxygen-independent host defense systems that may be important in the ultimate resolution of S. aureus infections.

A ClC chloride channel homolog and ornithine-containing membrane lipids of Rhizobium tropici CIAT899 are involved in symbiotic efficiency and acid tolerance

Rhizobium tropici CIAT899 is highly tolerant to several environmental stresses and is a good competitor for nodule occupancy of common bean plants in acid soils. Random transposon mutagenesis was performed to identify novel genes of this strain involved in symbiosis and stress tolerance. Here, we present a genetic analysis of the locus disrupted by the Tn5 insertion in mutant 899-PV9, which lead to the discovery of sycA, a homolog of the ClC family of chloride channels and Cl-/H+ exchange transporters. A nonpolar deletion in this gene caused serious deficiencies in nodule development, nodulation competitiveness, and N2 fixation on Phaseolus vulgaris plants, probably due to its reduced ability to invade plant cells and to form stable symbiosomes, as judged by electron transmission microscopy. A second gene (olsC), found downstream of sycA, is homologous to aspartyl/asparaginyl beta-hydroxylases and modifies two species of ornithine-containing lipids in vivo, presumably by hydroxylation at a still-unknown position. A mutant carrying a nonpolar deletion in olsC is symbiotically defective, whereas overexpressed OlsC in the complemented strain provokes an acid-sensitive phenotype. This is the first report of a ClC homolog being essential for the establishment of a fully developed N2-fixing root nodule symbiosis and of a putative beta-hydroxylase that modifies ornithine-containing membrane lipids of R. tropici CIAT899, which, in turn, are contributing to symbiotic performance and acid tolerance.

Regulatory enzymes of phosphatidylcholine biosynthesis: a personal perspective

Phosphatidylcholine is a prominent constituent of eukaryotic and some prokaryotic membranes. This Perspective focuses on the two enzymes that regulate its biosynthesis, choline kinase and CTP:phosphocholine cytidylyltransferase. These enzymes are discussed with respect to their molecular properties, isoforms, enzymatic activities, and structures, and the possible molecular mechanisms by which they participate in regulation of phosphatidylcholine levels in the cell.

Inorganic polyphosphate in Bacillus cereus: motility, biofilm formation, and sporulation

Chains of inorganic polyphosphate (poly-P) with hundreds of P(i) residues linked by phosphoanhydride bonds, as in ATP, are found in every bacterial, fungal, plant, and animal cell, in which they perform various functions. In the spore-forming Bacillus cereus, we have identified three principal enzymes and genes involved in the metabolism of poly-P, namely, (i) poly-P kinase (PPK), which synthesizes poly-P reversibly from ATP, (ii) exopolyphosphatase (PPX), which hydrolyzes poly-P to P(i), and (iii) poly-P/AMP phosphotransferase (PAP), which uses poly-P as a donor to convert AMP to ADP, reversibly. In the null mutant of ppk, poly-P levels are reduced to <5% of the WT; in the ppx mutant, the PPK activity is elevated 10-fold, and the accumulation of poly-P is elevated approximately 1,000-fold. All of the null mutants of ppk, ppx, and pap showed defects in motility and biofilm formation, but sporulation efficiency was impaired only in the ppx mutant. These enzymes and genes in B. cereus are nearly identical to those in the very closely related pathogen Bacillus anthracis, and, thus, they may provide attractive targets for the treatment of anthrax.

Bacterial inhibition of phosphatidylcholine synthesis triggers apoptosis in the brain

Streptococcus pneumoniae is the most common cause of bacterial meningitis of high mortality and morbidity. Neurological sequelae include paralysis, mental retardation, and learning disorders. In humans, neurons of the hippocampus undergo apoptosis as a result of meningitis. Phosphatidylcholine (PtdCho) is an essential component of mammalian cell membranes and PtdCho deficiency, either due to chemicals or altered nutrition, leads to apoptosis, especially in hippocampal neurons. We show that apoptosis of a variety of brain cells after pneumococcal infection arises from inhibition of PtdCho biosynthesis, the first such activity described for a bacterium. Apoptosis inhibitors did not prevent the bacterial-dependent inhibition of PtdCho biosynthesis. Supplementation with exogenous lyso-phosphatidylcholine prevents cell death and treatment of mice with cytidine diphosphocholine attenuates hippocampal damage during meningitis, even after the onset of infection. We conclude that bacterial inhibition of PtdCho biosynthesis activates an apoptotic cascade that is a causative event in pathogenesis and amenable to therapeutic intervention.

The Sinorhizobium meliloti ABC transporter Cho is highly specific for choline and expressed in bacteroids from Medicago sativa nodules

In Sinorhizobium meliloti, choline is the direct precursor of phosphatidylcholine, a major lipid membrane component in the Rhizobiaceae family, and glycine betaine, an important osmoprotectant. Moreover, choline is an efficient energy source which supports growth. Using a PCR strategy, we identified three chromosomal genes (choXWV) which encode components of an ABC transporter: ChoX (binding protein), ChoW (permease), and ChoV (ATPase). Whereas the best homology scores were obtained with components of betaine ProU-like systems, Cho is not involved in betaine transport. Site-directed mutagenesis of choX strongly reduced (60 to 75%) the choline uptake activity, and purification of ChoX, together with analysis of the ligand-binding specificity, showed that ChoX binds choline with a high affinity (KD, 2.7 microM) and acetylcholine with a low affinity (KD, 145 microM) but binds none of the betaines. Uptake competition experiments also revealed that ectoine, various betaines, and choline derivatives were not effective competitors for Cho-mediated choline transport. Thus, Cho is a highly specific high-affinity choline transporter. Choline transport activity and ChoX expression were induced by choline but not by salt stress. Western blotting experiments with antibodies raised against ChoX demonstrated the presence of ChoX in bacteroids isolated from nitrogen-fixing nodules obtained from Medicago sativa roots. The choX mutation did not have an effect on growth under standard conditions, and neither Nod nor Fix phenotypes were impaired in the mutant, suggesting that the remaining choline uptake system(s) still present in the mutant strain can compensate for the lack of Cho transporter.

Structure and function of choline kinase isoforms in mammalian cells

Choline kinase (CK) catalyzes the first phosphorylation reaction in the CDP-choline pathway for the biosynthesis of phosphatidylcholine (PC), yielding phosphocholine (P-Cho) from choline and ATP in the presence of Mg(2+). This enzyme exists in mammalian cells as at least three isoforms that are encoded by two separate genes termed ck-alpha and ck-beta. Each isoform is not active in its monomeric form. The active enzyme consists of either their homo- or hetero-dimeric (or oligomeric) forms. In recent years, the roles of CK in cell growth and cell stress/defense mechanisms have been intensely investigated. These functions of CK do not seem to be directly related to the net PC biosynthesis but predict another important role of this enzyme in certain cell physiology. This review summarizes briefly the recent progress of mammalian CK study which will include the gene structure of each isoform and its possible transcriptional regulation, the active configuration of the enzyme, induction of the particular isoform in chemically induced cell stress, and the possible role of this enzyme as well as of its reaction product, P-Cho, in cell growth and other cellular physiology.

A CDP-choline pathway for phosphatidylcholine biosynthesis in Treponema denticola

The genomes of Treponema denticola and Treponema pallidum contain a gene, licCA, which is predicted to encode a fusion protein containing choline kinase and CTP:phosphocholine cytidylyltransferase activities. Because both organisms have been reported to contain phosphatidylcholine, this raises the possibility that they use a CDP-choline pathway for the biosynthesis of phosphatidylcholine. This report shows that phosphatidylcholine is a major phospholipid in T. denticola, accounting for 35-40% of total phospholipid. This organism readily incorporated [14C]choline into phosphatidylcholine, indicating the presence of a choline-dependent biosynthetic pathway. The licCA gene was cloned, and recombinant LicCA had choline kinase and CTP:phosphocholine cytidylyltransferase activity. The licCA gene was disrupted in T. denticola by erythromycin cassette mutagenesis, resulting in a viable mutant. This disruption completely blocked incorporation of either [14C]choline or 32Pi into phosphatidylcholine. The rate of production of another phospholipid in T. denticola, phosphatidylethanolamine, was elevated considerably in the licCA mutant, suggesting that the elevated level of this lipid compensated for the loss of phosphatidylcholine in the membranes. Thus it appears that T. denticola does contain a licCA-dependent CDP-choline pathway for phosphatidylcholine biosynthesis.

Phosphatidylethanolamine is not essential for growth of Sinorhizobium meliloti on complex culture media

In addition to phosphatidylglycerol (PG), cardiolipin (CL), and phosphatidylethanolamine (PE), Sinorhizobium meliloti also possesses phosphatidylcholine (PC) as a major membrane lipid. The biosynthesis of PC in S. meliloti can occur via two different routes, either via the phospholipid N-methylation pathway, in which PE is methylated three times in order to obtain PC, or via the phosphatidylcholine synthase (Pcs) pathway, in which choline is condensed with CDP-diacylglycerol to obtain PC directly. Therefore, for S. meliloti, PC biosynthesis can occur via PE as an intermediate or via a pathway that is independent of PE, offering the opportunity to uncouple PC biosynthesis from PE biosynthesis. In this study, we investigated the first step of PE biosynthesis in S. meliloti catalyzed by phosphatidylserine synthase (PssA). A sinorhizobial mutant lacking PE was complemented with an S. meliloti gene bank, and the complementing DNA was sequenced. The gene coding for the sinorhizobial phosphatidylserine synthase was identified, and it belongs to the type II phosphatidylserine synthases. Inactivation of the sinorhizobial pssA gene leads to the inability to form PE, and such a mutant shows a greater requirement for bivalent cations than the wild type. A sinorhizobial PssA-deficient mutant possesses only PG, CL, and PC as major membrane lipids after growth on complex medium, but it grows nearly as well as the wild type under such conditions. On minimal medium, however, the PE-deficient mutant shows a drastic growth phenotype that can only partly be rescued by choline supplementation. Therefore, although choline permits Pcs-dependent PC formation in the mutant, it does not restore wild-type-like growth in minimal medium, suggesting that it is not only the lack of PC that leads to this drastic growth phenotype.

Pathways for phosphatidylcholine biosynthesis in bacteria

Phosphatidylcholine (PC) is the major membrane-forming phospholipid in eukaryotes with important structural and signalling functions. Although many prokaryotes lack PC, it can be found in significant amounts in membranes of rather diverse bacteria. Two pathways for PC biosynthesis are known in bacteria, the methylation pathway and the phosphatidylcholine synthase (PCS) pathway. In the methylation pathway, phosphatidylethanolamine is methylated three times to yield PC, in reactions catalysed by one or several phospholipid N-methyltransferases (PMTs). In the PCS pathway, choline is condensed directly with CDP-diacylglyceride to form PC in a reaction catalysed by PCS. Using cell-free extracts, it was demonstrated that Sinorhizobium meliloti, Agrobacterium tumefaciens, Rhizobium leguminosarum, Bradyrhizobium japonicum, Mesorhizobium loti and Legionella pneumophila have both PMT and PCS activities. In addition, Rhodobacter sphaeroides has PMT activity and Brucella melitensis, Pseudomonas aeruginosa and Borrelia burgdorferi have PCS activities. Genes from M. loti and L. pneumophila encoding a Pmt or a Pcs activity and the genes from P. aeruginosa and Borrelia burgdorferi responsible for Pcs activity have been identified. Based on these functional assignments and on genomic data, one might predict that if bacteria contain PC as a membrane lipid, they usually possess both bacterial pathways for PC biosynthesis. However, important pathogens such as Brucella melitensis, P. aeruginosa and Borrelia burgdorferi seem to be exceptional as they possess only the PCS pathway for PC formation.

Phospholipid synthesis in Borrelia burgdorferi: BB0249 and BB0721 encode functional phosphatidylcholine synthase and phosphatidylglycerolphosphate synthase proteins

Phospholipids are an important component of bacterial membranes.Borrelia burgdorferidiffers from many other bacteria in that it contains only two major membrane phospholipids: phosphatidylglycerol (PG) and phosphatidylcholine (PC).B. burgdorferiappears to lack enzymes required for synthesis of PC through the well-described methylation pathway. However,B. burgdorferidoes contain a gene (BB0249) with significant identity to a recently described phosphatidylcholine synthase gene (pcs) ofSinorhizobium meliloti. B. burgdorferialso contains a gene (BB0721) with significant identity to the gene (pgs) encoding phosphatidylglycerolphosphate synthase, an enzyme in the synthetic pathway of PG. Activity of BB0249 was confirmed by cloning the gene intoEscherichia coli, which does not produce PC. Transformation with a plasmid carryingBB0249resulted in production of PC byE. coli, but only in the presence of exogenously supplied choline, as would be predicted for a Pcs. Because loss of Pgs activity is lethal toE. coli, activity ofBB0721was confirmed by the ability of BB0721 to complement anE. coliPgs−mutant. A plasmid containingBB0721was transformed into a Pgs−mutant ofE. colicontaining a copy of the native gene on a temperature-regulated plasmid. The temperature-regulated plasmid was exchanged for a plasmid containingBB0721and it was shown that BB0721 was able to replace the lost Pgs function and restore bacterial growth. This study has established the existence and function of two critical enzymes in the synthesis of PC and PG inB. burgdorferi. Understanding of the biosynthetic pathways of PC and PG inB. burgdorferiis the first step in delineating the role of these phospholipids in the pathogenesis of Lyme disease.

Membrane lipids in plant-associated bacteria: their biosyntheses and possible functions

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.