Biosynthesis of phosphatidylcholine in bacteria

Phosphatidylcholine (PC) is the major membrane-forming phospholipid in eukaryotes and can be synthesized by either of two pathways, the methylation pathway or the CDP-choline pathway. Many prokaryotes lack PC, but it can be found in significant amounts in membranes of rather diverse bacteria and based on genomic data, we estimate that more than 10% of all bacteria possess PC. Enzymatic methylation of phosphatidylethanolamine via the methylation pathway was thought to be the only biosynthetic pathway to yield PC in bacteria. However, a choline-dependent pathway for PC biosynthesis has been discovered in Sinorhizobium meliloti. In this pathway, PC synthase, condenses choline directly with CDP-diacylglyceride to form PC in one step. A number of symbiotic (Rhizobium leguminosarum, Mesorhizobium loti) and pathogenic (Agrobacterium tumefaciens, Brucella melitensis, Pseudomonas aeruginosa, Borrelia burgdorferi and Legionella pneumophila) bacteria seem to possess the PC synthase pathway and we suggest that the respective eukaryotic host functions as the provider of choline for this pathway. Pathogens entering their hosts through epithelia (Streptococcus pneumoniae, Haemophilus influenzae) require phosphocholine substitutions on their cell surface components that are biosynthetically also derived from choline supplied by the host. However, the incorporation of choline in these latter cases proceeds via choline phosphate and CDP-choline as intermediates. The occurrence of two intermediates in prokaryotes usually found as intermediates in the eukaryotic CDP-choline pathway for PC biosynthesis raises the question whether some bacteria might form PC via a CDP-choline pathway.     

The role of sugar configuration in the acetolysis of 6-deoxyhexose methyl glycosides

The acetolysis of several perbenzylated 6-deoxyhexose methyl glycosides under two mild conditions (10 equiv ZnCl₂ in 2:1 v/v Ac₂O-AcOH at 5 °C; 10:10:1 v/v/v Ac₂O-AcOH-TFA at 70 °C) was studied. We focused on the effect of sugar configuration on the competition between mechanisms with activation at exocyclic or endocyclic oxygen site. No effect was detected in acetolysis using the TFA protocol promoting an exo-activation mechanism, which affords 1-O-Ac-pyranosides regardless of sugar configuration. On the contrary, it has a primary role in determining the endo- versus exo-product distribution on ZnCl₂-promoted acetolysis.     

Gene probes for the detection of 6-deoxyhexose metabolism in secondary metabolite-producing streptomycetes

DNA probes were designed from the streptomycin production genes strDELM of Streptomyces griseus involved in the biosynthesis of the 6-deoxyhexose (6DOH) dihydrostreptose which could detect the genomic fragments coding for 6DOH formation in other actinomycetes strains. In about 70% of the 43 strains tested at least one signal could be detected with strD-, strE- or strLM-specific probes. Evidence is presented that the hybridizing genes are mostly clustered and probably engaged in the formation of secondary metabolites. Because of the wide-spread use of 6DOH constituents in natural products these probes should allow to detect a vast array of different secondary metabolic gene clusters in actinomycetes.     

Analysis of genes involved in 6-deoxyhexose biosynthesis and transfer in Saccharopolyspora erythraea

Glycosylation represents an attractive target for protein engineering of novel antibiotics, because specific attachment of one or more deoxysugars is required for the bioactivity of many antibiotic and antitumour polyketides. However, proper assessment of the potential of these enzymes for such combinatorial biosynthesis requires both more precise information on the enzymology of the pathways and also improved Escherichia coli-actinomycete shuttle vectors. New replicative vectors have been constructed and used to express independently the dnmU gene of Streptomyces peucetius and the eryBVII gene of Saccharopolyspora erythraea in an eryBVII deletion mutant of Sac. erythraea. Production of erythromycin A was obtained in both cases, showing that both proteins serve analogous functions in the biosynthetic pathways to dTDP-L-daunosamine and dTDP-L-mycarose, respectively. Over-expression of both proteins was also obtained in S. lividans, paving the way for protein purification and in vitro monitoring of enzyme activity. In a further set of experiments, the putative desosaminyltransferase of Sac. erythraea, EryCIII, was expressed in the picromycin producer Streptomyces sp. 20032, which also synthesises dTDP-D-desosamine. The substrate 3-alpha-mycarosylerythronolide B used for hybrid biosynthesis was found to be glycosylated to produce erythromycin D only when recombinant EryCIII was present, directly confirming the enzymatic role of EryCIII. This convenient plasmid expression system can be readily adapted to study the directed evolution of recombinant glycosyltransferases.     

Biosynthesis of deoxyaminosugars in antibiotic-producing bacteria

Deoxyaminosugars comprise an important class of deoxysugars synthesized by a variety of different microorganisms; they can be structural components of lipopolysaccharides, extracellular polysaccharides, and secondary metabolites such as antibiotics. Genes involved in the biosynthesis of the deoxyaminosugars are often clustered and are located in the vicinity of other genes required for the synthesis of the final compound. Most of the gene clusters for aminosugar biosynthesis have common features, as they contain genes encoding dehydratases, isomerases, aminotransferases, methyltransferases, and glycosyltransferases. In the present mini-review, the proposed biosynthetic pathways for deoxyaminosugar components of both macrolide and non-macrolide antibiotics are highlighted. The possibilities for genetic manipulations of the deoxyaminosugar biosynthetic pathways aimed at production of novel secondary metabolites are discussed.     

Biosynthesis of ubiquinone in non-photosynthetic Gram-negative bacteria

1. The polyprenylphenol and quinone complements of the non-photosynthetic Gram-negative bacteria, Pseudomonas ovalis Chester, Proteus mirabilis and `Vibrio O1'' (Moraxella sp.), were investigated. 2. Ps. ovalis Chester and Prot. mirabilis were shown to contain 2-polyprenylphenols, 6-methoxy-2-polyprenylphenols, 6-methoxy-2-polyprenyl-1,4-benzoquinones, 5-demethoxyubiquinones, ubiquinones, an unidentified 1,4-benzoquinone [2-polyprenyl-1,4-benzoquinone (?)] and `epoxyubiquinones''. `Vibrio O1'' was shown to contain only 5-demethoxyubiquinones, ubiquinones and `epoxyubiquinones''. 3. It was established that in Ps. ovalis Chester 2-polyprenylphenols, 6-methoxy-2-polyprenylphenols, 6-methoxy-2-polyprenyl-1,4-benzoquinones, 5-demethoxyubiquinones and 2-polyprenyl-1,4-benzoquinones (?) are precursors of ubiquinones. 4. Intracellular distribution studies showed that in Ps. ovalis Chester ubiquinone and its prenylated precursors are localized entirely on the protoplast membrane. 5. Investigations into the oxygen requirements for ubiquinone biosynthesis by Ps. ovalis Chester showed that the organism could not convert p-hydroxybenzoic acid into ubiquinone in the absence of oxygen, although it could convert a limited amount into 2-polyprenylphenols. 6. Attempts were made to prepare cell-free preparations capable of synthesizing ubiquinone. Purified protoplast membranes of Ps. ovalis Chester were found to be incapable of carrying out this synthesis, even when supplemented with cytoplasm. With crushed-cell preparations of Ps. ovalis Chester, organism PC4 (Achromobacter sp.) and Escherichia coli, synthesis was observed, although this was attributable in part to a small number of intact cells present in the preparations.     

Biosynthesis of a sulfonolipid in gliding bacteria

Gliding bacteria of the genus Cytophaga synthesize sulfonolipids (1,2) that contain capnine (1-deoxy-15-methylhexadecasphinganine-1-sulfonic acid). Studies of the incorporation of radiolabeled compounds by C. johnsonae show that cysteate is utilized preferentially to both cystine and inorganic sulfate as a precursor of capnine sulfur and to both cystine and serine as a precursor of carbons 1 and 2 of capnine. The results are consistent with a pathway in which capnine is formed by condensation of cysteate with a fatty acyl CoA. Cystine, added as the sole sulfur source in the presence of glucose, provides the sulfur but not the carbon for capnine. Hence, these cells form cysteate not by direct oxidation of cystine (or cysteine), but by transfer of its sulfur to a different carbon compound.     

Biosynthesis and metabolism of arginine in bacteria

[This corrects the article on p. 351 in vol. 50.].     

Biosynthesis of inulinases by Bacillus bacteria

Biosynthesis of extracellular inulinase by bacteria Bacillus polymyxa 29, B. polymyxa 722, and B. subtilis 68 was studied. The optimal parameters for the producer growth were as follows: pH 7.0, 33-35 degrees C, the growth duration within 72 h. The presence of mineral reduced or of organic nitrogen was necessary for the enzyme biosynthesis. The inulinase biosynthesis was sharply activated in the presence of carbohydrates. B. polymyxa 722 and B. polymyxa 29 displayed the maximal activity on a starch-containing culture medium, the maximal activity of B. subtilis 68 was found in the presence of sucrose. Inulin did not induce the inulinase biosynthesis by the strains studied. The time course of bacteria growth and of the enzyme biosynthesis was studied.     

Biosynthesis and physiology of coenzyme Q in bacteria

Ubiquinone, also called coenzyme Q, is a lipid subject to oxido-reduction cycles. It functions in the respiratory electron transport chain and plays a pivotal role in energy generating processes. In this review, we focus on the biosynthetic pathway and physiological role of ubiquinone in bacteria. We present the studies which, within a period of five decades, led to the identification and characterization of the genes named ubi and involved in ubiquinone production in Escherichia coli. When available, the structures of the corresponding enzymes are shown and their biological function is detailed. The phenotypes observed in mutants deficient in ubiquinone biosynthesis are presented, either in model bacteria or in pathogens. A particular attention is given to the role of ubiquinone in respiration, modulation of two-component activity and bacterial virulence. This article is part of a Special Issue entitled: 18th European Bioenergetic Conference.