Identification of UDP-glucose as an intermediate in the biosynthesis of the membrane-derived oligosaccharides of Escherichia coli

UDP-glucose sensing P2Y14R: A novel target for inflammation

Uridine 5'-diphosphoglucose (UDP-G) as a preferential agonist, but also other UDP-sugars, such as UDP galactose, function as extracellular signaling molecules under conditions of cell injury and apoptosis. Consequently, UDP-G is regarded to function as a damage-associated molecular pattern (DAMP), regulating immune responses. UDP-G promotes neutrophil recruitment, leading to the release of pro-inflammatory chemokines. As a potent endogenous agonist with the highest affinity for the P2Y₁₄ receptor (R), it accomplishes an exclusive relationship between P2Y₁₄Rs in regulating inflammation via cyclic adenosine monophosphate (cAMP), nod-like receptor protein 3 (NLRP3) inflammasome, mitogen-activated protein kinases (MAPKs), and signal transducer and activator of transcription 1 (STAT1) pathways. In this review, we initially present a brief introduction into the expression and function of P2Y₁₄Rs in combination with UDP-G. Subsequently, we summarize emerging roles of UDP-G/P2Y₁₄R signaling pathways that modulate inflammatory responses in diverse systems, and discuss the underlying mechanisms of P2Y₁₄R activation in inflammation-related diseases. Moreover, we also refer to the applications as well as effects of novel agonists/antagonists of P2Y₁₄Rs in inflammatory conditions. In conclusion, due to the role of the P2Y₁₄R in the immune system and inflammatory pathways, it may represent a novel target for anti-inflammatory therapy.

Efficient production of the glycosylated derivatives of baicalein in engineered Escherichia coli

Baicalein-7-O-glucoside and baicalein-7-O-rhamnoside have been proven to possess many pharmacological activities and are potential candidate drug leads and herb supplements. However, their further development is largely limited due to low content in host plants. Few studies reported that both bioactive plant components are prepared through the bioconversion of baicalein that is considered as the common biosynthetic precursor of both compounds. Herein, we constructed a series of the engineered whole-cell bioconversion systems in which the deletion of competitive genes and the introduction of exogenous UDP-glucose supply pathway, glucosyltransferase, rhamnosyltransferase, and the UDP-rhamnose synthesis pathway are made. Using these engineered strains, the precursor baicalein is able to be transformed into baicalein-7-O-glucoside and baicalein-7-O-rhamnoside, with high-titer production, respectively. The further optimization of fermentation conditions led to the final production of 568.8 mg/L and 877.0 mg/L for baicalein-7-O-glucoside and baicalein-7-O-rhamnoside, respectively. To the best of our knowledge, it is the highest production in preparation of baicalein-7-O-glucoside from baicalein so far, while the preparation of baicalein-7-O-rhamnoside is the first reported via bioconversion approach. Our study provides a reference for the industrial production of high-value products baicalein-7-O-glucoside and baicalein-7-O-rhamnoside using engineered E. coli. KEY POINTS: • Integrated design for improving the intracellular UDP-glucose pool • High production of rare baicalein glycosides in the engineered E. coli • Baicalein-7-O-glucoside and baicalein-7-O-rhamnoside.

Donnan Potential across the Outer Membrane of Gram-Negative Bacteria and Its Effect on the Permeability of Antibiotics

The cell envelope structure of Gram-negative bacteria is unique, composed of two lipid bilayer membranes and an aqueous periplasmic space sandwiched in between. The outer membrane constitutes an extra barrier to limit the exchange of molecules between the cells and the exterior environment. Donnan potential is a membrane potential across the outer membrane, resulted from the selective permeability of the membrane, which plays a pivotal role in the permeability of many antibiotics. In this review, we discussed factors that affect the intensity of the Donnan potential, including the osmotic strength and pH of the external media, the osmoregulated periplasmic glucans trapped in the periplasmic space, and the displacement of cell surface charges. The focus of our discussion is the impact of Donnan potential on the cellular permeability of selected antibiotics including fluoroquinolones, tetracyclines, β-lactams, and trimethoprim.

Phage Resistance in Multidrug-Resistant Klebsiella pneumoniae ST258 Evolves via Diverse Mutations That Culminate in Impaired Adsorption

The therapeutic use of bacteriophage (phage) is garnering renewed interest in the setting of difficult-to-treat infections. Phage resistance is one major limitation of phage therapy; therefore, developing effective strategies to avert or lessen its impact is critical. Characterization of in vitro phage resistance may be an important first step in evaluating the relative likelihood with which phage-resistant populations emerge, the most likely phenotypes of resistant mutants, and the effect of certain phage cocktail combinations in increasing or decreasing the genetic barrier to resistance. If this information confers predictive power in vivo, then routine studies of phage-resistant mutants and their in vitro evolution should be a valuable means for improving the safety and efficacy of phage therapy in humans.

The evolution of phage resistance poses an inevitable threat to the efficacy of phage therapy. The strategic selection of phage combinations that impose high genetic barriers to resistance and/or high compensatory fitness costs may mitigate this threat. However, for such a strategy to be effective, the evolution of phage resistance must be sufficiently constrained to be consistent. In this study, we isolated lytic phages capable of infecting a modified Klebsiella pneumoniae clinical isolate and characterized a total of 57 phage-resistant mutants that evolved from their prolonged coculture in vitro. Single- and double-phage-resistant mutants were isolated from independently evolved replicate cocultures grown in broth or on plates. Among resistant isolates evolved against the same phage under the same conditions, mutations conferring resistance occurred in different genes, yet in each case, the putative functions of these genes clustered around the synthesis or assembly of specific cell surface structures. All resistant mutants demonstrated impaired phage adsorption, providing a strong indication that these cell surface structures functioned as phage receptors. Combinations of phages targeting different host receptors reduced the incidence of resistance, while, conversely, one three-phage cocktail containing two phages targeting the same receptor increased the incidence of resistance (relative to its two-phage, nonredundant receptor-targeting counterpart). Together, these data suggest that laboratory characterization of phage-resistant mutants is a useful tool to help optimize therapeutic phage selection and cocktail design.

Cellular responses during morphological transformation in Azospirillum brasilense and Its flcA knockout mutant

FlcA is a response regulator controlling flocculation and the morphological transformation of Azospirillum cells from vegetative to cyst-like forms. To understand the cellular responses of Azospirillum to conditions that cause morphological transformation, proteins differentially expressed under flocculation conditions in A. brasilense Sp7 and its flcA knockout mutant were investigated. Comparison of 2-DE protein profiles of wild-type (Sp7) and a flcA deletion mutant (Sp7-flcAΔ) revealed a total of 33 differentially expressed 2-DE gel spots, with 22 of these spots confidently separated to allow protein identification. Analysis of these spots by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and MASCOT database searching identified 48 proteins (≥10% emPAI in each spot). The functional characteristics of these proteins included carbon metabolism (beta-ketothiolase and citrate synthase), nitrogen metabolism (Glutamine synthetase and nitric oxide synthase), stress tolerance (superoxide dismutase, Alkyl hydroperoxidase and ATP-dependent Clp protease proteolytic subunit) and morphological transformation (transducer coupling protein). The observed differences between Sp7 wild-type and flcA⁻ strains enhance our understanding of the morphological transformation process and help to explain previous phenotypical observations. This work is a step forward in connecting the Azospirillum phenome and genome.

One-pot three-enzyme synthesis of UDP-Glc, UDP-Gal, and their derivatives

A UTP-glucose-1-phosphate uridylyltransferase (SpGalU) and a galactokinase (SpGalK) were cloned from Streptococcus pneumoniae TIGR4 and were successfully used to synthesize UDP-galactose (UDP-Gal), UDP-glucose (UDP-Glc), and their derivatives in an efficient one-pot reaction system. The reaction conditions for the one-pot multi-enzyme synthesis were optimized and nine UDP-Glc/Gal derivatives were synthesized. Using this system, six unnatural UDP-Gal derivatives, including UDP-2-deoxy-Galactose and UDP-GalN3 which were not accepted by other approach, can be synthesized efficiently in a one pot fashion. More interestingly, this is the first time it has been reported that UDP-Glc can be synthesized in a simpler one-pot three-enzyme synthesis reaction system.

A retrospective: use of Escherichia coli as a vehicle to study phospholipid synthesis and function

Although the study of individual phospholipids and their synthesis began in the 1920s first in plants and then mammals, it was not until the early 1960s that Eugene Kennedy using Escherichia coli initiated studies of bacterial phospholipid metabolism. With the base of information already available from studies of mammalian tissue, the basic blueprint of phospholipid biosynthesis in E. coli was worked out by the late 1960s. In 1970s and 1980s most of the enzymes responsible for phospholipid biosynthesis were purified and many of the genes encoding these enzymes were identified. By the late 1990s conditional and null mutants were available along with clones of the genes for every step of phospholipid biosynthesis. Most of these genes had been sequenced before the complete E. coli genome sequence was available. Strains of E. coli were developed in which phospholipid composition could be changed in a systematic manner while maintaining cell viability. Null mutants, strains in which phospholipid metabolism was artificially regulated, and strains synthesizing foreign lipids not found in E. coli have been used to this day to define specific roles for individual phospholipid. This review will trace the findings that have led to the development of E. coli as an excellent model system to study mechanisms underlying the synthesis and function of phospholipids that are widely applicable to other prokaryotic and eukaryotic systems. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Functional metagenomics reveals novel salt tolerance loci from the human gut microbiome

Metagenomics is a powerful tool that allows for the culture-independent analysis of complex microbial communities. One of the most complex and dense microbial ecosystems known is that of the human distal colon, with cell densities reaching up to 10(12) per gram of faeces. With the majority of species as yet uncultured, there are an enormous number of novel genes awaiting discovery. In the current study, we conducted a functional screen of a metagenomic library of the human gut microbiota for potential salt-tolerant clones. Using transposon mutagenesis, three genes were identified from a single clone exhibiting high levels of identity to a species from the genus Collinsella (closest relative being Collinsella aerofaciens) (COLAER_01955, COLAER_01957 and COLAER_01981), a high G+C, Gram-positive member of the Actinobacteria commonly found in the human gut. The encoded proteins exhibit a strong similarity to GalE, MurB and MazG. Furthermore, pyrosequencing and bioinformatic analysis of two additional fosmid clones revealed the presence of an additional galE and mazG gene, with the highest level of genetic identity to Akkermansia muciniphila and Eggerthella sp. YY7918, respectively. Cloning and heterologous expression of the genes in the osmosensitive strain, Escherichia coli MKH13, resulted in increased salt tolerance of the transformed cells. It is hoped that the identification of atypical salt tolerance genes will help to further elucidate novel salt tolerance mechanisms, and will assist our increased understanding how resident bacteria cope with the osmolarity of the gastrointestinal tract.

Effects of galU mutation on Pseudomonas syringae-plant interactions

Bacterial galU coding for a uridine diphosphate-glucose pyrophosphorylase plays an important role in carbohydrates biosynthesis, including synthesis of lipopolysaccharides (LPS), membrane-derived oligosaccharides, and capsular polysaccharides. In this study, we characterized the galU mutant of Pseudomonas syringae pv. syringae 61 (Psy61), a necrotizing plant pathogen whose pathogenicity depends on a functional type III secretion system (T3SS), and showed that the Psy61 galU mutant had reduced biofilm formation ability, was nonmotile, and had an assembled T3SS structure but failed to elicit hypersensitive response in resistant plants and necrotic lesions in susceptible plants. Moreover, the defective LPS and other pathogen-associated molecular patterns (PAMPs) on the surface of the Psy61 galU mutant were capable of inducing PAMP-triggered immunity, which severely compromised the ability of the Psy61 galU mutant to survive in planta. Our results demonstrated that the complete LPS protected plant-pathogenic bacteria from host innate immunity, similar to what was found in animal pathogens, prior to the translocation of T3S effectors and bacterial multiplication.

Escherichia coli AspP activity is enhanced by macromolecular crowding and by both glucose-1,6-bisphosphate and nucleotide-sugars

Escherichia coli ADP-sugar pyrophosphatase (AspP) is a "Nudix" hydrolase that catalyzes the hydrolytic breakdown of ADP-glucose linked to glycogen biosynthesis. Moderate increases of AspP activity in the cell are accompanied by significant reductions of the glycogen content. In vitro analyses showed that AspP activity is strongly enhanced by macromolecular crowding and by both glucose-1,6-bisphosphate and nucleotide-sugars, providing a first set of indicative evidences that AspP is a highly regulated enzyme. To our knowledge, AspP is the sole bacterial enzyme described to date which is activated by both G1,6P(2) and nucleotide-sugars.

An agr-like two-component regulatory system in Lactobacillus plantarum is involved in production of a novel cyclic peptide and regulation of adherence

We have analyzed a locus on the annotated Lactobacillus plantarum WCFS1 genome that showed homology to the staphylococcal agr quorum-sensing system and designated it lam for Lactobacillus agr-like module. Production of the lamBDCA transcript was shown to be growth phase dependent. Analysis of a response regulator-defective mutant (Delta)lamA) in an adherence assay showed that lam regulates adherence of L. plantarum to a glass surface. Global transcription analysis of the wild-type and (Delta)lamA strains in early, mid-, and late log phase of growth was performed using a clone-based microarray. Remarkably, only a small set of genes showed significant differences in transcription profiles between the wild-type and lamA mutant strains. The microarray analysis confirmed that lamBDCA is autoregulatory and showed that lamA is involved in regulation of expression of genes encoding surface polysaccharides, cell membrane proteins, and sugar utilization proteins. The lamBD genes encoding the putative autoinducing peptide precursor (LamD) and its processing protein (LamB) were overexpressed using the nisin-controlled expression system, and culture supernatants were analyzed by liquid chromatography/mass spectrometry (LC/MS) to identify overproduced LamD-derived peptides. In this way, a cyclic thiolactone pentapeptide that possesses a ring structure similar to those of autoinducing peptides of the staphylococcal agr system was identified. The peptide was designated LamD558, and its sequence (CVGIW) matched the annotated precursor peptide sequence. Time course analysis of wild-type culture supernatants by LC/MS indicated that LamD558 production was increased markedly from mid-log to late log growth phase. This is the first example of an agr-like system in nonpathogenic bacteria that encodes a cyclic thiolactone autoinducing peptide and is involved in regulation of adherence.

Yeast glycogenin (Glg2p) produced in Escherichia coli is simultaneously glucosylated at two vicinal tyrosine residues but results in a reduced bacterial glycogen accumulation

Saccharomyces cerevisiae possesses two glycogenin isoforms (designated as Glg1p and Glg2p) that both contain a conserved tyrosine residue, Tyr232. However, Glg2p possesses an additional tyrosine residue, Tyr230 and therefore two potential autoglucosylation sites. Glucosylation of Glg2p was studied using both matrix-assisted laser desorption ionization and electrospray quadrupole time of flight mass spectrometry. Glg2p, carrying a C-terminal (His6) tag, was produced in Escherichia coli and purified. By tryptic digestion and reversed phase chromatography a peptide (residues 219-246 of the complete Glg2p sequence) was isolated that contained 4-25 glucosyl residues. Following incubation of Glg2p with UDPglucose, more than 36 glucosyl residues were covalently bound to this peptide. Using a combination of cyanogen bromide cleavage of the protein backbone, enzymatic hydrolysis of glycosidic bonds and reversed phase chromatography, mono- and diglucosylated peptides having the sequence PNYGYQSSPAM were generated. MS/MS spectra revealed that glucosyl residues were attached to both Tyr232 and Tyr230 within the same peptide. The formation of the highly glucosylated eukaryotic Glg2p did not favour the bacterial glycogen accumulation. Under various experimental conditions Glg2p-producing cells accumulated approximately 30% less glycogen than a control transformed with a Glg2p lacking plasmid. The size distribution of the glycogen and extractable activities of several glycogen-related enzymes were essentially unchanged. As revealed by high performance anion exchange chromatography, the intracellular maltooligosaccharide pattern of the bacterial cells expressing the functional eukaryotic transgene was significantly altered. Thus, the eukaryotic glycogenin appears to be incompatible with the bacterial initiation of glycogen biosynthesis.

General assay for sugar nucleotidyltransferases using electrospray ionization mass spectrometry

An electrospray ionization mass spectrometry-based assay has been developed to study the class of enzymes called sugar nucleotidyltransferases that couple sugar-1-phosphates and nucleotide triphosphates to form Leloir pathway glycosyl donors. The recombinant Escherichia coli and the commercially available yeast uridine-diphosphoglucose pyrophosphorylases were used as model systems. This technique allows the simultaneous and direct detection of the substrates and products without separation and, as described, is as sensitive as traditional coupled techniques. More importantly, the assay is capable of easily measuring kinetic values and inhibition constants for a range of natural and nonnatural substrates. This new assay was used to show for the first time that the reaction of the commercially available yeast uridine-diphosphoglucose pyrophosphorylase preparation is competitively inhibited by adenosine 5'-triphosphate (ATP), an observation that indicates a single active site that accepts both uridine 5'-triphosphate and ATP substrates.

Metabolic block at early stages of the glycolytic pathway activates the Rcs phosphorelay system via increased synthesis of dTDP-glucose in Escherichia coli

A mutational block in the early stages of the glycolytic pathway facilitates the degradation of the ptsG mRNA encoding the major glucose transporter IICBGlc in Escherichia coli. The degradation is RNase E dependent and is correlated with the accumulation of either glucose-6-P or fructose-6-P (Kimata et al., 2001, EMBO J 20: 3587-3595; Morita et al., 2003, J Biol Chem 278: 15608-15614). In this paper, we investigate additional physiological effects resulting from the accumulation of glucose-6-P caused by a mutation in pgi encoding phosphoglucose isomerase, focusing on changes in gene expression. The addition of glucose to the pgi strain caused significant growth inhibition, in particular in the mlc background. Cell growth then gradually resumed as the level of IICBGlc decreased. We found that the transcription of the cps operon, encoding a series of proteins responsible for the synthesis of colanic acid, was markedly but transiently induced under this metabolic stress. Both genetic and biochemical studies revealed that the metabolic stress induces cps transcription by activating the RcsC/YojN/RcsB signal transduction system. Overexpression of glucose-6-P dehydrogenase eliminated both growth inhibition and cps induction by reducing the glucose-6-P level. Mutations in genes responsible for the synthesis of glucose-1-P and/or dTDP-glucose eliminated the activation of the Rcs system by the metabolic stress. Taken together, we conclude that an increased synthesis of dTDP-glucose activates the Rcs phosphorelay system, presumably by affecting the synthesis of oligosaccharides for enterobacterial common antigen and O-antigen.

UDP-glucose is a potential intracellular signal molecule in the control of expression of sigma S and sigma S-dependent genes in Escherichia coli

The sigma S subunit of RNA polymerase is the master regulator of a regulatory network that controls stationary-phase induction as well as osmotic regulation of many genes in Escherichia coli. In an attempt to identify additional regulatory components in this network, we have isolated Tn10 insertion mutations that in trans alter the expression of osmY and other sigma S-dependent genes. One of these mutations conferred glucose sensitivity and was localized in pgi (encoding phosphoglucose isomerase). pgi::Tn10 strains exhibit increased basal levels of expression of osmY and otsBA in exponentially growing cells and reduced osmotic inducibility of these genes. A similar phenotype was also observed for pgm and galU mutants, which are deficient in phosphoglucomutase and UDP-glucose pyrophosphorylase, respectively. This indicates that the observed effects on gene expression are related to the lack of UDP-glucose (or a derivative thereof), which is common to all three mutants. Mutants deficient in UDP-galactose epimerase (galE mutants) and trehalose-6-phosphate synthase (otsA mutants) do not exhibit such an effect on gene expression, and an mdoA mutant that is deficient in the first step of the synthesis of membrane-derived oligosaccharides, shows only a partial increase in the expression of osmY. We therefore propose that the cellular content of UDP-glucose serves as an internal signal that controls expression of osmY and other sigma S-dependent genes. In addition, we demonstrate that pgi, pgm, and galU mutants contain increased levels of sigma S during steady-state growth, indicating that UDP-glucose interferes with the expression of sigma S itself.

UTP: alpha-D-glucose-1-phosphate uridylyltransferase of Escherichia coli: isolation and DNA sequence of the galU gene and purification of the enzyme

The galU gene of Escherichia coli, thought to encode the enzyme UTP:alpha-D-glucose-1-phosphate uridylyltransferase, had previously been mapped to the 27-min region of the chromosome (J. A. Shapiro, J. Bacteriol. 92:518-520, 1966). By complementation of the membrane-derived oligosaccharide biosynthetic defect of strains with a galU mutation, we have now identified a plasmid containing the galU gene and have determined the nucleotide sequence of this gene. The galU gene is located immediately downstream of the hns gene, and its open reading frame would be transcribed in the direction opposite that of the hns gene (i.e., clockwise on the E. coli chromosome). The nucleotide sequences of five galU mutations were also determined. The enzyme UTP:alpha-D-glucose-1-phosphate uridylyltransferase was purified from a strain containing the galU gene on a multicopy plasmid. The amino-terminal amino acid sequence (10 residues) of the purified enzyme was identical to the predicted amino acid sequence (after the initiating methionine) of the galU-encoded open reading frame. The functional enzyme appears to be a tetramer of the galU gene product.

Homology between a genetic locus (mdoA) involved in the osmoregulated biosynthesis of periplasmic glucans in Escherichia coli and a genetic locus (hrpM) controlling pathogenicity of Pseudomonas syringae

Membrane-derived oligosaccharides (MDO) of Escherichia coli are representative members of a family of glucans found in the periplasmic space of Gram-negative bacteria. The two genes forming the mdoGH operon are necessary for the synthesis of MDO. The nucleotide sequence (4759 bp) and the transcriptional start of this operon were determined. Both gene products were further characterized by gene fusion analysis. MdoG is a 56 kDa periplasmic protein whose function remains to be determined. MdoH, whose presence was shown to be necessary for normal glucosyl transferase activity, is a 97 kDa protein spanning the cytoplasmic membrane. To our surprise, these proteins are not homologous to the periplasmic glucan biosynthetic enzymes previously characterized in the Rhizobiaceae family. However, a considerable homology (69% identical nucleotides out of 2816) was discovered between mdoGH and the two genes present at the hrpM locus of the phytopathogenic bacterium Pseudomonas syringae pv. syringae. Functions of these genes remain mysterious but they are known to be required for both the expression of disease symptoms on host plants and the development of the hypersensitive reaction on non-host plants (Mills and Mukhopadhyay, 1990). These results confirm the importance of periplasmic glucans for the physiological ecology of Gram-negative bacteria.

Isolation and characterization of Escherichia coli mutants blocked in production of membrane-derived oligosaccharides

We report a new procedure for the facile selection of mutants of Escherichia coli that are blocked in the production of membrane-derived oligosaccharides. Four phenotypic classes were identified, including two with a novel array of characteristics. The mutations mapped to two genetic loci. Mutations in the mdoA region near 23 min are in two distinct genes, only one of which is needed for the membrane-localized glucosyltransferase that catalyzes the synthesis of the beta-1,2-glucan backbone of membrane-derived oligosaccharides. Another set of mutations mapped near 27 min closely linked to osmZ; these appear to be in the galU gene.

Convenient preparative synthesis of [14C]trehalose from [14C]glucose by intact Escherichia coli cells

At high osmolarity, Escherichia coli synthesizes trehalose intracellularly, irrespective of the nature of the carbon source. Synthesis proceeds via the transfer of UDP-glucose to glucose 6-phosphate, yielding trehalose 6-phosphate, followed by its dephosphorylation to trehalose (H.M. Giaeyer, B.O. Styrvold, I. Kaasen, and A.R. Strøm, J. Bacteriol. 170:2841-2849, 1988). This reaction was exploited to preparatively synthesize [14C]trehalose from exogenous [14C]glucose by using intact bacteria of a mutant (DF214) that could not metabolize glucose. The total yield of radiochemically pure trehalose from glucose was routinely more than 50%.

Correlation of 3,4-dihydroxybutyl 1-phosphonate resistance with a defect in cardiolipin synthesis in Escherichia coli

Escherichia coli treated for 1 h with 100 microM rac-3,4-dihydroxybutyl 1-phosphonate (DBP), a glycerol-3-phosphate analog, die when sorted at 5 degrees C, whereas the viability of untreated cells is relatively unaffected. This observation formed the basis of a selection procedure that was used to isolate mutants that are partially resistant to DBP. One such mutant, strain 6204, is constitutive for DBP transport, exhibits a particularly high degree of cold resistance, has the same doubling time as the parent, and is similar to the parent strain in terms of incorporation of DBP into the lipid fraction. Glycerol-3-phosphate and phosphatidylglycerol phosphate synthetases obtained from strain 6204 and its parent were identical in terms of DBP recognition. The parent strain is killed when incubated in the presence of a combination of 70 microM rac-DBP and 0.25% deoxycholate, whereas strain 6204 continues to grow, albeit more slowly, in the presence of this combination. Strain 6204 can be distinguished from the parent strain on agar plates (low phosphate minimal medium with glucuronate as the sole carbon source) containing 15 microM rac-DBP. The insertion of Tn10 near the 6204 mutation has facilitated genetic manipulations. All phenotypic effects attributed to strain 6204 appear to be due to a single mutation. Genetic analysis indicates that Tn10, inserted near the gene responsible for DBP resistance, maps in the vicinity of 27 min. Three-factor crosses reveal a gene order of hemA-Dbpr-Tn10(zch)-trp. The only gene for phosphoglyceride metabolism known to map in this region is the gene associated with cardiolipin synthetase, cls. Genetic results suggest that the mutation responsible for DBP resistance maps in or very near cls. Analysis of the lipids isolated from untreated strain 6204 (and from each of the transductants prepared by P1 vir-mediated transfer of DBP resistance of wild-type strains) reveals that cardiolipin synthesis is defective. These results strongly suggest that the mutation responsible for DBP resistance has its primary effect on cardiolipin synthesis. To further test this hypothesis, strains with an authentic cls mutation were constructed and examined for resistance to DBP. These strains had growth properties that were identical with those of strain 6204. Wild-type strains and mutants defective in cardiolipin synthesis were treated with DBP and 20 mM magnesium or calcium chloride. Simultaneous treatment of either cell type with DBP and divalent cation not only failed to stimulate growth but, quite the contrary, had a marked synergistic growth inhibitory effect.

Diglyceride kinase mutants of Escherichia coli: inner membrane association of 1,2-diglyceride and its relation to synthesis of membrane-derived oligosaccharides

Mutants of Escherichia coli defective in diglyceride kinase contain 10 to 20 times more sn-1,2-diglyceride than normal cells. This material constitutes about 8% of the total lipid in such strains. We now report that this excess diglyceride is recovered in the particulate fraction, primarily in association with the inner, cytoplasmic membrane. The diglyceride kinase of wild-type cells was recovered in the same inner membrane fractions. The conditions employed for the preparation of the membranes did not appear to cause significant redistribution of lipids and proteins. The biochemical reactions leading to the formation of diglyceride in E. coli are not known. To determine whether diglyceride formation requires concurrent synthesis of the membrane-derived oligosaccharides (H. Schulman and E. P. Kennedy, J. Biol. Chem. 252:4250-4255, 1977), we have constructed a double mutant defective in both the kinase (dgk) and phosphoglucose isomerase (pgi). When oligosaccharide synthesis was inhibited in this organism by growing the cells on amino acids as the sole carbon source, the diglyceride was no longer present in large amounts. When glucose was also added to the medium, the pgi mutation was bypassed, oligosaccharide synthesis resumed, and diglyceride again accumulated. These findings suggest that diglyceride may arise during the transfer of the sn-glycero-1-P moiety from phosphatidylglycerol (and possibly cardiolipin) to the oligosaccharides. In wild-type cells the kinase permits the cyclical reutilization of diglyceride molecules for phospholipid biosynthesis.

Localization of membrane-derived oligosaccharides in the outer envelope of Escherichia coli and their occurrence in other Gram-negative bacteria

The glucose-containing, membrane-derived oligosaccharides of Escherichia coli are localized in the external envelope of that organism, most probably in the periplasmic space. The membrane-derived oligosaccharides appear to be generally occurring cell constituents of gram-negative (but not gram-positive) bacteria.