An essential function for acyl carrier protein in the biosynthesis of membrane-derived oligosaccharides of Escherichia coli

Identification of enzymatic functions of osmo-regulated periplasmic glucan biosynthesis proteins from Escherichia coli reveals a novel glycoside hydrolase family

Most Gram-negative bacteria synthesize osmo-regulated periplasmic glucans (OPG) in the periplasm or extracellular space. Pathogenicity of many pathogens is lost by knocking out opgG, an OPG-related gene indispensable for OPG synthesis. However, the biochemical functions of OpgG and OpgD, a paralog of OpgG, have not been elucidated. In this study, structural and functional analyses of OpgG and OpgD from Escherichia coli revealed that these proteins are β-1,2-glucanases with remarkably different activity from each other, establishing a new glycoside hydrolase family, GH186. Furthermore, a reaction mechanism with an unprecedentedly long proton transfer pathway among glycoside hydrolase families is proposed for OpgD. The conformation of the region that forms the reaction pathway differs noticeably between OpgG and OpgD, which explains the observed low activity of OpgG. The findings enhance our understanding of OPG biosynthesis and provide insights into functional diversity for this novel enzyme family.

(p)ppGpp and moonlighting RNases influence the first step of lipopolysaccharide biosynthesis in Escherichia coli

The outer membrane (OM) protects Gram-negative bacteria from harsh environmental conditions and provides intrinsic resistance to many antimicrobial compounds. The asymmetric OM is characterized by phospholipids in the inner leaflet and lipopolysaccharides (LPS) in the outer leaflet. Previous reports suggested an involvement of the signaling nucleotide ppGpp in cell envelope homeostasis in Escherichia coli. Here, we investigated the effect of ppGpp on OM biosynthesis. We found that ppGpp inhibits the activity of LpxA, the first enzyme of LPS biosynthesis, in a fluorometric in vitro assay. Moreover, overproduction of LpxA resulted in elongated cells and shedding of outer membrane vesicles (OMVs) with altered LPS content. These effects were markedly stronger in a ppGpp-deficient background. We further show that RnhB, an RNase H isoenzyme, binds ppGpp, interacts with LpxA, and modulates its activity. Overall, our study uncovered new regulatory players in the early steps of LPS biosynthesis, an essential process with many implications in the physiology and susceptibility to antibiotics of Gram-negative commensals and pathogens.

Acyl Carrier Protein 3 Is Involved in Oxidative Stress Response in Pseudomonas aeruginosa

The human opportunistic pathogen Pseudomonas aeruginosa expresses three acyl carrier proteins (ACPs): AcpP, Acp1, and Acp3. The function of AcpP in membrane fatty acid synthesis (FAS) was confirmed recently, but the physiological roles of Acp1 and Acp3 remain unclear. To address this, we investigated the physiological role of Acp3 in P. aeruginosa. We found that expression of Acp3 dramatically increases in the log phase of cell growth and that its transcription is under the control of the QS regulators LasR and RhlR. Deletion of acp3 from P. aeruginosa strain PAO1 results in thicker biofilm formation, increased resistance of the strain to hydrogen peroxide, and higher persistence in a mouse infection model. Tandem affinity purification (TAP) experiments revealed several novel protein-binding partners of Acp3, including KatA, the major catalase in P. aeruginosa. Acp3 was found to repress the catalase activity of KatA and, consistent with inhibition by Acp3, less reactive oxygen species are present in the acp3 deletion strain. Overall, our study reveals that Acp3 has a distinct function from that of the canonical AcpP and may be involved in the oxidative stress response.

A nutrient-dependent division antagonist is regulated post-translationally by the Clp proteases in Bacillus subtilis

BACKGROUND

Changes in nutrient availability have dramatic and well-defined impacts on both transcription and translation in bacterial cells. At the same time, the role of post-translational control in adaptation to nutrient-poor environments is poorly understood. Previous studies demonstrate the ability of the glucosyltransferase UgtP to influence cell size in response to nutrient availability. Under nutrient-rich medium, interactions with its substrate UDP-glucose promote interactions between UgtP and the tubulin-like cell division protein FtsZ in Bacillus subtilis, inhibiting maturation of the cytokinetic ring and increasing cell size. In nutrient-poor medium, reductions in UDP-glucose availability favor UgtP oligomerization, sequestering it from FtsZ and allowing division to occur at a smaller cell mass.

RESULTS

Intriguingly, in nutrient-poor conditions UgtP levels are reduced ~ 3-fold independent of UDP-glucose. B. subtilis cells cultured under different nutrient conditions indicate that UgtP accumulation is controlled through a nutrient-dependent post-translational mechanism dependent on the Clp proteases. Notably, all three B. subtilis Clp chaperones appeared able to target UgtP for degradation during growth in nutrient-poor conditions.

CONCLUSIONS

Together these findings highlight conditional proteolysis as a mechanism for bacterial adaptation to a rapidly changing nutritional landscape.

Osmoregulated Periplasmic Glucans

Among all the systems developed by enterobacteria to face osmotic stress, only osmoregulated periplasmic glucans (OPGs) were found to be modulated during osmotic fluxes. First detected in 1973 by E.P. Kennedy's group in a study of phospholipid turnover in Escherichia coli, OPGs have been shown across alpha, beta, and gamma subdivisions of the proteobacteria. Discovery of OPG-like compounds in the epsilon subdivision strongly suggested that the presence of periplasmic glucans is essential for almost all proteobacteria. This article offers an overview of the different classes of OPGs. Then, the biosynthesis of OPGs and their regulation in E. coli and other species are discussed. Finally, the biological role of OPGs is developed. Beyond structural function, OPGs are involved in pathogenicity, in particular, by playing a role in signal transduction pathways. Recently, OPG synthesis proteins have been suggested to control cell division and growth rate.

Biosynthesis of Membrane Lipids

The pathways in Escherichia coli and (largely by analogy) S. enterica remain the paradigm of bacterial lipid synthetic pathways, although recently considerable diversity among bacteria in the specific areas of lipid synthesis has been demonstrated. The structural biology of the fatty acid synthetic proteins is essentially complete. However, the membrane-bound enzymes of phospholipid synthesis remain recalcitrant to structural analyses. Recent advances in genetic technology have allowed the essentialgenes of lipid synthesis to be tested with rigor, and as expected most genes are essential under standard growth conditions. Conditionally lethal mutants are available in numerous genes, which facilitates physiological analyses. The array of genetic constructs facilitates analysis of the functions of genes from other organisms. Advances in mass spectroscopy have allowed very accurate and detailed analyses of lipid compositions as well as detection of the interactions of lipid biosynthetic proteins with one another and with proteins outside the lipid pathway. The combination of these advances has resulted in use of E. coli and S. enterica for discovery of new antimicrobials targeted to lipid synthesis and in deciphering the molecular actions of known antimicrobials. Finally,roles for bacterial fatty acids other than as membrane lipid structural components have been uncovered. For example, fatty acid synthesis plays major roles in the synthesis of the essential enzyme cofactors, biotin and lipoic acid. Although other roles for bacterial fatty acids, such as synthesis of acyl-homoserine quorum-sensing molecules, are not native to E. coli introduction of the relevant gene(s) synthesis of these foreign molecules readily proceeds and the sophisticated tools available can used to decipher the mechanisms of synthesis of these molecules.

Evaluation of the Role of the opgGH Operon in Yersinia pseudotuberculosis and Its Deletion during the Emergence of Yersinia pestis

The opgGH operon encodes glucosyltransferases that synthesize osmoregulated periplasmic glucans (OPGs) from UDP-glucose, using acyl carrier protein (ACP) as a cofactor. OPGs are required for motility, biofilm formation, and virulence in various bacteria. OpgH also sequesters FtsZ in order to regulate cell size according to nutrient availability. Yersinia pestis (the agent of flea-borne plague) lost the opgGH operon during its emergence from the enteropathogen Yersinia pseudotuberculosis. When expressed in OPG-negative strains of Escherichia coli and Dickeya dadantii, opgGH from Y. pseudotuberculosis restored OPGs synthesis, motility, and virulence. However, Y. pseudotuberculosis did not produce OPGs (i) under various growth conditions or (ii) when overexpressing its opgGH operon, its galUF operon (governing UDP-glucose), or the opgGH operon or Acp from E. coli. A ΔopgGH Y. pseudotuberculosis strain showed normal motility, biofilm formation, resistance to polymyxin and macrophages, and virulence but was smaller. Consistently, Y. pestis was smaller than Y. pseudotuberculosis when cultured at ≥ 37°C, except when the plague bacillus expressed opgGH. Y. pestis expressing opgGH grew normally in serum and within macrophages and was fully virulent in mice, suggesting that small cell size was not advantageous in the mammalian host. Lastly, Y. pestis expressing opgGH was able to infect Xenopsylla cheopis fleas normally. Our results suggest an evolutionary scenario whereby an ancestral Yersinia strain lost a factor required for OPG biosynthesis but kept opgGH (to regulate cell size). The opgGH operon was presumably then lost because OpgH-dependent cell size control became unnecessary.

Bacterial lipids: metabolism and membrane homeostasis

Membrane lipid homeostasis is a vital facet of bacterial cell physiology. For decades, research in bacterial lipid synthesis was largely confined to the Escherichia coli model system. This basic research provided a blueprint for the biochemistry of lipid metabolism that has largely defined the individual steps in bacterial fatty acid and phospholipids synthesis. The advent of genomic sequencing has revealed a surprising amount of diversity in the genes, enzymes and genetic organization of the components responsible for bacterial lipid synthesis. Although the chemical steps in fatty acid synthesis are largely conserved in bacteria, there are surprising differences in the structure and cofactor requirements for the enzymes that perform these reactions in Gram-positive and Gram-negative bacteria. This review summarizes how the explosion of new information on the diversity of biochemical and genetic regulatory mechanisms has impacted our understanding of bacterial lipid homeostasis. The potential and problems of developing therapeutics that block pathogen phospholipid synthesis are explored and evaluated. The study of bacterial lipid metabolism continues to be a rich source for new biochemistry that underlies the variety and adaptability of bacterial life styles.

The length of the bound fatty acid influences the dynamics of the acyl carrier protein and the stability of the thioester bond

Acyl carrier proteins involved in fatty acid biosynthesis have been shown to exhibit a high degree of conformational flexibility, in that they are able to sequester fatty acid intermediates between 4 and 18 carbons in length. This flexibility has been observed in X-ray and NMR structures of acyl carrier proteins attached to different fatty acids. NMR studies comparing decanoyl-ACP and stearoyl-ACP indicated that ACP exhibits more dynamic motions when bound to longer fatty acids. We have used complementary chemical and NMR methods as an approach to improving our understanding of the effect of fatty acid length on the dynamics of acyl carrier protein. A chemical assay of the accessibility of the acyl thioester to solvent revealed a positive correlation between chain length and rate of hydrolysis. Surprisingly, this linear correlation was biphasic, with accelerated hydrolysis observed for fatty acids longer than 15 carbons. To further understand the motions associated with this acceleration, we collected ¹⁵N relaxation dispersion data for 14:0-, 15:0-, and 16:0-ACP. The greatest dispersions were exhibited by residues that form the entrance to the fatty acid binding pocket. In addition, these dispersions were observed to increase with the length of the fatty acid. Because the exchange rates derived from fitting the data to a two-state model varied from residue to residue, a more complex motional model appears to be required to adequately explain the dynamics. Thus, acyl-ACP offers an interesting system for future investigations of complex protein motions on the micro- and millisecond time scales.

Linear osmoregulated periplasmic glucans are encoded by the opgGH locus of Pseudomonas aeruginosa

Osmoregulated periplasmic glucans (OPGs) are produced by many proteobacteria and are important for bacterial-host interactions. The opgG and opgH genes involved in the synthesis of OPGs are the most widely distributed genes in proteobacterial genomes. Two other non-homologous genes, both named ndvB, are also involved in OPG biosynthesis in several species. The Pseudomonas aeruginosa genome possesses two ORFs, PA5077 and PA5078, that show similarity to opgH and opgG of Pseudomonas syringae, respectively, and one ORF, PA1163, similar to ndvB of Sinorhizobium meliloti. Here, we report that the opgGH locus of P. aeruginosa PA14 is involved in the synthesis of linear polymers with beta-1,2-linked glucosyl residues branched with a few beta-1,6 glucosyl residues. Succinyl residues also substitute this glucose backbone. Transcription of opgGH is repressed by high osmolarity. Low osmolarity promotes the formation of highly structured biofilms, but biofilm development is slower and the area of biomass is reduced under high osmolarity. Biofilm development of an opgGH mutant grown under low osmolarity presents a similar phenotype to the wild-type biofilm grown under high osmolarity. These results suggest that OPGs are important for biofilm formation under conditions of low osmolarity. A previous study suggested that the P. aeruginosa ndvB gene is involved in the resistance of biofilms to antibiotics. We have shown that ndvB is not involved in the biosynthesis of the OPG described here, and opgGH do not appear to be involved in the resistance of P. aeruginosa PA14 biofilms to antibiotics.

Structural Studies of Fatty Acyl-(Acyl Carrier Protein) Thioesters Reveal a Hydrophobic Binding Cavity that Can Expand to Fit Longer Substrates

A knowledge of the structures of acyl chain loaded species of the acyl carrier protein (ACP) as used in fatty acid biosynthesis and a range of other metabolic events, is essential for a full understanding of the molecular recognition at the heart of these processes. To date the only crystal structure of an acylated species of ACP is that of a butyryl derivative of Escherichia coli ACP. We have now determined the structures of a family of acylated E. coli ACPs of varying acyl chain length. The acyl moiety is attached via a thioester bond to a phosphopantetheine linker that is in turn bound to a serine residue in ACP. The growing acyl chain can be accommodated within a central cavity in the ACP for transport during the elongation stages of lipid synthesis through changes in the conformation of a four alpha-helix bundle. The results not only clarify the means by which a substrate of varying size and complexity is transported in the cell but also suggest a mechanism by which interacting enzymes can recognize the loaded ACP through recognition of surface features including the conformation of the phosphopantetheine linker.

Solution structures of spinach acyl carrier protein with decanoate and stearate

Acyl carrier protein (ACP) is a cofactor in a variety of biosynthetic pathways, including fatty acid metabolism. Thus, it is of interest to determine structures of physiologically relevant ACP-fatty acid complexes. We report here the NMR solution structures of spinach ACP with decanoate (10:0-ACP) and stearate (18:0-ACP) attached to the 4'-phosphopantetheine prosthetic group. The protein in the fatty acid complexes adopts a single conformer, unlike apo- and holo-ACP, which interconvert in solution between two major conformers. The protein component of both 10:0- and 18:0-ACP adopts the four-helix bundle topology characteristic of ACP, and a fatty acid binding cavity was identified in both structures. Portions of the protein close in space to the fatty acid and the 4'-phosphopantetheine were identified using filtered/edited NOESY experiments. A docking protocol was used to generate protein structures containing bound fatty acid for 10:0- and 18:0-ACP. In both cases, the predominant structure contained fatty acid bound down the center of the helical bundle, in agreement with the location of the fatty acid binding pockets. These structures demonstrate the conformational flexibility of spinach ACP and suggest how the protein changes to accommodate its myriad binding partners.

Gene-specific random mutagenesis of Escherichia coli in vivo: isolation of temperature-sensitive mutations in the acyl carrier protein of fatty acid synthesis

Acyl carrier proteins (ACPs) are very small acidic proteins that play a key role in fatty acid and complex lipid synthesis. Moreover, recent data indicate that the acyl carrier protein of Escherichia coli has a large protein interaction network that extends beyond lipid synthesis. Despite extensive efforts over many years, no temperature-sensitive mutants with mutations in the structural gene (acpP) that encodes ACP have been isolated. We report the isolation of three such mutants by a new approach that utilizes error-prone PCR mutagenesis, overlap extension PCR, and phage lambda Red-mediated homologous recombination and that should be generally applicable. These mutants plus other experiments demonstrate that ACP function is essential for the growth of E. coli. Each of the mutants was efficiently modified with the phosphopantetheinyl moiety essential for the function of ACP in lipid synthesis, and thus lack of function at the nonpermissive temperature cannot be attributed to a lack of prosthetic group attachment. All of the mutant proteins were largely stable at the nonpermissive temperature except the A68T/N73D mutant protein. Fatty acid synthesis in strains that carried the D38V or A68T/N73D mutations was inhibited upon a shift to the nonpermissive temperature and in the latter case declined to a small percentage of the rate of the wild-type strain.

NMR studies of Escherichia coli acyl carrier protein: dynamic and structural differences of the apo- and holo-forms

Two indicators of conformational variability of Escherichia coli acyl carrier protein (ACP) have been investigated, namely backbone dynamics and chemical shift variations of ACP. Hydrophobic interactions between the 4'-PP prosthetic group and the hydrophobic pocket enclosed by the amphipathic helices resulted in chemical shift perturbations in the residues near the prosthetic group binding sites and contact sites in the hydrophobic pockets upon conversion from apo- to holo-forms. At pH 7.9, destabilization of ACP due to negative charge repulsions and the deprotonated state of His 75 resulted in observed chemical shift changes in the C-terminal region. Model-free analysis showed that the alpha(1)alpha(2) loop region near the prosthetic group binding site in ACP shows the greatest flexibility (lowest S(2) values) and this result may suggest these flexibilities are required for structural rearrangements when the acyl chain binds to the prosthetic group of ACP. Flexibility of ACP shown in this study is essential for its ability to interact with functionally different enzyme partners specifically and weakly in the rapid delivery of acyl chain from one partner to another.

Preparation of isotopically labeled spinach acyl-acyl carrier protein for NMR structural studies

Acyl carrier proteins (ACPs) are important protein cofactors in fatty acid biosynthesis, but their acylated forms have not been well-studied. To permit detailed nuclear magnetic resonance studies of acylated spinach ACP isoform I, we have developed a new expression plasmid for recombinant production of the apo-protein and modified protocols for purifying the protein product and acylating it to form acyl-ACP. To solve plasmid stability problems associated with growth in minimal media, the ampicillin resistance gene from pSACP-2a was replaced with the tetA(C) gene from pBR322. The resulting plasmid, pSACP-2t, supported overexpression of apo-ACP in Escherichia coli BL21(DE3) cells in M9 medium containing 15NH4Cl as the sole nitrogen source. Apo-ACP was purified to homogeneity by means of polyethylene glycol precipitation and anion exchange. Two in vitro synthetic routes were used to produce acyl-ACPs. In one route, apo-ACP was converted to the holo form and the acyl form by a published protocol that employs a discrete enzymatic reaction for each step. As an alternative route to produce decanoyl-ACP, apo-ACP was directly converted to the acyl form by using holo-ACP synthase along with the non-natural substrate decanoyl-CoA. Two-dimensional 1H-15N NMR spectroscopy of decanoyl-ACP and stearoyl-ACP revealed that changes in the length of the covalently attached fatty acid do not affect the secondary structure of the protein but do influence the local conformation and dynamics.

The genetics of glycosylation in Gram-negative bacteria

In recent years there has been a dramatic increase in reports of glycosylation of proteins in various Gram-negative systems including Neisseria meningitidis, Neisseria gonorrhoeae, Campylobacter jejuni, Pseudomonas aeruginosa, Escherichia coli, Caulobacter crescentus, Aeromonas caviae and Helicobacter pylori. Although this growing list contains many important pathogens (reviewed by Benz and Schmidt [Mol. Microbiol. 45 (2002) 267-276]) and the glycosylations are found on proteins important in pathogenesis such as pili, adhesins and flagella the precise role(s) of the glycosylation of these proteins remains to be determined. Furthermore, the details of the glycosylation biosynthetic process have not been determined in any of these systems. The definition of the precise role of glycosylation and the mechanism of biosynthesis will be facilitated by a detailed understanding of the genes involved.

X-ray crystallographic studies on butyryl-ACP reveal flexibility of the structure around a putative acyl chain binding site

Acyl carrier protein (ACP) is an essential cofactor in biosynthesis of fatty acids and many other reactions that require acyl transfer steps. We have determined the first crystal structures of an acylated form of ACP from E. coli, that of butyryl-ACP. Our analysis of the molecular surface of ACP reveals a plastic hydrophobic cavity in the vicinity of the phosphopantethylated Ser36 residue that is expanded and occupied by the butyryl and beta-mercaptoethylamine moieties of the acylated 4'-phosphopantetheine group in one of our crystal forms. In the other form, the cavity is contracted, and we propose that the protein has adopted the conformation after delivery of substrate into the active site of a partner enzyme.

Putative ACP phosphodiesterase gene (acpD) encodes an azoreductase

An FMN-dependent NADH-azoreductase of Escherichia coli was purified and analyzed for identification of the gene responsible for azo reduction by microorganisms. The N-terminal sequence of the azoreductase conformed to that of the acpD gene product, acyl carrier protein phosphodiesterase. Overexpression of the acpD gene provided the E. coli with a large amount of the 23-kDa protein and more than 800 times higher azoreductase activity. The purified gene product exhibited activity corresponding to that of the native azoreductase. The reaction followed a ping-pong mechanism requiring 2 mol of NADH to reduce 1 mol of methyl red (4'-dimethylaminoazobenzene-2-carboxylic acid) into 2-aminobenzoic acid and N,N'-dimethyl-p-phenylenediamine. On the other hand, the gene product could not convert holo-acyl carrier protein into the apo form under either in vitro or in vivo conditions. These data indicate that the acpD gene product is not acyl carrier protein phosphodiesterase but an azoreductase.

Topological analysis of the membrane-bound glucosyltransferase, MdoH, required for osmoregulated periplasmic glucan synthesis in Escherichia coli

The MdoH protein is essential for synthesis of the osmoregulated periplasmic glucans, known as membrane-derived oligosaccharides (MDOs), in Escherichia coli. Mutants lacking MdoH are deficient in a glucosyltransferase activity assayed in vitro. The MdoH protein is the product of the second gene of an operon, and it has been shown to span the cytoplasmic membrane. The MdoH protein comprises 847 amino acids and is poorly expressed as observed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. We have experimentally measured the topological organization of MdoH within the membrane by construction of fusions to beta-lactamase as a reporter. Analysis of 51 different MdoH-beta-lactamase fusions suggested that the MdoH protein crosses the cytoplasmic membrane eight times, with the N and C termini in the cytoplasm. Moreover, a 310-amino-acid domain is present in the cytoplasm between the second and third transmembrane segments. It was deduced from the measurement of the MDO biosynthetic activity of truncated or fused MdoH proteins that almost all the C-terminal residues are necessary for this activity. The model of the MdoH protein in the membrane suggests that this protein could be directly involved in the translocation of nascent polyglucose chains to the periplasmic space.

Domains of Escherichia coli acyl carrier protein important for membrane-derived-oligosaccharide biosynthesis

Acyl carrier protein participates in a number of biosynthetic pathways in Escherichia coli: fatty acid biosynthesis, phospholipid biosynthesis, lipopolysaccharide biosynthesis, activation of prohemolysin, and membrane-derived oligosaccharide biosynthesis. The first four pathways require the protein's prosthetic group, phosphopantetheine, to assemble an acyl chain or to transfer an acyl group from the thioester linkage to a specific substrate. By contrast, the phosphopantetheine prosthetic group is not required for membrane-derived oligosaccharide biosynthesis, and the function of acyl carrier protein in this biosynthetic scheme is currently unknown. We have combined biochemical and molecular biological approaches to investigate domains of acyl carrier protein that are important for membrane-derived oligosaccharide biosynthesis. Proteolytic removal of the first 6 amino acids from acyl carrier protein or chemical synthesis of a partial peptide encompassing residues 26 to 50 resulted in losses of secondary and tertiary structure and consequent loss of activity in the membrane glucosyltransferase reaction of membrane-derived oligosaccharide biosynthesis. These peptide fragments, however, inhibited the action of intact acyl carrier protein in the enzymatic reaction. This suggests a role for the loop regions of the E. coli acyl carrier protein and the need for at least two regions of the protein for participation in the glucosyltransferase reaction. We have purified acyl carrier protein from eight species of Proteobacteria (including representatives from all four subgroups) and characterized the proteins as active or inhibitory in the membrane glucosyltransferase reaction. The complete or partial amino acid sequences of these acyl carrier proteins were determined. The results of site-directed mutagenesis to change amino acids conserved in active, and altered in inactive, acyl carrier proteins suggest the importance of residues Glu-4, Gln-14, Glu-21, and Asp-51. The first 3 of these residues define a face of acyl carrier protein that includes the beginning of the loop region, residues 16 to 36. Additionally, screening for membrane glucosyltransferase activity in membranes from bacterial species that had acyl carrier proteins that were active with E. coli membranes revealed the presence of glucosyltransferase activity only in the species most closely related to E. coli. Thus, it seems likely that only bacteria from the Proteobacteria subgroup gamma-3 have periplasmic glucans synthesized by the mechanism found in E. coli.

Mitochondrial acyl carrier protein is involved in lipoic acid synthesis in Saccharomyces cerevisiae

The yeast gene, ACP1, encoding the mitochondrial acyl carrier protein, was deleted by gene replacement. The resulting acp1-deficient mutants had only 5-10% of the wild-type lipoic acid content remaining, and exhibited a respiratory-deficient phenotype. Upon meiosis, the lipoate deficiency co-segregated with the acp1 deletion. The role of ACP1 in long-chain fatty acid synthesis was studied in fast and fas2 null mutants completely lacking cytoplasmic fatty acid synthase. When grown on odd-chain (13:0 and 15:0) fatty acids, these cells showed less than 1% of C-16 and C-18 acids in their total lipids. Mitochondrial ACP is therefore suggested to be involved with the biosynthesis of octanoate, a precursor to lipoic acid.

Identification of a cotton fiber-specific acyl carrier protein cDNA by differential display

Transcripts from immature fibers and stripped ovules (fibers removed) of cotton (Gossypium hirsutum L.) were compared by differential display to identify cDNA fragments that represent mRNAs that are expressed primarily in cotton fibers. Eight independent fiber-specific cDNA fragments were isolated. One of these cDNAs had strong sequence similarity with acyl carrier protein (ACP). A full-length cDNA for the cotton fiber-specific ACP was isolated using a PCR cDNA library screening technique. This 713 bp cDNA has an open reading frame that encodes a 136 amino acid polypeptide. Overall nucleotide and amino acid sequence identities with other plant ACP gene sequences averaged 66% and 60% respectively. A 19 amino acid sequence surrounding the prosthetic group attachment site is nearly identical to other plant ACP genes. Northern blot analyses showed that transcripts homologous to this fiber-specific ACP cDNA were predominantly expressed during the elongation stage of fiber development. Initial genomic Southern blot analysis indicated that a single copy of the fiber-specific ACP gene may be present in both the cotton A and D genomes, since diploid Gossypium species with A or D genomes gave identical bands. We speculate that this putative fiber-specific ACP may play an important role in rapidly elongating cotton fibers by contributing to the synthesis of membrane lipids. It is also apparent that during the evolution of cotton a member of the ACP gene family has been recruited for specific expression in cotton fibers.

Why do mitochondria synthesize fatty acids? Evidence for involvement in lipoic acid production

The function of acyl carrier protein (ACP) in mitochondria isolated from pea leaves has been investigated. When pea leaf mitochondria were labeled with [2-14C] malonic acid in vitro, radioactivity was incorporated into fatty acids, and, simultaneously, ACP was acylated. [1-14C]Acetate was much less effective as a precursor for fatty acid synthesis, suggesting that mitochondria do not possess acetyl-CoA carboxylase. The incorporation of radioactivity from [2-14C]malonate into fatty acids and the labeling of ACP were inhibited by cerulenin and required ATP and Mg2+. These findings indicate that plant mitochondria contain not only ACP, but all enzymes required for de novo fatty acid synthesis. Over 30% of the radioactive products from pea mitochondria labeled with [2-14C]malonate were recovered in H protein, which is a subunit of glycine decarboxylase and contains lipoic acid as an essential constituent. In similar experiments, the H protein of Neurospora mitochondria was also labeled by [2-14C]malonate. The labeling of pea H protein was inhibited by addition of cerulenin into the assay medium. Together, these findings indicate that ACP is involved in the de novo synthesis of fatty acids in plant mitochondria and that a major function of this pathway is production of lipoic acid precursors.

Biosynthesis of teichuronic acid in the bacterial cell wall. Purification and characterization of the glucosyltransferase of Micrococcus luteus

This report describes what is, to our knowledge, the first purification to near homogeneity of an enzyme involved in the biosynthesis of the teichuronic acid of Micrococcus luteus cell walls. The glucosyltransferase of M. luteus, which participates in the biosynthesis of teichuronic acid, was solubilized from cytoplasmic membrane fragments by extraction with buffer solutions containing the detergents Thesit (dodecyl alcohol polyoxyethylene ether; 1 mg/ml) and 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (0.5 mg/ml). The detergent-solubilized enzyme was purified 150-fold, with a recovery of 13% by adsorbent column chromatography, ion-exchange chromatography, gel filtration, and preparative nondenaturing gradient polyacrylamide gel electrophoresis. On the basis of its mobility on native gradient gel, the glucosyltransferase was estimated to have a molecular mass of 440 kDa. The purified native enzyme was a multisubunit protein consisting of subunits of two sizes; their molecular masses were determined to be 52.5 and 54 kDa, respectively, by observation of the mobility of the protein bands in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The isoelectric point of the enzyme was approximately 5.

Desiccation tolerance of prokaryotes

The removal of cell-bound water through air drying and the addition of water to air-dried cells are forces that have played a pivotal role in the evolution of the prokaryotes. In bacterial cells that have been subjected to air drying, the evaporation of free cytoplasmic water (Vf) can be instantaneous, and an equilibrium between cell-bound water (Vb) and the environmental water (vapor) potential (psi wv) may be achieved rapidly. In the air-dried state some bacteria survive only for seconds whereas others can tolerate desiccation for thousands, perhaps millions, of years. The desiccated (anhydrobiotic) cell is characterized by its singular lack of water--with contents as low as 0.02 g of H2O g (dry weight)-1. At these levels the monolayer coverage by water of macromolecules, including DNA and proteins, is disturbed. As a consequence the mechanisms that confer desiccation tolerance upon air-dried bacteria are markedly different from those, such as the mechanism of preferential exclusion of compatible solutes, that preserve the integrity of salt-, osmotically, and freeze-thaw-stressed cells. Desiccation tolerance reflects a complex array of interactions at the structural, physiological, and molecular levels. Many of the mechanisms remain cryptic, but it is clear that they involve interactions, such as those between proteins and co-solvents, that derive from the unique properties of the water molecule. A water replacement hypothesis accounts for how the nonreducing disaccharides trehalose and sucrose preserve the integrity of membranes and proteins. Nevertheless, we have virtually no insight into the state of the cytoplasm of an air-dried cell. There is no evidence for any obvious adaptations of proteins that can counter the effects of air drying or for the occurrence of any proteins that provide a direct and a tangible contribution to cell stability. Among the prokaryotes that can exist as anhydrobiotic cells, the cyanobacteria have a marked capacity to do so. One form, Nostoc commune, encompasses a number of the features that appear to be critical to the withstanding of a long-term water deficit, including the elaboration of a conspicuous extracellular glycan, synthesis of abundant UV-absorbing pigments, and maintenance of protein stability and structural integrity. There are indications of a growing technology for air-dried cells and enzymes. Paradoxically, desiccation tolerance of bacteria has virtually been ignored for the past quarter century. The present review considers what is known, and what is not known, about desiccation, a phenomenon that impinges upon every facet of the distributions and activities of prokaryotic cells.

Serine residue 45 of nodulation protein NodF from Rhizobium leguminosarum bv. viciae is essential for its biological function

A system for testing the role of the Rhizobium nodF gene in the production of host-specific lipochitin oligosaccharides and in nodulation was developed. We show that a mutant nodF gene, in which the codon for serine residue 45 was changed to that for threonine, still expresses NodF, which, however, is no longer functional.

Desiccation tolerance of prokaryotes

The removal of cell-bound water through air drying and the addition of water to air-dried cells are forces that have played a pivotal role in the evolution of the prokaryotes. In bacterial cells that have been subjected to air drying, the evaporation of free cytoplasmic water (Vf) can be instantaneous, and an equilibrium between cell-bound water (Vb) and the environmental water (vapor) potential (psi wv) may be achieved rapidly. In the air-dried state some bacteria survive only for seconds whereas others can tolerate desiccation for thousands, perhaps millions, of years. The desiccated (anhydrobiotic) cell is characterized by its singular lack of water--with contents as low as 0.02 g of H2O g (dry weight)-1. At these levels the monolayer coverage by water of macromolecules, including DNA and proteins, is disturbed. As a consequence the mechanisms that confer desiccation tolerance upon air-dried bacteria are markedly different from those, such as the mechanism of preferential exclusion of compatible solutes, that preserve the integrity of salt-, osmotically, and freeze-thaw-stressed cells. Desiccation tolerance reflects a complex array of interactions at the structural, physiological, and molecular levels. Many of the mechanisms remain cryptic, but it is clear that they involve interactions, such as those between proteins and co-solvents, that derive from the unique properties of the water molecule. A water replacement hypothesis accounts for how the nonreducing disaccharides trehalose and sucrose preserve the integrity of membranes and proteins. Nevertheless, we have virtually no insight into the state of the cytoplasm of an air-dried cell. There is no evidence for any obvious adaptations of proteins that can counter the effects of air drying or for the occurrence of any proteins that provide a direct and a tangible contribution to cell stability. Among the prokaryotes that can exist as anhydrobiotic cells, the cyanobacteria have a marked capacity to do so. One form, Nostoc commune, encompasses a number of the features that appear to be critical to the withstanding of a long-term water deficit, including the elaboration of a conspicuous extracellular glycan, synthesis of abundant UV-absorbing pigments, and maintenance of protein stability and structural integrity. There are indications of a growing technology for air-dried cells and enzymes. Paradoxically, desiccation tolerance of bacteria has virtually been ignored for the past quarter century. The present review considers what is known, and what is not known, about desiccation, a phenomenon that impinges upon every facet of the distributions and activities of prokaryotic cells.

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.

Regulation of fatty acid biosynthesis in Escherichia coli

Our understanding of fatty acid biosynthesis in Escherichia coli has increased greatly in recent years. Since the discovery that the intermediates of fatty acid biosynthesis are bound to the heat-stable protein cofactor termed acyl carrier protein, the fatty acid synthesis pathway of E. coli has been studied in some detail. Interestingly, many advances in the field have aided in the discovery of analogous systems in other organisms. In fact, E. coli has provided a paradigm of predictive value for the synthesis of fatty acids in bacteria and plants and the synthesis of bacterial polyketide antibiotics. In this review, we concentrate on four major areas of research. First, the reactions in fatty acid biosynthesis and the proteins catalyzing these reactions are discussed in detail. The genes encoding many of these proteins have been cloned, and characterization of these genes has led to a better understanding of the pathway. Second, the function and role of the two essential cofactors in fatty acid synthesis, coenzyme A and acyl carrier protein, are addressed. Finally, the steps governing the spectrum of products produced in synthesis and alternative destinations, other than membrane phospholipids, for fatty acids in E. coli are described. Throughout the review, the contribution of each portion of the pathway to the global regulation of synthesis is examined. In no other organism is the bulk of knowledge regarding fatty acid metabolism so great; however, questions still remain to be answered. Pursuing such questions should reveal additional regulatory mechanisms of fatty acid synthesis and, hopefully, the role of fatty acid synthesis and other cellular processes in the global control of cellular growth.

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.

beta-Furfuryl-beta-Glucoside: An Endogenous Activator of Higher Plant UDP-Glucose:(1-3)-beta-Glucan Synthase : Biological Activity, Distribution, and in Vitro Synthesis

In a recent paper (P Ohana, DP Delmer, JC Steffens, DE Matthews, R Mayer, M Benziman [1991] J Biol Chem 266: 13472-13475), we described the purification and structural characterization of beta-furfuryl-beta-glucoside (FG), an endogenous activator of plant UDP-glucose:(1-->3)-beta-glucan (callose) synthase. In the present report, we provide evidence that FG specifically stimulates callose synthase. The effects of FG on the kinetic properties of callose synthase were studied, and we ascertained that FG, or at least a very similar compound, is present in other plant systems. Chemically synthesized alpha-furfuryl-beta-glucoside also stimulates callose synthase, exhibiting a slightly higher K(a) of 80 micromolar, compared with 50 micromolar for FG. In addition, we have identified and partially characterized an enzyme that catalyzes the synthesis of FG using beta-furfuryl alcohol and UDP-glucose as substrates. A model for the regulation of callose synthesis in vivo, involving changes in intracellular compartmentation of FG and Ca(2+), is proposed.

The mdoA locus of Escherichia coli consists of an operon under osmotic control

In Escherichia coli, the 5 kb mdoA locus is involved in the osmotically controlled biosynthesis of periplasmic membrane-derived oligosaccharides (MDOs). The structure of this locus was analysed by in vitro cassette insertion, transposon mutagenesis, and gene-fusion analysis. A 'neo' cassette, derived from the neomycin phosphotransferase II region of transposon Tn5, was inserted into mdoA, borne by a multicopy plasmid. This plasmid was shown to complement two previously described mdoA mutations, depending on the orientation of the exogenous gene. Thus, the gene altered by these mutations could be expressed under the control of the exogenous promoter. Moreover, the 'neo' cassette inactivated another, uncharacterized, mdo gene, because when this insertion was transferred into the chromosome MDO synthesis was abolished. The existence of a second gene was confirmed by complementation analysis with a collection of Tn1000 insertions into mdoA. Two groups were defined, and the two genes are organized into an operon (mdoGH). This conclusion was reached because Tn1000 insertions in the first gene displayed a polar effect on the expression of the second gene. An active gene fusion was obtained on a multicopy plasmid between the beginning of mdoH and lacZ. The hybrid beta-galactosidase activity followed the same osmotically controlled response as that described for of MDO synthesis. This regulation was unaffected by the presence, or absence, of MDOs in the periplasm. Finally, the amount of mdoA-specific mRNAs, determined by dot blot hybridization, decreased when the osmolarity of the growth medium increased.

A novel membrane-bound glucosyltransferase from Bradyrhizobium japonicum

Bacteria within the family Rhizobiaceae are distinguished by their ability to infect higher plants. The cell envelope carbohydrates of these bacteria are believed to be involved in the plant infection process. One class of cell envelope carbohydrate, the cyclic beta-1,2-glucans, is synthesized by species within two genera of this family, Agrobacterium and Rhizobium. In contrast, species of the genus Bradyrhizobium, a third genus within this family, appear to lack the capacity for cyclic beta-1,2-glucan biosynthesis. Instead, these bacteria synthesize cyclic glucans containing beta-1,6 and beta-1,3 glycosidic linkages (K.J. Miller, R.S. Gore, R. Johnson, A.J. Benesi, and V.N. Reinhold, J. Bacteriol. 172:136-142, 1990). We now report the initial characterization of a novel membrane-bound glucosyltransferase activity from Bradyrhizobium japonicum USDA 110. Analysis of the product of this glucosyltransferase activity revealed the following: the presence of beta-1,3 and beta-1,6 glycosidic linkages, an average molecular weight of 2,100, and no detectable reducing terminal residues. The glucosyltransferase activity was found to have an apparent Km of 50 microM for for UDP-glucose, and activity was stimulated optimally by Mn2+ ions. On the basis of the structural properties of the in vitro glucan product, it is possible that this membrane-bound glucosyltransferase activity may be responsible for the biosynthesis of cyclic beta-1,6-beta-1,3-glucans by this organism.

Isolation of the Rhizobium leguminosarum NodF nodulation protein: NodF carries a 4'-phosphopantetheine prosthetic group

Rhizobium species produce a protein product of the nodF gene that has a limited but recognizable homology to the well-characterized acyl carrier protein (ACP) of Escherichia coli. NodF functions together with NodE in generating a host-specific response to the plant host in the interchange of signals leading to the effective nodulation of roots (H.P. Spaink, J. Weinman, M.A. Djordjevic, C.A. Wijffelman, R.J.H. Okker, and B. J.J. Lugtenberg, EMBO J. 8:2811-2818, 1989; B. Scheres, C. van de Wiel, A. Zalensky, B. Horvath, H. Spaink, H. van Eck, F. Zwartkruis, A.M. Wolters, T. Gloudemans, A. van Kammen, and T. Bisseling, Cell 60:281-294, 1990). The nodFE region of Rhizobium leguminosarum has been cloned into a multicopy plasmid and has been shown in R. leguminosarum to code for a flavonoid-inducible protein that is effectively labeled by radioactive beta-alanine added to the growth medium. After purification, the labeled protein migrates as a single band with an apparent molecular weight of 5,000 during sodium dodecyl sulfate-polyacrylamide gel electrophoresis, more rapidly than E. coli ACP. In contrast, in native gels the protein is resolved into two bands, both identified as NodF by analysis of the amino terminus and both migrating more slowly than E. coli ACP. Pulse-chase experiments with labeled beta-alanine suggested that the slower-moving band may be the precursor of the faster band. The NodF protein carries a 4'-phosphopantetheine as a prosthetic group. A NodF fusion protein under the control of the lac promoter is expressed in E. coli and is labeled with beta-alanine, indicating that it is recognized by the ACP synthase of E. coli. The ACP phosphodiesterase of E. coli, which catalyzes the release of phosphopantetheine from E. coli ACP, does not remove phosphopantetheine from NodF.

Pantothenic acid in health and disease

In summary, the vitamin pantothenic acid is an integral part of the acylation carriers, CoA and acyl carrier protein (ACP). The vitamin is readily available from diverse dietary sources, a fact which is underscored by the difficulty encountered in attempting to induce pantothenate deficiency. Although pantothenic acid deficiency has not been linked with any particular disease, deficiency of the vitamin results in generalized malaise clinically. In view of the fact that pantothenate is required for the synthesis of CoA, it is surprising that tissue CoA levels are not altered in pantothenate deficiency. This suggests that the cell is equipped to conserve its pantothenate content, possibly by a recycling mechanism for utilizing pantothenate obtained from degradation of pantothenate-containing molecules. Although the steps involved in the conversion of pantothenate to CoA have been characterized, much remains to be done to understand the regulation of CoA synthesis. In particular, in view of what is known about the in vitro regulation of pantothenate kinase, it is surprising that the enzyme is active in vivo, since factors that are known to inhibit the enzyme are present in excess of the concentrations known to inhibit the enzyme. Thus, other physiological regulatory factors (which are largely unknown) must counteract the effects of these inhibitors, since the pantothenate-to-CoA conversion is operative in vivo. Another step in the biosynthetic pathway that may be rate limiting is the conversion of 4'-phosphopantetheine (4'-PP) to dephospho-CoA, a step catalyzed by 4'-phosphopantetheine adenylyl-transferase. In mammalian systems, this step may occur in the mitochondria or in the cytosol. The teleological significance of these two pathways remains to be established, particularly since mitochondria are capable of transporting CoA from the cytosol. Altered homeostasis of CoA has been observed in diverse disease states including starvation, diabetes, alcoholism, Reye syndrome (RS), medium-chain acyl CoA dehydrogenase deficiency, vitamin B12 deficiency, and certain tumors. Hormones, such as glucocorticoids, insulin, and glucagon, as well as drugs, such as clofibrate, also affect tissue CoA levels. It is not known whether the abnormal metabolism observed in these conditions is the result of altered CoA metabolism or whether CoA levels change in response to hormonal or nonhormonal perturbations brought about in these conditions. In other words, a cause-effect relation remains to be elucidated. It is also not known whether the altered CoA metabolism (be it cause or result of abnormal metabolism) can be implicated in the manifestations of a disease. Besides CoA, pantothenic acid is also an integral part of the ACP molecule.(ABSTRACT TRUNCATED AT 400 WORDS)

Isolation and characterization of the constitutive acyl carrier protein from Rhizobium meliloti

Rhizobium species produce an inducible acyl carrier protein (ACP), encoded by the nodF gene, that somehow functions in an exchange of cell signals between bacteria and specific plant hosts, leading to nodulation of plant roots and symbiotic nitrogen fixation, as well as a constitutive ACP needed for the synthesis of essential cell lipids. The periplasmic cyclic glucans of Rhizobium spp. are also involved in specific rhizobium-plant interaction. These glucans are strongly similar to the periplasmic membrane-derived oligosaccharides (MDO) of Escherichia coli. E. coli ACP is an essential component of a membrane-bound transglucosylase needed for the biosynthesis of MDO, raising the possibility that either or both of the rhizobial ACPs might have a similar function. We have now isolated the constitutive ACP of R. meliloti and determined its primary structure. We have also examined its function, together with those of ACPs from E. coli, Rhodobacter sphaeroides, and spinach, in the MDO transglucosylase system and as substrate for the E. coli ACP acylase enzyme. All four ACPs act as acceptors of acyl residues, but only the E. coli ACP functions in the transglucosylase system.

Isolation and properties of acyl carrier protein phosphodiesterase of Escherichia coli

The acyl carrier protein (ACP) phosphodiesterase of Escherichia coli catalyzes the hydrolytic cleavage of the 4'-phosphopantetheine residue from ACP, with the generation of apo-ACP (P. R. Vagelos and A. R. Larrabee, J. Biol. Chem. 242:1776-1781, 1967). Although it has been postulated to play a role in the regulation of fatty acid synthesis, presently available evidence makes this unlikely, and its physiological function requires further investigation. We have now purified the enzyme from E. coli more than 3,000-fold and have identified it as a protein of Mr 25,000, as judged from its migration during electrophoresis in gels containing sodium dodecyl sulfate. The enzyme has remarkable thermostability, being protected against irreversible inactivation at 90 degrees C by the presence of sodium dodecyl sulfate. A partial sequence of the amino terminus of the enzyme is as follows: H2N-Ser-Lys-Val-Leu-Val-Leu-Lys-Ser-?-Ile-Leu-Ala-Gly-Tyr-Ser-. Other properties of the enzyme are also described.

Lack of inhibition by colicin M suggests bactoprenol independence of MDO biosynthesis

Biosynthesis of membrane-derived oligosaccharides (MDO), located in the periplasmic space of Escherichia coli, was not inhibited by colicin M, an inhibitor of bactoprenyl phosphate regeneration. This result suggests that bactoprenol does not serve as a lipid carrier of MDO oligosaccharides across the cytoplasmic membrane.

Site-directed mutagenesis of the spinach acyl carrier protein-I prosthetic group attachment site

Site-directed mutagenesis was used to change the phosphopantetheine attachment site (Ser38) of spinach acyl carrier protein I (ACP-I) from a serine to a threonine or cysteine residue. 1. Although the native ACP-I is fully phosphopantethenylated when expressed in Escherichia coli, the TH-ACP-I and CY-ACP-I mutants were found to be completely devoid of the phosphopantetheine group. Therefore, the E. coli holoACP synthase requires serine for in vivo phosphopantetheine addition to spinach ACP-I. 2. Spinach holoACP synthase was completely inactive in vitro with either the TH-ACP-I or CY-ACP-I mutants. In addition, TH-ACP-I and CY-ACP-I were strong inhibitors of spinach holoACP synthase. 3. The mutant ACPs were weak or ineffective as inhibitors of spinach fatty acid synthesis and spinach oleoyl-ACP hydrolase. 4. Compared to holoACP-I, the mutant apoACP-I analogs had: (a) altered mobility in SDS and native gel electrophoresis, (b) altered binding to anti-(spinach ACP-I) antibodies and (c) altered isoelectric points. The combined physical, immunological and enzyme inhibition data indicate that attachment of the phosphopantheine prosthetic group alters ACP conformation.

Molecular cloning and expression of a locus (mdoA) implicated in the biosynthesis of membrane-derived oligosaccharides in Escherichia coli

Mutants of Escherichia coli defective in the mdoA locus are blocked at an early stage in the biosynthesis of membrane-derived oligosaccharides. The mdoA locus has now been cloned into multicopy plasmids. A 5 kb DNA fragment is necessary to complement mdoA mutations. Cells harbouring the mdoA+ plasmid produced three to four times more MDO than wild-type cells. MDO overproduction did not affect the degree of MDO substitution with sn-1-phosphoglycerol residues. The biosynthesis of MDO remained under osmotic control in overproducing strains.

Membrane-binding sites for acyl carrier protein in Escherichia coli

We report that membrane vesicles of Escherichia coli contain protein-binding sites for acyl carrier protein. Scatchard analysis of the binding indicates a dissociation constant around 0.35 micrometers and a maximum number of protein-binding sites around 50 pmol per mg of membrane protein. Binding is on the inner membrane while the outer membrane is devoid of binding sites. These results are consistent with the fact that some acyl carrier protein-dependent enzymes implicated in phospholipid- and membrane-derived oligosaccharide biosynthesis are localized in the cytoplasmic membrane.

Three-dimensional structure of acyl carrier protein in solution determined by nuclear magnetic resonance and the combined use of dynamical simulated annealing and distance geometry

The solution conformation of acyl carrier protein from Escherichia coli (77 residues) has been determined on the basis of 423 interproton-distance restraints and 32 hydrogen-bonding restraints derived from NMR measurements. A total of nine structures were computed using a hybrid approach combining metric matrix distance geometry and dynamic simulated annealing. The polypeptide fold is well defined with an average backbone atomic root-mean-square difference of 0.20 +/- 0.03 nm between the final nine converged structures and the mean structure obtained by averaging their coordinates. The principal structural motif is composed of three helices: 1 (residues 3-12), 2 (residues 37-47) and 4 (residues 65-75) which line a hydrophobic cavity. Helices 2 and 4 are approximately parallel to each other and anti-parallel at an angle of approximately equal to 150 degrees to helix 1. The smaller helix 3 (residues 56-63) is at an angle of approximately equal to 100 degrees to helix 4.