The DNA sequences encoding plsB and dgk loci of Escherichia coli

Eugene P. Kennedy's Legacy: Defining Bacterial Phospholipid Pathways and Function

In the 1950's and 1960's Eugene P. Kennedy laid out the blueprint for phospholipid biosynthesis in somatic cells and Escherichia coli, which have been coined the Kennedy Pathways for phospholipid biosynthesis. His research group continued to make seminal contributions in the area of phospholipids until his retirement in the early 1990's. During these years he mentored many young scientists that continued to build on his early discoveries and who also mentored additional scientists that continue to make important contributions in areas related to phospholipids and membrane biogenesis. This review will focus on the initial E. coli Kennedy Pathways and how his early contributions have laid the foundation for our current understanding of bacterial phospholipid genetics, biochemistry and function as carried on by his scientific progeny and others who have been inspired to study microbial phospholipids.

Therapeutic Targets in Chlamydial Fatty Acid and Phospholipid Synthesis

Chlamydia trachomatis is an obligate intracellular pathogen with a reduced genome reflecting its host cell dependent life style. However, C. trachomatis has retained all of the genes required for fatty acid and phospholipid synthesis that are present in free-living bacteria. C. trachomatis assembles its cellular membrane using its own biosynthetic machinery utilizing glucose, isoleucine, and serine. This pathway produces disaturated phospholipid molecular species containing a branched-chain 15-carbon fatty acid in the 2-position, which are distinct from the structures of host phospholipids. The enoyl reductase step (FabI) is a target for antimicrobial drug discovery, and the developmental candidate, AFN-1252, blocks the activity of CtFabI. The x-ray crystal structure of the CtFabI•NADH•AFN-1252 ternary complex reveals the interactions between the drug, protein, and cofactor. AFN-1252 treatment of C. trachomatis-infected HeLa cells at any point in the infection cycle reduces infectious titers, and treatment at the time of infection prevents the first cell division. Fatty acid synthesis is essential for C. trachomatis proliferation within its eukaryotic host, and CtFabI is a validated therapeutic target against C. trachomatis.

Unraveling the expression of genes involved in the biosynthesis pathway of cardiolipin and phosphatidylethanolamine in Salmonella Hadar grown under static magnetic field 200 mT

We aimed in this work to evaluate the effect of static magnetic field 200 mT (SMF) on the expression of genes involved in the biosynthetic pathway of cardiolipin: g3pd, plsB, cdsA, pgsA, pgpA, cls and phosphatidylethanolamine: pssA and psd in Salmonella enterica subsp enterica serovar Hadar. Bacteria were exposed to a SMF during 3, 6 and 9 h. RNA extraction was followed by Reverse Transcriptase Polymerase Chain Reaction RT-PCR. The relative quantification of mRNA expression levels using 16S rRNA doesn't change during the time exposure. RT-PCR was done for two exposure experiments. The gene expression using RT-PCR present no significant difference in case of plsB, cdsA, pgpA, pgsA and psd genes during the different exposure times. However, a significant increase was observed in the expression of g3pd and pssA genes after 6 h and for cls gene after 3 h of exposure, but any variation was notified after 9 h of exposure. So we can conclude from this study that cls, g3pd and pssA genes are required in the adaptation of Salmonella Hadar to SMF.

Phylogenetic analysis of the diacylglycerol kinase family of proteins and identification of multiple highly-specific conserved inserts and deletions within the catalytic domain that are distinctive characteristics of different classes of DGK homologs

Diacylglycerol kinase (DGK) family of proteins, which phosphorylates diacylglycerol into phosphatidic acid, play important role in controlling diverse cellular processes in eukaryotic organisms. Most vertebrate species contain 10 different DGK isozymes, which are grouped into 5 different classes based on the presence or absence of specific functional domains. However, the relationships among different DGK isozymes or how they have evolved from a common ancestor is unclear. The catalytic domain constitutes the single largest sequence element within the DGK proteins that is commonly and uniquely shared by all family members, but there is limited understanding of the overall function of this domain. In this work, we have used the catalytic domain sequences to construct a phylogenetic tree for the DGK family members from representatives of the main vertebrate classes and have also examined the distributions of various DGK isozymes in eukaryotic phyla. In a tree based on catalytic domain sequences, the DGK homologs belonging to different classes formed strongly supported clusters which were separated by long branches, and the different isozymes within each class also generally formed monophyletic groupings. Further, our analysis of the sequence alignments of catalytic domains has identified >10 novel sequence signatures consisting of conserved signature indels (inserts or deletions, CSIs) that are distinctive characteristics of either particular classes of DGK isozymes, or are commonly shared by members of two or more classes of DGK isozymes. The conserved indels in protein sequences are known to play important functional roles in the proteins/organisms where they are found. Thus, our identification of multiple highly specific CSIs that are distinguishing characteristics of different classes of DGK homologs points to the existence of important differences in the catalytic domain function among the DGK isozymes. The identified CSIs in conjunction with the results of blast searches on species distribution of DGK isozymes also provide useful insights into the evolutionary relationships among the DGK family of proteins.

Phylogenetic analysis of glycerol 3-phosphate acyltransferases in opisthokonts reveals unexpected ancestral complexity and novel modern biosynthetic components

Glycerolipid synthesis represents a central metabolic process of all forms of life. In the last decade multiple genes coding for enzymes responsible for the first step of the pathway, catalyzed by glycerol 3-phosphate acyltransferase (GPAT), have been described, and characterized primarily in model organisms like Saccharomyces cerevisiae and mice. Notoriously, the fungal enzymes share low sequence identity with their known animal counterparts, and the nature of their homology is unclear. Furthermore, two mitochondrial GPAT isoforms have been described in animal cells, while no such enzymes have been identified in Fungi. In order to determine if the yeast and mammalian GPATs are representative of the set of enzymes present in their respective groups, and to test the hypothesis that metazoan orthologues are indeed absent from the fungal clade, a comparative genomic and phylogenetic analysis was performed including organisms spanning the breadth of the Opisthokonta supergroup. Surprisingly, our study unveiled the presence of ‘fungal’ orthologs in the basal taxa of the holozoa and ‘animal’ orthologues in the basal holomycetes. This includes a novel clade of fungal homologues, with putative peroxisomal targeting signals, of the mitochondrial/peroxisomal acyltransferases in Metazoa, thus potentially representing an undescribed metabolic capacity in the Fungi. The overall distribution of GPAT homologues is suggestive of high relative complexity in the ancestors of the opisthokont clade, followed by loss and sculpting of the complement in the descendent lineages. Divergence from a general versatile metabolic model, present in ancestrally deduced GPAT complements, points to distinctive contributions of each GPAT isoform to lipid metabolism and homeostasis in contemporary organisms like humans and their fungal pathogens.

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.

Phosphatidic acid synthesis in bacteria

Membrane phospholipid synthesis is a vital facet of bacterial physiology. Although the spectrum of phospholipid headgroup structures produced by bacteria is large, the key precursor to all of these molecules is phosphatidic acid (PtdOH). Glycerol-3-phosphate derived from the glycolysis via glycerol-phosphate synthase is the universal source for the glycerol backbone of PtdOH. There are two distinct families of enzymes responsible for the acylation of the 1-position of glycerol-3-phosphate. The PlsB acyltransferase was discovered in Escherichia coli, and homologs are present in many eukaryotes. This protein family primarily uses acyl-acyl carrier protein (ACP) endproducts of fatty acid synthesis as acyl donors, but may also use acyl-CoA derived from exogenous fatty acids. The second protein family, PlsY, is more widely distributed in bacteria and utilizes the unique acyl donor, acyl-phosphate, which is produced from acyl-ACP by the enzyme PlsX. The acylation of the 2-position is carried out by members of the PlsC protein family. All PlsCs use acyl-ACP as the acyl donor, although the PlsCs of the γ-proteobacteria also may use acyl-CoA. Phospholipid headgroups are precursors in the biosynthesis of other membrane-associated molecules and the diacylglycerol product of these reactions is converted to PtdOH by one of two distinct families of lipid kinases. The central importance of the de novo and recycling pathways to PtdOH in cell physiology suggest that these enzymes are suitable targets for the development of antibacterial therapeutics in Gram-positive pathogens. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Prokaryotic diacylglycerol kinase and undecaprenol kinase

Prokaryotic diacylglycerol kinase (DAGK) and undecaprenol kinase (UDPK) are the lone members of a family of multispan membrane enzymes that are very small, lack relationships to any other family of proteins-including water soluble kinases-and exhibit an unusual structure and active site architecture. Escherichia coli DAGK plays an important role in recycling diacylglycerol produced as a by-product of biosynthesis of molecules located in the periplasmic space. UDPK seems to play an analogous role in gram-positive bacteria, where its importance is evident because UDPK is essential for biofilm formation by the oral pathogen Streptococcus mutans. DAGK has also long served as a model system for studies of membrane protein biocatalysis, folding, stability, and structure. This review explores our current understanding of the microbial physiology, enzymology, structural biology, and folding of the prokaryotic DAGK family, which is based on over 40 years of studies.

The Vibrio cholerae fatty acid regulatory protein, FadR, represses transcription of plsB, the gene encoding the first enzyme of membrane phospholipid biosynthesis

Glycerol-3-phosphate (sn-glycerol-3-P, G3P) acyltransferase catalyses the first committed step in the biosynthesis of membrane phospholipids, the acylation of G3P to form 1-acyl G3P (lysophosphatidic acid). The paradigm G3P acyltransferase is the Escherichia coli plsB gene product which acylates position-1 of G3P using fatty acids in thioester linkage to either acyl carrier protein (ACP) or CoA as acyl donors. Although the E. coli plsB gene was discovered about 30 years ago, no evidence for transcriptional control of its expression has been reported. However A.E. Kazakov and co-workers (J Bacteriol 2009; 191: 52-64) reported the presence of a putative FadR binding site upstream of the candidate plsB genes of Vibrio cholerae and three other Vibrio species suggesting that plsB might be regulated by FadR, a GntR family transcription factor thus far known only to regulate fatty acid synthesis and degradation. We report that the V. cholerae plsB homologue restored growth of E. coli strain BB26-36 which is a G3P auxotroph due to an altered G3P acyltransferase activity. The plsB promoter was also mapped and the predicted FadR-binding palindrome was found to span positions -19 to -35, upstream of the transcription start site. Gel shift assays confirmed that both V. cholerae FadR and E. coli FadR bound the V. cholerae plsB promoter region and binding was reversed upon addition of long-chain fatty acyl-CoA thioesters. The expression level of the V. cholerae plsB gene was elevated two- to threefold in an E. coli fadR null mutant strain indicating that FadR acts as a repressor of V. cholerae plsB expression. In both E. coli and V. cholerae the β-galactosidase activity of transcriptional fusions of the V. cholerae plsB promoter to lacZ increased two- to threefold upon supplementation of growth media with oleic acid. Therefore, V. cholerae co-ordinates fatty acid metabolism with 1-acyl G3P synthesis.

Thematic review series: glycerolipids. Mammalian glycerol-3-phosphate acyltransferases: new genes for an old activity

Glycerol-3-phosphate acyltransferases (GPATs; EC2.3.1.15) catalyze the first step in the de novo synthesis of neutral lipids (triglycerides) and glycerophospholipids. The existence of multiple enzyme isoforms with GPAT activity was predicted many years ago when GPAT activities with distinct kinetic profiles and sensitivity to inhibitors were characterized in two subcellular compartments, mitochondria and microsomes. We now know that mammals have at least four GPAT isoforms with distinct tissue distribution and function. GPAT1 is the major mitochondrial GPAT isoform and is characterized by its resistance to sulfhydryl-modifying reagents, such as N-ethylmaleimide (NEM). GPAT2 is a minor NEM-sensitive mitochondrial isoform. The activity referred to as microsomal GPAT is encoded by two closely related genes, GPAT3 and GPAT4. GPAT isoforms are important regulators of cellular triglyceride and phospholipid content, and may channel fatty acids toward particular metabolic fates. Overexpression and knock-out studies suggest that GPAT isoforms can play important roles in the development of hepatic steatosis, insulin resistance, and obesity; GPAT isoforms are also important for lactation. This review summarizes the current state of knowledge on mammalian GPAT isoforms.

LPT1 encodes a membrane-bound O-acyltransferase involved in the acylation of lysophospholipids in the yeast Saccharomyces cerevisiae

Phospholipids are major components of cellular membranes that participate in a range of cellular processes. Phosphatidic acid (PA) is a key molecule in the phospholipid biosynthetic pathway. In Saccharomyces cerevisiae, SLC1 has been identified as the gene encoding lysophosphatidic acid acyltransferase, which catalyzes PA synthesis. However, despite the importance of PA, disruption of SLC1 does not affect cell viability (Nagiec, M. M., Wells, G. B., Lester, R. L., and Dickson, R. C. (1993) J. Biol. Chem. 268, 22156-22163). We originally aimed to identify the acetyl-CoA:lyso platelet-activating factor acetyltransferase (lysoPAF AT) gene in yeast. Screening of a complete set of yeast deletion clones (4741 homozygous diploid clones) revealed a single mutant strain, YOR175c, with a defect in lysoPAF AT activity. YOR175c has been predicted to be a member of the membrane-bound O-acyltransferase superfamily, and we designated the gene LPT1. An Lpt1-green fluorescent protein fusion protein localized at the endoplasmic reticulum. Other than lysoPAF AT activity, Lpt1 catalyzed acyltransferase activity with a wide variety of lysophospholipids as acceptors, including lysophosphatidic acid, lysophosphatidylcholine, lysophosphatidylethanolamine, lysophosphatidylglycerol, lysophosphatidylinositol, and lysophosphatidylserine. A liquid chromatography-mass spectrometry analysis indicated that lysophosphatidylcholine and lysophosphatidylethanolamine accumulated in the Deltalpt1 mutant strain. Although the Deltalpt1 mutant strain did not show other detectable defects, the Deltalpt1 Deltaslc1 double mutant strain had a synthetic lethal phenotype. These results indicate that, in concert with Slc1, Lpt1 plays a central role in PA biosynthesis, which is essential for cell viability.

Uniformly aligned full-length membrane proteins in liquid crystalline bilayers for structural characterization

High-resolution solid-state NMR spectra of three full-length membrane proteins uniformly aligned in lipid bilayers between glass slides are observed at high magnetic field. The resolution of the specific amino acid labeled samples shows promise for large membrane protein structure determination utilizing aligned samples and shows resonance patterns known as PISA wheels. The tilt angles of the transmembrane helices are extracted from the resonance patterns in PISEMA spectra.

Topology and active site of PlsY: the bacterial acylphosphate:glycerol-3-phosphate acyltransferase

The most widely distributed biosynthetic pathway to initiate phosphatidic acid formation in bacterial membrane phospholipid biosynthesis involves the conversion of acyl-acyl carrier protein to acylphosphate by PlsX and the transfer of the acyl group from acylphosphate to glycerol 3-phosphate by an integral membrane protein, PlsY. The membrane topology of Streptococcus pneumoniae PlsY was determined using the substituted cysteine accessibility method. PlsY has five membrane-spanning segments with the amino terminus and two short loops located on the external face of the membrane. Each of the three larger cytoplasmic domains contains a highly conserved sequence motif. Site-directed mutagenesis revealed that each conserved domain was critical for PlsY catalysis. Motif 1 had an essential serine and arginine residue. Motif 2 had the characteristics of a phosphate-binding loop. Mutations of the conserved glycines in motif 2 to alanines resulted in a Km defect for glycerol 3-phosphate binding leading to the conclusion that this motif corresponded to the glycerol 3-phosphate binding site. Motif 3 contained a conserved histidine and asparagine that were important for activity and a glutamate that was critical to the structural integrity of PlsY. PlsY was noncompetitively inhibited by palmitoyl-CoA. These data define the membrane architecture and the critical active site residues in the PlsY family of bacterial acyltransferases.

Channeling of eukaryotic diacylglycerol into the biosynthesis of plastidial phosphatidylglycerol

Plastidial glycolipids contain diacylglycerol (DAG) moieties, which are either synthesized in the plastids (prokaryotic lipids) or originate in the extraplastidial compartment (eukaryotic lipids) necessitating their transfer into plastids. In contrast, the only phospholipid in plastids, phosphatidylglycerol (PG), contains exclusively prokaryotic DAG backbones. PG contributes in several ways to the functions of chloroplasts, but it is not known to what extent its prokaryotic nature is required to fulfill these tasks. As a first step toward answering this question, we produced transgenic tobacco plants that contain eukaryotic PG in thylakoids. This was achieved by targeting a bacterial DAG kinase into chloroplasts in which the heterologous enzyme was also incorporated into the envelope fraction. From lipid analysis we conclude that the DAG kinase phosphorylated eukaryotic DAG forming phosphatidic acid, which was converted into PG. This resulted in PG with 2-3 times more eukaryotic than prokaryotic DAG backbones. In the newly formed PG the unique Delta3-trans-double bond, normally confined to 3-trans-hexadecenoic acid, was also found in sn-2-bound cis-unsaturated C18 fatty acids. In addition, a lipidomics technique allowed the characterization of phosphatidic acid, which is assumed to be derived from eukaryotic DAG precursors in the chloroplasts of the transgenic plants. The differences in lipid composition had only minor effects on measured functions of the photosynthetic apparatus, whereas the most obvious phenotype was a significant reduction in growth.

Cloning and characterization of mouse lung-type acyl-CoA:lysophosphatidylcholine acyltransferase 1 (LPCAT1). Expression in alveolar type II cells and possible involvement in surfactant production

Phosphatidylcholine (1,2-diacyl-sn-glycero-3-phosphocholine, PC), is an important constituent of biological membranes. It is also the major component of serum lipoproteins and pulmonary surfactant. In the remodeling pathway of PC biosynthesis, 1-acyl-sn-glycero-3-phosphocholine (LPC) is converted to PC by acyl-CoA:lysophosphatidylcholine acyltransferase (LPCAT, EC 2.3.1.23). Whereas LPCAT activity has been detected in several tissues, the structure and detailed biochemical information on the enzyme have not yet been reported. Here, we present the cloning and characterization of a cDNA for mouse lung-type LPCAT (LPCAT1). The cDNA encodes an enzyme of 60 kDa, with three putative transmembrane domains. When expressed in Chinese hamster ovary cells, mouse LPCAT1 exhibited Ca(2+)-independent activity with a pH optimum between 7.4 and 10. LPCAT1 demonstrated a clear preference for saturated fatty acyl-CoAs, and 1-myristoyl- or 1-palmitoyl-LPC as acyl donors and acceptors, respectively. Furthermore, the enzyme was predominantly expressed in the lung, in particular in alveolar type II cells. Thus, the enzyme might synthesize phosphatidylcholine in pulmonary surfactant and play a pivotal role in respiratory physiology.

Identification of two lysophosphatidic acid acyltransferase genes with overlapping function in Pseudomonas fluorescens

Phosphatidic acid (PA) is known to be a crucial phospholipid intermediate in cell membrane biosynthesis. In Escherichia coli, this molecule is produced from lysophosphatidic acid (LPA) by LPA acyltransferase (EC 2.3.1.51), encoded by plsC. E. coli possesses only one such LPA acyltransferase and a plsC mutant is non-permissive for growth at elevated temperatures. This study describes the identification and characterization of two genes from Pseudomonas fluorescens F113 that encode enzymes with LPA acyltransferase activity. One of the genes, hdtS, was previously described, whereas patB is a novel gene. In addition, a putative lyso-ornithine lipid acyltransferase was also identified. All three proteins possess conserved acyltransferase domains and are homologous to PlsC and to LPA acyltransferases identified in Neisseria meningitidis. Functional analysis determined that both HdtS and PatB are functional LPA acyltransferases, as both complemented an E. coli plsC mutant. Mutants lacking each of the putative acyltransferases were constructed and analysed. Growth defects were observed for hdtS and patB single mutants, and a double hdtSpatB mutant could not be constructed. To determine precise roles in phospholipid synthesis, fatty acid methyl ester analysis was carried out. The hdtS mutant displayed a profile consistent with a defect in LPA acyltransferase activity, whereas no such phenotype was observed in the patB mutant, indicating that hdtS encodes the primary LPA acyltransferase in the cell. The presence of at least two genes specifying LPA acyltransferase activity may have implications for the function and survival of P. fluorescens in diverse environments.

Secondary structure and backbone dynamics of Escherichia coli diacylglycerol kinase, as revealed by site-directed solid-state 13C NMR

To gain insight into secondary structure and backbone dynamics, we have recorded (13)C NMR spectra of [3-(13)C]Ala-, [1-(13)C]Val-labeled Escherichia coli diacylglycerol kinase (DGK), using cross-polarization-magic angle spinning (CP-MAS) and single-pulse excitation with dipolar decoupled-magic angle spinning (DD-MAS) methods. DGK was either solubilized in n-decyl-beta-maltoside (DM) micelle or integrated into 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) or 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers. Surprisingly, the (13)C NMR spectra were broadened to yield rather featureless peaks at physiological temperatures, both in DM solution or lipid bilayers at liquid crystalline phase, due to interference of motional frequencies of DGK with frequencies of magic angle spinning (MAS) or proton decoupling (10(4) or 10(5) Hz, respectively). In gel phase lipids, however, up to six distinct (13)C NMR peaks were well-resolved due to lowered fluctuation frequencies (<10(5) Hz) for the transmembrane region, the amphipathic alpha-helices and loops. While DGK can be tightly packed in gel phase lipids, DGK is less tightly packed at physiological temperatures, where it becomes more mobile. The fact that the enzymatic activity is low under conditions where motion is restricted and high when conformational fluctuations can occur suggests that acquisition of low frequency backbone motions, on the microsecond to millisecond time scale, may facilitate the efficient enzymatic activity of DGK.

Schizosaccharomyces pombe cells deficient in triacylglycerols synthesis undergo apoptosis upon entry into the stationary phase

Triacylglycerols (TAG) are important energy storage molecules for nearly all eukaryotic organisms. In this study, we found that two gene products (Plh1p and Dga1p) are responsible for the terminal step of TAG synthesis in the fission yeast Schizosaccharomyces pombe through two different mechanisms: Plh1p is a phospholipid diacylglycerol acyltransferase, whereas Dga1p is an acyl-CoA:diacylglycerol acyltransferase. Cells with both dga1+ and plh1+ deleted (DKO cells) lost viability upon entry into the stationary phase and demonstrated prominent apoptotic markers. Exponentially growing DKO cells also underwent dramatic apoptosis when briefly treated with diacylglycerols (DAGs) or free fatty acids. We provide strong evidence suggesting that DAG, not sphingolipids, mediates fatty acids-induced lipoapoptosis in yeast. Lastly, we show that generation of reactive oxygen species is essential to lipoapoptosis.

Bacillus subtilis diacylglycerol kinase (DgkA) enhances efficient sporulation

The sn-1,2-diacylglycerol kinase homologue gene, dgkA, is a sporulation gene indispensable for the maintenance of spore stability and viability in Bacillus subtilis. After 6 h of growth in resuspension medium, the endospore morphology of the dgkA mutant by standard phase-contrast microscopy was normal; however, after 9 h, the endospores appeared mostly dark by phase-contrast microscopy, suggesting a defect in the spores. Moreover, electron microscopic studies revealed an abnormal cortex structure in mutant endospores 6 h after the onset of sporulation, an indication of cortex degeneration. In addition, a significant decrease in the dipicolinic acid content of mutant spores was observed. We also found that dgkA is expressed mainly during the vegetative phase. It seems likely that either the DgkA produced during growth prepares the cell for an essential step in sporulation or the enzyme persists into sporulation and performs an essential function.

Physiological and nutritional regulation of enzymes of triacylglycerol synthesis

Although triacylglycerol stores play the critical role in an organism's ability to withstand fuel deprivation and are strongly associated with such disorders as diabetes, obesity, and atherosclerotic heart disease, information concerning the enzymes of triacylglycerol synthesis, their regulation by hormones, nutrients, and physiological conditions, their mechanisms of action, and the roles of specific isoforms has been limited by a lack of cloned cDNAs and purified proteins. Fortunately, molecular tools for several key enzymes in the synthetic pathway are becoming available. This review summarizes recent studies of these enzymes, their regulation under varying physiological conditions, their purported roles in synthesis of triacylglycerol and related glycerolipids, the possible functions of different isoenzymes, and the evidence for specialized cellular pools of triacylglycerol and glycerolipid intermediates.

Properties and functions of diacylglycerol kinases

Diacylglycerol kinases (DGKs) phosphorylate the second-messenger diacylglycerol (DAG) to phosphatidic acid (PA). The family of DGKs is well conserved among most species. Nine mammalian isotypes have been identified, and are classified into five subgroups based on their primary structure. DGKs contain a conserved catalytic domain and an array of other conserved motifs that are likely to play a role in lipid-protein and protein-protein interactions in various signalling pathways dependent on DAG and/or PA production. DGK is therefore believed to be activated at the (plasma) membrane where DAG is generated. Some isotypes are found associated with and/or regulated by small GTPases of the Rho family, presumably acting in cytoskeletal rearrangements. Others are (also) found in the nucleus, in association with other regulatory enzymes of the phosphoinositide cycle, and have an effect on cell cycle progression. Most DGK isotypes show high expression in the brain, often in distinct brain regions, suggesting that each individual isotype has a unique function.

Genes expressed in Pseudomonas putida during colonization of a plant-pathogenic fungus

In vivo expression technology (IVET) was employed to study colonization of Phytophthora parasitica by a biological control bacterium, Pseudomonas putida 06909, based on a new selection marker. The pyrB gene, which encodes aspartate transcarbamoylase, an enzyme used for pyrimidine biosynthesis, was cloned from P. putida 06909. A pyrB-disrupted mutant did not grow in pyrimidine-deficient media unless it was complemented with pyrBC' behind an active promoter. Thirty clones obtained from P. putida 06909 that were expressed on fungal hyphae but not on culture media were isolated by IVET based on the promoterless transcriptional fusion between pyrBC' and lacZ. Nineteen of these clones were induced during late-stage bacterial growth in vitro, while 11 of the clones were expressed only when they were inoculated onto fungal hyphae. Restriction analysis of these 11 clones revealed that there were five unique clones. Sequence analyses of three of the five unique clones showed that the 3' ends of the clones fused to pyrB were similar to genes encoding diacylglycerol kinase (DAGK), bacterial ABC transporters, and outer membrane porins. The sequences of the two other clones were not similar to the sequences of any of the genes in the database used. A LuxR family response regulator was found upstream of DAGK, and a LysR family response regulator was found upstream of the ABC transporter. The location of the inducible promoter of two clones suggested that DAGK and the ABC transporter are induced and may play a role in colonization of the fungus P. parasitica by P. putida 06909.

Rat sn-glycerol-3-phosphate acyltransferase: molecular cloning and characterization of the cDNA and expressed protein

Rat mitochondrial glycerol-3-phosphate acyltransferase (GPAT) cDNA was cloned and characterized. We identified a cDNA containing an open reading frame of 828 amino acids that had an 89% homology with the coding region of the previously characterized mouse mitochondrial GPAT and a predicted amino acid sequence that was 96% identical. The rat 5' UTR was only 159 nucleotides, in contrast to the 926 nucleotide 5' UTR of the mouse cDNA and had an internal deletion of 167 nucleotides. GPAT was expressed in Sf21 insect cells, and specific inhibitors strongly suggest that, like the Escherichia coli GPAT, the recombinant mitochondrial GPAT and the mitochondrial GPAT isoform in rat liver contain critical serine, histidine, and arginine residues.

Active sites of diacylglycerol kinase from Escherichia coli are shared between subunits

We show that residues from different subunits participate in forming the active site of the trimeric membrane protein diacylglycerol kinase (DGK) from Escherichia coli. Five likely active-site mutants were identified: A14Q, N72S, E76L, K94L, and D95N. All five of these mutants possessed significantly impaired catalytic function, without evidence of gross structural alterations as judged by their similar near-UV and far-UV circular dichroism spectra. We found that mixtures of either A14Q or E76L with N72S or K94L possessed much greater activity than the mutant proteins by themselves, suggesting that Ala14 and Glu76 may be on one half-site while Asn72 and Lys94 are on another half-site. Consistent with the shared site model, we also found that (1) peak activity of A14Q and N72S subunit mixtures occur at equimolar concentrations; (2) the maximum activity of the A14Q and N72S mixture was 20% of the wild-type enzyme, in good agreement with the theoretical maximum of 25%; (3) the activity of mutant subunits could not be recovered by mixing with the wild-type subunits; (4) a double mutant, A14Q/N72S, bearing mutations in both putative half-sites was found to inactivate wild-type subunits; (5) the concentration dependence of inactivation by the A14Q/N72S mutant could be well described by a shared site model for a trimeric protein. Unexpectedly, we found that the single mutant D95N behaved in a manner similar to the double mutant, A14Q/N72S, inactivating wild-type subunits. We propose that Asp95 plays a role in more than one active site.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

Nutritional and hormonal regulation of enzymes in fat synthesis: studies of fatty acid synthase and mitochondrial glycerol-3-phosphate acyltransferase gene transcription

The activities of critical enzymes in fatty acid and triacylglycerol biosynthesis are tightly controlled by different nutritional, hormonal, and developmental conditions. Feeding previously fasted animals high-carbohydrate, low-fat diets causes a dramatic induction of enzymes-such as fatty acid synthase (FAS) and mitochondrial glycerol-3-phosphate acyltransferase (GPAT)-involved in fatty acid and triacylglycerol synthesis. During fasting and refeeding, transcription of these two enzymes is coordinately regulated by nutrients and hormones, such as glucose, insulin, glucagon, glucocorticoids, and thyroid hormone. Insulin stimulates transcription of the FAS and mitochondrial GPAT genes, and glucagon antagonizes the insulin effect through the cis-acting elements within the promoters and their bound trans-acting factors. This review discusses advances made in the understanding of the transcriptional regulation of FAS and mitochondrial GPAT genes, with emphasis on elucidation of the mechanisms by which multiple nutrients and hormones achieve their effects.

Acylation of Escherichia coli hemolysin: a unique protein lipidation mechanism underlying toxin function

The pore-forming hemolysin (HlyA) of Escherichia coli represents a unique class of bacterial toxins that require a posttranslational modification for activity. The inactive protoxin pro-HlyA is activated intracellularly by amide linkage of fatty acids to two internal lysine residues 126 amino acids apart, directed by the cosynthesized HlyC protein with acyl carrier protein as the fatty acid donor. This action distinguishes HlyC from all bacterial acyltransferases such as the lipid A, lux-specific, and nodulation acyltransferases, and from eukaryotic transferases such as N-myristoyl transferases, prenyltransferases, and thioester palmitoyltransferases. Most lipids directly attached to proteins may be classed as N-terminal amide-linked and internal ester-linked acyl groups and C-terminal ether-linked isoprenoid groups. The acylation of HlyA and related toxins does not equate to these but does appear related to a small number of eukaryotic proteins that include inflammatory cytokines and mitogenic and cholinergic receptors. While the location and structure of lipid moieties on proteins vary, there are common effects on membrane affinity and/or protein-protein interactions. Despite being acylated at two residues, HlyA does not possess a "double-anchor" motif and does not have an electrostatic switch, although its dependence on calcium binding for activity suggests that the calcium-myristoyl switch may have relevance. The acyl chains on HlyA may provide anchorage points onto the surface of the host cell lipid bilayer. These could then enhance protein-protein interactions either between HlyA and components of a host signal transduction pathway to influence cytokine production or between HlyA monomers to bring about oligomerization during pore formation.

Mutations in aarE, the ubiA homolog of Providencia stuartii, result in high-level aminoglycoside resistance and reduced expression of the chromosomal aminoglycoside 2'-N-acetyltransferase

The aarE1 allele was identified on the basis of the resulting phenotype of increased aminoglycoside resistance. The aarE1 mutation also resulted in a small-colony phenotype and decreased levels of aac(2')-Ia mRNA. The deduced AarE gene product displayed 61% amino acid identity to the Escherichia coli UbiA protein, an octaprenyltransferase required for the second step of ubiquinone biosynthesis. Complementation experiments in both Providencia stuartii and E. coli demonstrated that aarE and ubiA are functionally equivalent.

The Saccharomyces cerevisiae TGL2 gene encodes a protein with lipolytic activity and can complement an Escherichia coli diacylglycerol kinase disruptant

Escherichia coli cells with a disrupted diacylglycerol kinase gene are unable to grow on media containing arbutin due to a lethal accumulation of diacylglycerol. In order to isolate genes from the yeast Saccharomyces cerevisiae involved in diacylglycerol metabolism we complemented an E. coli diacylglycerol kinase disruptant with a yeast genomic library and transformants were selected capable of growing in the presence of arbutin. Using this method, a gene (TGL2) was isolated coding for a protein resembling lipases from Pseudomonas. After expression of the TGL2 gene in E. coli, lipolytic activity towards triacylglycerols and diacylglycerols with short-chain fatty acids could be measured. Therefore, it is very likely that the TGL2 gene can complement the E. coli diacylglycerol kinase disruptant, because it encodes a protein that degrades the diacylglycerol accumulated after growth in the presence of arbutin. Disruption of the TGL2 gene in S. cerevisiae did not result in a detectable phenotype. The role of the Tgl2 protein in lipid degradation in yeast is still unclear.

A gene (plsD) from Clostridium butyricum that functionally substitutes for the sn-glycerol-3-phosphate acyltransferase gene (plsB) of Escherichia coli

The sn-glycerol-3-phosphate acyltransferase (plsB) of Escherichia coli is a key regulatory enzyme that catalyzes the first committed step in phospholipid biosynthesis. We report the initial characterization of a novel gene (termed plsD) from Clostridium butyricum, cloned based on its ability to complement the sn-glycerol-3-phosphate auxotrophic phenotype of a plsB mutant strain of E. coli. Unlike the 83-kDa PlsB acyltransferase from E. coli, the predicted plsD open reading frame encoded a protein of 26.5 kDa. Two regions of strong homology to other lipid acyltransferases, including PlsB and PlsC analogs from mammals, plants, yeast, and bacteria, were identified. PlsD was most closely related to the 1-acyl-sn-glycerol-3-phosphate acyltransferase (plsC) gene family but did not complement the growth of plsC(Ts) mutants. An in vivo metabolic labeling experiment using a plsB plsX plsC(Ts) strain of E. coli confirmed that the plsD expression restored the ability of the cells to synthesize 1-acyl-glycerol-3-phosphate. However, glycerol-3-phosphate acyltransferase activity was not detected in vitro in assays using either acyl-acyl carrier protein or acyl coenzyme A as the substrate.

sn-Glycerol-3-phosphate acyltransferase from Escherichia coli

This review attempts to capture the history of research involved in the understanding of lipid metabolism via investigation of the sn-glycerol-3-phosphate acyltransferase (glycerol-P acyltransferase), the first step in the synthesis of lipids in E. coli. We will review the original identification of this enzymatic activity and its subsequent characterization. The biochemical and genetic regulation of this enzyme and gene are discussed, as well as the unique structural characterization of this integral membrane protein. Future perspectives regarding the regulatory and structural aspects of this key enzyme are discussed.

A method for assessing the stability of a membrane protein

The integral membrane protein diacylglycerol kinase (DGK) from Escherichia coli has been reversibly unfolded in a protein/detergent/mixed micelle system by varying the molar ratio of n-decyl beta-D-maltoside (DM) and sodium dodecyl sulfate (SDS). Unfolding was monitored by circular dichroism (CD) and ultraviolet (UV) absorbance spectroscopy. When unfolding is monitored by measuring changes in absorbance at 294 nm, two distinct denaturation phases are observed, indicative of a stable intermediate. When CD is used as a conformational probe, the resulting denaturation curve contains only one major transition, which corresponds to the first unfolding phase observed by absorbance changes. The unfolding behavior of several mutant proteins in which the tryptophan residues were selectively replaced made it possible to assign the first unfolding phase to a denaturation event in a cytoplasmic domain and the second phase to denaturation of the membrane-embedded portion of the protein. The denaturation curves fit well to a model which assumes two cooperative transitions and a linear relationship between unfolding free energy and SDS concentration. Extrapolation back to zero denaturant indicates an unfolding free energy of 6 kcal/mol for the cytoplasmic domain and 16 kcal/mol for the transmembrane domain. The high apparent stability of the transmembrane domain could explain the high degree of tolerance to amino acid substitutions seen for DGK and other membrane proteins. The approach described in this paper may be applicable to other membrane protein systems.

Essential role for diacylglycerol in protein transport from the yeast Golgi complex

Yeast phosphatidylinositol transfer protein (Sec14p) is required for the production of secretory vesicles from the Golgi. This requirement can be relieved by inactivation of the cytosine 5'-diphosphate (CDP)-choline pathway for phosphatidylcholine biosynthesis, indicating that Sec14p is an essential component of a regulatory pathway linking phospholipid metabolism with vesicle trafficking (the Sec14p pathway). Sac1p (refs 7 and 8) is an integral membrane protein related to inositol-5-phosphatases such as synaptojanin, a protein found in rat brain. Here we show that defects in Sac1p also relieve the requirement for Sec14p by altering phospholipid metabolism so as to expand the pool of diacylglycerol (DAG) in the Golgi. Moreover, although short-chain DAG improves secretory function in strains with a temperature-sensitive Sec14p, expression of diacylglycerol kinase from Escherichia coli further impairs it. The essential function of Sec14p may therefore be to maintain a sufficient pool of DAG in the Golgi to support the production of secretory vesicles.

CHILLING SENSITIVITY IN PLANTS AND CYANOBACTERIA: The Crucial Contribution of Membrane Lipids

The contribution of membrane lipids, particularly the level of unsaturation of fatty acids, to chilling sensitivity of plants has been intensively discussed for many years. We have demonstrated that the chilling sensitivity can be manipulated by modulating levels of unsaturation of fatty acids of membrane lipids by the action of acyl-lipid desaturases and glycerol-3-phosphate acyltransferase. This review covers recent studies on genetic manipulation of these enzymes in transgenic tobacco and cyanobacteria with special emphasis on the crucial importance of the unsaturation of membrane lipids in protecting the photosynthetic machinery from photoinhibition under cold conditions. Furthermore, we review the molecular mechanism of temperature-induced desaturation of fatty acids and introduce our hypothesis that changes in the membrane fluidity is the initial event of the expression of desaturase genes.

Isolation and characterisation of a maize cDNA that complements a 1-acyl sn-glycerol-3-phosphate acyltransferase mutant of Escherichia coli and encodes a protein which has similarities to other acyltransferases

We selected cDNA plasmid clones that corrected the temperature-sensitive phenotype of Escherichia coli strain JC201, which is deficient in 1-acyl-sn-glycerol-3-phosphate acyltransferase activity. A plasmid-based maize endosperm cDNA library was used for complementation and a plasmid that enabled the cells to grow at 44 degrees C on ampicillin was isolated. Addition of this plasmid (pMAT1) to JC201 restored 1-acyl-sn-glycerol-3-phosphate acyltransferase activity to the cells. Total phospholipid labelling showed that the substrate for the enzyme, lysophosphatidic acid, accumulated in JC201 and was further metabolised to phosphatidylethanolamine in complemented cells. Membranes isolated from such cells were able to convert lysophosphatidic acid to phosphatidic acid in acyltransferase assays. The cDNA insert of pMAT1 contains one long open reading frame of 374 amino acids which encodes a protein of relative molecular weight 42,543. The sequence of this protein is most similar to SLC1, which is thought to be able to acylate glycerol at the sn-2 position during synthesis of inositol-containing lipids. Homologies between the SLC1 protein, the 1-acyl-sn-glycerol-3-phosphate acyltransferase of E. coli (PlsC) and the maize ORF were found with blocks of conserved amino acids, whose spacing was conserved between the three proteins, identifiable.

Membrane topology of Escherichia coli diacylglycerol kinase

The topology of Escherichia coli diacylglycerol kinase (DAGK) within the cytoplasmic membrane was elucidated by a combined approach involving both multiple aligned sequence analysis and fusion protein experiments. Hydropathy plots of the five prokaryotic DAGK sequences available were uniform in their prediction of three transmembrane segments. The hydropathy predictions were experimentally tested genetically by fusing C-terminal deletion derivatives of DAGK to beta-lactamase and beta-galactosidase. Following expression, the enzymatic activities of the chimeric proteins were measured and used to determine the cellular location of the fusion junction. These studies confirmed the hydropathy predictions for DAGK with respect to the number and approximate sequence locations of the transmembrane segments. Further analysis of the aligned DAGK sequences detected probable alpha-helical N-terminal capping motifs and two amphipathic alpha-helices within the enzyme. The combined fusion and sequence data indicate that DAGK is a polytopic integral membrane protein with three transmembrane segments with the N terminus of the protein in the cytoplasm, the C terminus in the periplasmic space, and two amphipathic helices near the cytoplasmic surface.

Molecular characterization of a STreptococcus mutans mutant altered in environmental stress responses

A mutant defective in aciduricity, GS5Tn1, was constructed following mutagenesis of Streptococcus mutans GS5 with the conjugative transposon Tn916. The mutant grew poorly at acidic pH levels and was sensitive to high osmolarity and elevated temperatures. These properties resulted from a single insertion of Tn916 into the GS5 chromosome, and the DNA fragment harboring the transposon was isolated into the cosmid vector, charomid 9-20. Spontaneous excision of Tn916 from the cosmid revealed that Tn916 inserted into a 8.6-kb EcoRI fragment. On the basis of the restriction analyses of insert fragments, it was found that Tn916 inserted into a 0.9-kb EcoRI-XbaI fragment. Nucleotide sequence analysis of this fragment indicated the presence of two open reading frames, ORF1 and ORF2. By using a marker rescue strategy, a 6.0-kb HindIII fragment including the target site for Tn916 insertion and the 5' end of ORF1 was isolated and sequenced. The deduced amino acid sequences of ORF1 and ORF2 showed significant homology with the diacylglycerol kinase and Era proteins, respectively, from Escherichia coli. Nucleotide sequence analysis of the Tn916 insertion junction region in the GS5Tn1 chromosome revealed that the transposon inserted near the 3' terminus of ORF1. Restoration of ORF1 to its original sequence in mutant GS5Tn1 was carried out following transformation with integration vector pVA891 containing an intact ORF1. The resultant transformant showed wild-type levels of aciduricity as well as resistance to elevated temperatures and high osmolarity. These results suggest that the S. mutans homolog of diacylglycerol kinase is important for adaptation of the organism to several environmental stress signals.

Expression and identification of p90 as the murine mitochondrial glycerol-3-phosphate acyltransferase

Glycerol-3-phosphate acyltransferase (GPAT) catalyzes the initial and committed step in glycerolipid biosynthesis. Mitochondrial GPAT, unlike the microsomal isozyme, prefers saturated fatty acids as a substrate. We have recently reported cloning of a cDNA to an unidentified 6.8-kb mRNA by a differential hybridization. The mRNA contains an open reading frame of 827 amino acids (p90) with 30% sequence homology in a 300 amino acid stretch to Escherichia coli GPAT. The 6.8-kb mRNA was induced dramatically when fasted mice were refed a high-carbohydrate diet. Here, we have expressed the open reading frame as trpE fusion proteins and used them to generate antibodies. The antibodies recognized a polypeptide of 90 kDa (p90) when the 6.8-kb cDNA sequence was used for in vitro transcription and translation. By Western blot analysis using these antibodies, we detected p90 in mitochondrial fractions of liver, and the p90 level was increased by refeeding. The increase in the p90 level correlated with the increase in mitochondrial GPAT activity. Moreover, p90 was not detectable in 3T3-L1 preadipocytes but markedly increased during adipose conversion. This increase was consistent with the 11-fold increase we observed in N-ethylmaleimide (NEM)-resistant mitochondrial GPAT activity during adipocyte differentiation. In addition, we have expressed p90 in CHO cells by stable transfection. The transfected genes in both correct and reverse orientations produced distinct 3.9-kb transcripts owing to the truncation of a part of the noncoding regions of the endogenous 6.8-kb mRNA before insertion into the pMSXND vector. The transfected CHO cells were treated with 2-aminopurine, an agent that increases expression of exogenous genes.(ABSTRACT TRUNCATED AT 250 WORDS)

The 92-min region of the Escherichia coli chromosome: location and cloning of the ubiA and alr genes

A cosmid (pND320) bearing 42.5 kb of Escherichia coli chromosomal DNA, including the genes between xylE and ssb near minute 92 on the linkage map, was isolated by selection for complementation of a dnaB mutation. Known nucleotide (nt) sequences were used to align restriction maps in this region to the physical map of the chromosome (coordinates 4319.5 to 4362 kb), and to locate precisely and define the orientations of 19 genes. Predicted physical linkage of sequenced genes across unsequenced gaps of defined length was confirmed by the nt sequence analysis of fragments subcloned from pND320. Mutant complementation by plasmids showed that ubiA is located between malM and plsB. A previously sequenced long open reading frame that encodes the C-terminal portion of the E. coli ubiA product (4-hydroxybenzoate polyprenyltransferase, HPTase) shows a high degree of sequence identity with the corresponding segment of yeast HPTase (the COQ2 gene product). Comparison of homologous regions from E. coli and Salmonella typhimurium was used to locate precisely the gene alr that encodes alanine racemase (ARase) between dnaB and tyrB. Subcloning of alr downstream from tandem bacteriophage lambda promoters produced a plasmid that directed high-level overproduction of a soluble approx. 40-kDa protein with ARase activity.

Amplification of the bacA gene confers bacitracin resistance to Escherichia coli

An Escherichia coli genomic library was constructed in order to facilitate selection for genes which confer bacitracin resistance through amplification. One of the plasmids from the library, plasmid pXV62, provided a high level of bacitracin resistance for E. coli. Deletion and nucleotide sequence analyses of bacitracin resistance plasmid pXV62 revealed that a single open reading frame, designated the bacA gene, was sufficient for antibiotic resistance. The bacA gene mapped to approximately 67 min on the E. coli chromosome by proximity to a previously mapped locus. The deduced amino acid sequence of the bacA-encoded protein suggests an extremely hydrophobic protein of 151 amino acids, approximately 65% of which were nonpolar amino acids. E. coli cells containing plasmid pXV62 have increased isoprenol kinase activity. The physical characteristics of the deduced protein and enhanced lipid kinase activity suggest that the bacA gene may confer resistance to bacitracin by phosphorylation of undecaprenol.

The gene and the RNA for the precursor to the plastid-located glycerol-3-phosphate acyltransferase of Arabidopsis thaliana

The gene and the RNA from Arabidopsis thaliana for the plastid-located glycerol-3-phosphate acyltransferase (GPAT; EC 2.3.1.15) and their encoded product have been studied. The gene (designated ATS1) was isolated by screening a lambda DASH genomic library for cross-hybridization with a radiolabeled probe prepared from cDNA for GPAT from squash. cDNA clones representing the mRNA were isolated by screening a lambda ZAPII cDNA library for hybridization with a radiolabeled probe prepared from a DNA fragment of ATS1. The nucleotide sequences of the gene and the cDNA were determined, and the 5' end of the RNA was mapped by primer extension. Sequences similar to the TATA box, polyadenylation sequences and intron-splicing sequences were found at the expected locations. The pre-mRNA was 3288 nucleotides long and contained 5' and 3'-untranslated sequences of 57 and 442 nucleotides, respectively. The coding sequence of 1377 nucleotides was interrupted by 11 introns of 1412 nucleotides in total and the 3'-untranslated sequence contained another intron of 94 nucleotides. The open-reading frame encoded a polypeptide of 459 amino acid residues, the amino acid sequence of which was highly homologous to those of precursors to plastid-located GPATs from squash and pea. The enzymatic activity of a gene product that was over-produced in Escherichia coli confirmed the identity of the gene.

Structural characterization of ordered arrays of sn-glycerol-3-phosphate acyltransferase from Escherichia coli

Overproduction of the sn-glycerol-3-phosphate acyltransferase in Escherichia coli leads to incorporation of this integral membrane protein into ordered tubular arrays within the cell. Freeze-fracture-etch shadowing was performed on suspensions of partially purified tubules and whole bacteria. This procedure revealed the presence of ridges and grooves defining a set of long-pitch left-handed helical ridges. The long-pitch helices represented chains of acyltransferase dimers. Tubules observed within the cell were often closely packed, with an apparent alignment of grooves and ridges in adjacent tubules. Fracture planes passing through the tubules indicated the presence of a bilayer structure, with some portion of the enzyme being associated with the membrane. The major portion of the enzyme extended from the hydrophilic surface, forming a large globular structure that, in favorable views, displayed a central cavity facing the cytoplasm. Computer analysis of shadowed tubules revealed that the left-handed helices were six stranded, with a pitch of 1,050 A (105.0 nm) and a spacing of 75 A (7.5 nm) between acyltransferase dimers along the chains. Analysis of the predicted secondary structure failed to reveal obvious transmembrane segments, suggesting that very little of the protein was inserted into the bilayer.

Abolishment of bradykinin-induced calcium oscillations in ras-transformed fibroblasts by the expression of 80 kDa diacylglycerol kinase

Our previous study showed bradykinin-induced periodic Ca2+ changes (Ca2+ oscillations) in v-Ki-ras-transformed NIH/3T3 (DT) cells in which protein kinase C (PKC) activity is partially down-regulated by a sustained high level of 1,2-diacylglycerol (DAG) [FEBS Lett. (1991) 281, 263-266]. In the present study, DAG kinase with 80 kDa mass (80K DGK) has been successfully transfected in DT cells, which exhibited enhanced cellular DAG kinase activities, decreased cellular DAG contents, and increased PKC activities compared to the control vector-transfected cells. Furthermore, these DGK-transfectants showed strong inhibition in bradykinin-induced Ca2+ oscillations. The results suggest that the sustained DAG increase down-regulates the PKC activity, thereby leading to the induction of Ca2+ oscillations in DT cells.

Ubiquinone biosynthesis. Cloning of the genes coding for chorismate pyruvate-lyase and 4-hydroxybenzoate octaprenyl transferase from Escherichia coli

Chorismate pyruvate-lyase activity was detected in extracts of Escherichia coli. 4-Hydroxybenzoate was identified as the product of the enzymatic reaction by chemical derivatization and GC-MS analysis. The ubiC gene, coding for the chorismate pyruvate-lyase, was cloned and sequenced. The molecular weight of the gene product was calculated as 18,776 Da and confirmed by expression of the protein in E. coli minicells. The ubiA gene, coding for the 4-hydroxybenzoate octaprenyl transferase, was identified by sequence homology and complementation of a ubiA- strain. It is located directly downstream of ubiC in a typical operon structure.

Cloning and sequencing of Escherichia coli ubiC and purification of chorismate lyase

In Escherichia coli, chorismate lyase catalyzes the first step in ubiquinone biosynthesis, the conversion of chorismate to 4-hydroxybenzoate. 4-Hydroxybenzoate is converted to 3-octaprenyl-4-hydroxybenzoate by 4-hydroxybenzoate octaprenyltransferase. These two enzymes are encoded by ubiC and ubiA, respectively, and have been reported to map near one another at 92 min on the E. coli chromosome. We have cloned the ubiCA gene cluster and determined the nucleotide sequence of ubiC and a portion of ubiA. The nucleotide sequence abuts with a previously determined sequence that encodes a large portion of ubiA. ubiC was localized by subcloning, and overproducing plasmids were constructed. Overexpression of ubiC allowed the purification of chorismate lyase to homogeneity, and N-terminal sequence analysis of chorismate lyase unambiguously defined the beginning of the ubiC coding region. Although chorismate lyase showed no significant amino acid sequence similarity to 4-amino-4-deoxychorismate lyase (4-amino-4-deoxychroismate----4-aminobenzoate), the product of E. coli pabC, chorismate lyase overproduction could complement the growth requirement for 4-aminobenzoate of a pabC mutant strain. Of the several enzymes that convert chorismate to intermediates of E. coli biosynthetic pathways, chorismate lyase is the last to be isolated and characterized.