Recently, three distinct isoforms of phospholipase D (PLD) were identified in Arabidopsis thaliana. PLD α represents the well-known form found in plants, while PLD β and γ have been only recently discovered (Pappan et al., 1997b; Qin et al., 1997). These isoforms differ in substrate selectivity and cofactors required for activity. Here, I report that PLD β and γ isoforms were active toward N-acylphosphatidylethanolamine (NAPE), but PLD α was not. The ability of PLD β and γ to hydrolyze NAPE marks a key difference from PLD α. N-acylethanolamines (NAE), the hydrolytic products of NAPE by PLD β and γ, inhibited PLD α from castor bean and cabbage. Inhibition of PLD α by NAE was dose-dependent and inversely proportional to acyl chain length and degree of unsaturation. Enzyme kinetic analysis suggested non-competitive inhibition of PLD α by NAE 14:0. In addition, a 1.2-kb tobacco (Nicotiana tabacum L.) cDNA fragment was isolated that possessed a 74% amino acid identity to Arabidopsis PLD β indicating that this isoform is expressed in tobacco cells. Collectively, these results provide evidence for NAE producing PLD activities and suggest a possible regulatory role for NAE with respect to PLD α.

Two phage lambda clones encompassing human tRNA genes have been isolated from a human gene library harbored in bacteriophage lambda Charon-UA. One of the clones (designated as hLeuU) containing a 20-kb human DNA fragment was isolated and found to contain a cluster of four tRNA genes. An 8.2-kb Hindlll fragment encompassing the four tRNA genes was isolated from the 20-kb fragment and subcloned into pBR322 for restriction mapping and DNA sequence analysis. The four tRNA genes are arranged as two tandem pairs with the first pair containing a proline tRNAAGQ gene and a leucine tRNAAAQ gene and the second pair containing another proline tRNAAGG gene and a threonine tRNAuQU gene. The two pairs are separated about 3 kb from each other, and the leucine tRNAAAG gene is of opposite polarity from the other three tRNA genes. The tRNA transcription units were sequenced by a unidirectional deletion dideoxyribonucleotide chain-termination method in the M13mpl8 and 19 vectors. The coding regions of the four tRNA genes contain characteristic internal split promoter sequences and do not encode intervening sequences nor the CCA trinucleotide found in mature tRNAs. The proline t R N A A G G gene is separated from the leucine t R …

Glucose phosphate isomerase (GPI) was purified from human placenta utilizing cross-linked spherical particle phosphocellulose. In three steps, GPI could be purified approximately 5500 fold with greater than 50% recovery. The purified enzyme exhibited four bands upon non-denaturing PAGE and isoelectric focusing (IEF) when stained with GPI specific activity stain. The four isozymes were isolated by preparative IEF. The isoelectric points of the isozymes were determined. Sodium dodecyl sulfate (SDS) gel electrophoresis showed two types of subunits with different molecular weights. Structural analyses showed both types of subunits had blocked amino termini. Other properties of the isozymes and subunits, including immunological reactivity, pH stability, peptide mapping and amino acid composition, were also established.

A description of the nature of the transition state structure for phosphoryl transfer in the cAPK reaction requires a measurement of the primary 180 isotope effect at the serine hydroxyl acceptor. Since it is difficult to obtain primary 180 isotope effect directly, the 15N/1 4N ratio of the a-amine of the C-terminal glycine in the peptide Leu Arg-Lys-Ala-Ser-Leu-Gly (when serine is phosphorylated) was used to represent on the phosphorylation at serine. 15N Glycine, ' 4N-Glycine and 180 serine were synthesized and used to synthesize two peptides, one containing 1 80-serine/' 5 N glycine and second 1 60-serine/1 4N-glycine. Methods were developed for hydrolyzing the peptides and quantitatively isolating glycine. Partitioning results suggest that catalytic rate was slow compare to substrate dissociation. The 180 primary isotope effect will be determined in the near future using the method developed herein.

N-Acylphosphatidylethanoiamine (NAPE) is synthesized in the microsomes of cotton seedlings by a mechanism that is possibly unique to plants, the ATP-, Ca2+-, and CoA-independent acylation ofphosphatidylethanolamine (PE) with unesterified free fatty acids (FFAs), catalyzed by NAPE synthase. A photoreactive free fatty acid analogue, 12-[(4- azidosalicyl)amino]dodecanoic acid (ASD), and its 125I-labeled derivative acted as substrates for the NAPE synthase enzyme.

To define the overall kinetic and chemical mechanism of adenosine 3',5'-monophosphate dependent protein kinase catalytic subunit, the mechanism in the direction of MgADP phosphorylation was determined, using studies of initial velocity in the absence and presence of dead-end inhibitors. The kinetic mechanism was determined as a function of uncomplexed Mg^2+ (Mg_f) at pH 7.2 and as a function of pH at low (0.5 mM) Mg_f. At pH 7.2 data are consistent with a random kinetic mechanism in the direction of MgADP phosphorylation with both pathways allowed: the pathway in which MgADP binds to enzyme prior to phosphorylated peptide (PSP) and that in which PSP binds before MgADP. One or the other pathway predominates, depending on Mg_f concentration. At 0.5 mM Mg_f, the mechanism is steady-state ordered with the pathway where PSP binds first preferred; at 10 mM Mg_f, the mechanism is equilibrium ordered, and the pathway in which MgADP binds first preferred. This change in mechanism to equilibrium ordered is due to an increase in affinity of enzyme for MgADP and a decrease in affinity for PSP. There is also a pH-dependent change in mechanism at 0.5 mM Mg_f. At pH 6 the mechanism is equilibrium ordered with the pathway …

Legumes play an important role in agriculture as major food sources for humans and as feed for animals. Bioavailable nitrogen is a limiting nutrient for crop growth. Legumes are important because they can form a symbiotic relationship with soil bacteria called rhizobia that results in nitrogen-fixing root nodules. In this symbiosis, rhizobia provide nitrogen to the legumes and the legumes provide carbon sources to the rhizobia. The Medicago truncatula NPF1.7/NIP/LATD gene is essential for root nodule development and also for proper development of root architecture. Work in our lab on the MtNPF1.7/MtNIP/LATD gene has established that it encodes a nitrate transporter and strongly suggests it has another function. Mtnip-1/latd mutants have pleiotropic defects, which are only partially explained by defects in nitrate transport. MtNPF1.7/NIP/LATD is a member of the large and diverse NPF/NRT1(PTR) transporter family. NPF/NRT1(PTR) members have been shown to transport other compounds in addition to nitrate: nitrite, amino acids, di- and tri-peptides, dicarboxylates, auxin, abscisic acid and glucosinolates. In Arabidopsis thaliana, the AtNPF6.3/NRT1.1( CHL1) transporter was shown to transport auxin as well as nitrate. Atchl1 mutants have defects in root architecture, which may be explained by defects in auxin transport and/or nitrate sensing. Considering the pleiotropic phenotypes observed …

Glyoxalase 11, the second enzyme of the glyoxalase system, hydrolyzes S-D-lactoylglutathione (SLG) to regenerate glutathione (GSH) and liberate free D-lactate. It was found that GTP binds with Gil from rat liver and inhibits Gil activity. Preincubation experiments showed that the binding is relatively tight, since more than 15 minutes are required to release GTP from the complex following dilution. Inhibition kinetics studies indicate that GTP is a "partially competitive inhibitor"; Thus, it would appear that the binding sites for substrate (SLG) and inhibitor (GTP) are different, but spatially close. Glyoxalase 11 binds to a GTP affinity medium, and with polyacrylamide gel electrophoresis, Gil has a higher relative mobility when GTP is present (ATP has no effect). The functional consequences of GTP binding with a specific site on Gil are still unclear. It is speculated that Gil may interact with tubulin by serving as a dissociable GTP carrier, delivering GTP to the tubulinGTP binding site, and thus facilitating tubulin polymerization.

Commonly, when it is desirable to replace an essential gene with an allelic series of mutated genes, or genes with altered expression patterns, the complementing constructs are introduced into heterozygous plants, followed by the selection of homozygous null segregants. To overcome this laborious and time-consuming step, the newly developed two-component system utilizes a site-specific recombinase to excise a wild-type copy of the gene of interest from transformed tissues. In the first component (the first vector), a wild-type version of the gene is placed between target sequences recognized by FLP recombinase from the yeast 2 μm plasmid. This construct is transformed into a plant heterozygous for a null mutation at the endogenous locus, and progeny plants carrying the excisable complementing gene and segregating homozygous knockout at the endogenous locus are selected. The second component (the second vector) carries the experimental gene along with the FLP gene. When this construct is introduced, FLP recombinase excises the complementing gene, leaving the experimental gene as the only functional copy. The FLP gene is driven by an egg apparatus specific enhancer (EASE) to ensure excision of the complementing cDNA in the egg cell and zygote following floral-dip transformation. The utility of this system is being …

The kinetic mechanism of activation of the NAD-malic enzyme by fumarate and the transition state structure for the oxidation malate for the NAD-malic enzyme reaction have been studied. Fumarate exerts its activating effect by decreasing the off-rate for malate from the E:Mg:malate and E:Mg:NAD:malate complexes. The activation by fumarate results in a decrease in K_imalate and an increase in V/K_malate by about 2-fold, while the maximum velocity remains constant. A discrimination exists between active and activator sites for the binding of dicarboxylic acids. Activation by fumarate is proposed to have physiologic importance in the parasite. The hydride transfer transition state for the NAD-malic enzyme reaction is concerted with respect to solvent isotope sensitive and hydride transfer steps. Two protons are involved in the solvent isotope sensitive step, one with a normal fractionation factor, another with an inverse fractionation factor. A structure for the transition state for hydride transfer in the NAD-malic enzyme reaction is proposed.

To elaborate on the function(s) of the ENOD8 gene in the nodules of M. truncatula, several different experimental approaches were used. A census of the ENOD8 genes was first completed indicating that only ENOD8.1 (nt10554-12564 of GenBank AF463407) is highly expressed in nodule tissues. A maltose binding protein-ENOD8 fusion protein was made with an E. coli recombinant system. A variety of biochemical assays were undertaken with the MBP-ENOD8 recombinant protein expressed in E. coli, which did not yield the esterase activity observed for ENOD8 protein nodule fractions purified from M. sativa, tested on general esterase substrates, α-naphthyl acetate, and p-nitrophenylacetate. Attempts were also made to express ENOD8 in a Pichia pastoris system; no ENOD8 protein could be detected from Pichia pastoris strains which were transformed with the ENOD8 expression cassette. Additionally, it was shown that the ENOD8 protein can be recombinantly synthesized by Nicotiana benthamiana in a soluble form, which could be tested for activity toward esterase substrates, bearing resemblance to nodule compounds, such as the Nod factor. Transcription localization studies using an ENOD8 promoter gusA fusion indicated that ENOD8 is expressed in the bacteroid-invaded zone of the nodule. The ENOD8 protein was also detected in that same zone by …

Symbiotic nitrogen fixation occurs in plants harboring nitrogen-fixing bacteria within the plant tissue. The most widely studied association is between the legumes and rhizobia. In this relationship the plant (legumes) provides the bacteria (rhizobia) with reduced carbon derived from photosynthesis in exchange for reduced atmospheric nitrogen. This allows the plant to survive in soil, which is low in available of nitrogen. Rhizobia infect and enter plant root and reside in organs known as nodules. In the nodules the bacteria fix atmospheric nitrogen. The association between the legume, Medicago truncatula and the bacteria Sinorhizobium meliloti, has been studied in detail. Medicago mutants that have defects in nodulation help us understand the process of nitrogen fixation better. One such mutant is the Mtnip-1. Mtnip-1 plants respond to S. meliloti by producing abnormal nodules in which numerous aberrant infection threads are produced, with very rare rhizobial release into host plant cells. The mutant plant Mtnip-1 has an abnormal defense-like response in root nodules as well as defects in lateral root development. Three alleles of the Mtnip/latd mutants, Mtnip-1, Mtlatd and Mtnip-3 show different degrees of severity in their phenotype. Phylogenetic analysis showed that MtNIP/LATD encodes a protein belonging to the NRT1(PTR) family of …

Fatty acid amide hydrolase (FAAH) terminates the endocannabinoid signaling pathway that regulates numerous neurobehavioral processes in animals by hydrolyzing a class of lipid mediators, N-acylethanolamines (NAEs). Recent identification of an Arabidopsis FAAH homologue (AtFAAH) and several studies, especially those using AtFAAH overexpressing and knock-out lines suggest that a FAAH-mediated pathway exists in plants for the metabolism of endogenous NAEs. Here, I provide evidence to support this concept by identifying candidate FAAH cDNA sequences in diverse plant species. NAE amidohydrolase assays confirmed that several of the proteins encoded by these cDNAs indeed catalyzed the hydrolysis of NAEs in vitro. Kinetic parameters, inhibition properties, and substrate specificities of the plant FAAH enzymes were very similar to those of mammalian FAAH. Five amino acid residues determined to be important for catalysis by rat FAAH were absolutely conserved within the plant FAAH sequences. Site-directed mutation of each of the five putative catalytic residues in AtFAAH abolished its hydrolytic activity when expressed in Escherichia coli. Contrary to overexpression of native AtFAAH in Arabidopsis that results in enhanced seedling growth, and in seedlings that were insensitive to exogenous NAE, overexpression of the inactive AtFAAH mutants showed no growth enhancement and no NAE tolerance. However, both active …

N-Acylethanolamines (NAEs) are fatty acid derivatives in plants that negatively influence seedling growth. N-Lauroylethanolamine (NAE 12:0), one type of NAE, inhibits root length, increases radial swelling of root tips and reduces root hair numbers in a dose dependent manner in Arabidopis thaliana L. (ecotype Columbia). A forward genetics approach was employed by screening a population of T-DNA “activation-tagged” developed by the Salk Institute lines for NAE resistance to identify potential genes involved in NAE signaling events in Arabidopsis thaliana L. (ecotype Columbia). Seeds of the activation tagged lines were grown at 0, 25, 30, 50, 75 and 100 µM N-lauroylethanolamime (NAE 12:0). Ten plants which displayed NAE tolerance (NRA) seedling phenotypes, compared with wildtype (Columbia, Col-0) seedlings were identified. I focused on one mutant line, identified as NRA 25, where the tolerance to NAE 12:0 appears to be mediated by a single dominant, nuclear gene. Thermal asymmetric interlaced (TAIL) PCR identified the location of the T-DNA insert as 3.86 kbp upstream of the locus At1g68510. Quantitative PCR indicated that the transcript level corresponding to At1g68510 is upregulated approximately 20 fold in the mutant relative to wildtype. To determine whether the NAE tolerance in NRA 25 is associated with overexpression of …

Two swine kidney proteins (PI 4.8 and 5.8) both possessing 9-prostaglandin ketoreductase (9-PGKR) and 15-hydroxyprostaglandin dehydrogenase (15-PGDH) activities were purified to homogeneity. Purification increased specific activities in parallel. Molecular weight, subunit size, amino acid composition, coenzyme and substrate specificity and antigenicity of both proteins were similar. Gel filtration and SDS-polyacrylamide gel electrophoresis molecular weights of 29,500 and 29,000, respectively, suggested a single subunit. Although a variety of prostaglandins served as substrates, the best for 15-PGDH was PGB, while PGA_1-GSH showed the lowest Km for 9-PGKR. Rabbit antibody against the PI 5.8 protein crossreacted with both purified renal enzymes and with extracts from rat spleen, lung, heart, aorta, and liver.

Data obtained from isotope exchange at equilibrium, exchange of inorganic phosphate against forward reaction flux, and positional isotope exchange of 18O from the (βγ-bridge position of pyrophosphate to a (β-nonbridge position all indicate that the pyrophosphate-dependent phosphofructokinase from Propionibacterium freudenreichii has a rapid equilibrium random kinetic mechanism. All exchange reactions are strongly inhibited at high concentrations of the fructose 6-phosphate/Pi and MgPPi/Pi substrate-product pairs and weakly inhibited at high concentrations of the MgPPi/fructose 1,6-bisphosphate pair suggesting three dead-end complexes, E:F6P:Pi, E:MgPPi:Pi, and E:FBP:MgPPi. Neither back-exchange by [32p] nor positional isotope exchange of 18O-bridge-labeled pyrophosphate was observed under any conditions, suggesting that either the chemical interconversion step or a step prior to it limits the overall rate of the reaction. Reduction of the pyridoxal 5'-phosphate-inactivated enzyme with NaB[3H]4 indicates that about 7 lysines are modified in free enzyme and fructose 1,6-bisphosphate protects 2 of these from modification. The pH dependence of the enzyme-reactant dissociation constants suggests that the phosphates of fructose 6-phosphate, fructose 1,6-bisphosphate, inorganic phosphate, and Mg-pyrophosphate must be completely ionized and that lysines are present in the vicinity of the 1- and 6-phosphates of the sugar phosphate and bisphosphates probably directly coordinated to these phosphates. The pH dependence of …

A novel molecular mechanism of autophosphorylation-dependent activation of the ser/thr S6/H4 kinase isolated from human placenta is described. Phosphopeptide mapping of the enzyme was used to determine the rate and extent of site-specific autophosphorylation. These data were correlated to phosphotransferase activity of the protein kinase. The results indicated that a sequential phosphorylation of two sites in the catalytic domain is required for maximum activation. Kinetic analysis determined that site 1 is modified by an intramolecular phosphorylation, and site 2 is modified by an intermolecular phosphorylation. On the basis of these data a model is proposed in which autophosphorylation of the pseudosubstrate domain and on a serine residue in subdomain VIII are both required for maximum activation of the S6/H4 kinase.

Methodology was developed which allowed the rapid and routine quantitation of subpicomole quantities of diadenosine-5ʹ,5ʹʹʹ-P¹,P⁴-tetraphosphate (Ap₄A) in cultured mammalian cells. This methodology includes the rapid extraction of cellular nucleotides in cold alkali, resolution of Ap₄A from the bulk of cellular materials on a highly specific boronate affinity resin, and quantitation of the dinucleotide in a coupled bioluminescence assay utilizing venom phosphodiesterase and firefly luciferase. The sensitivity and selectivity of this assay is demonstrated and contrasted with previously developed techniques. This assay was used to examine the role of Ap₄A in DNA replication and the cellular stress response.

The mechanism by which nicotinamide adenine dinucleotide (NAD) stimulates the activity of adenylate cyclase (AC) in canine plasma membrane has been studied. Using [3 2P]-NAD, the activation by NAD was correlated with the radiolabeling of the stimulatory guanosine triphosphate (GTP) binding protein Gsa. Further characterization demonstrated that the modification occurred only in the presence of G-protein activators and that arginine residue(s) were modified by ADP-ribose by the action of a mono-ADP-ribosyltransferase. Inhibitors of the transferase blocked both the modification of Gsa and the activation of AC. Collectively, these studies suggest that ADP-ribosylation of Gsa by an endogenous mono-ADP-ribosyltransferase may regulate cardiac AC.

Legumes are unique among plants for their ability to fix atmospheric nitrogen with the help of soil bacteria rhizobia. Medicago truncatula is used as a model legume to study different aspects of symbiotic nitrogen fixation. M. truncatula, in association with its symbiotic partner Sinorhizobium meliloti, fix atmospheric nitrogen into ammonia, which the plant uses for amino acid biosynthesis and the bacteria get reduced photosynthate in return. M. truncatula NPF1.7 previously called MtNIP/LATD is required for symbiotic nitrogen fixing root nodule development and for normal root architecture. Mutations in MtNPF1.7 have defects in these processes. MtNPF1.7 encodes a member of the NPF family of transporters. Experimental results showing that MtNPF1.7 functioning as a high-affinity nitrate transporter are its expression restoring chlorate susceptibility to the Arabidopsis chl1-5 mutant and high nitrate transport in Xenopus laevis oocyte system. However, the weakest Mtnip-3 mutant allele also displays high-affinity nitrate transport in X. laevis oocytes and chlorate susceptibility to the Atchl1-5 mutant, suggesting that MtNPF1.7 might have another biochemical function. Experimental evidence shows that MtNPF1.7 also functions in hormone signaling. Constitutive expression of MtNPF1.7 in several species including M. truncatula results in plants with a robust growth phenotype. Using a synthetic auxin reporter, the presence …

Cytoplasmic 15-hydroxyprostaglandin dehydrogenase from swine kidney was purified to specific activity of 1.2 U per mg protein, by chromatographic techniques. Native molecular weight of enzyme was estimated at 45,000. Enzyme was inhibited by sulfhydryls, diuretics, and various fatty acids. Substrate studies indicated NAD+ specificity and ability to catabolize prostaglandins, except prostaglandin B and thromboxane B. Initial velocity studies gave intersecting plots conforming to a sequential mechanism. 15-keto-prostaglandin exhibited linear noncompetitive production inhibition with respect to either prostaglandin or NAD+; NAD yielded linear competitive production inhibition with respect to NADH. Results, and those of dead-end inhibition and alternated substrate studies, are consistent with an ordered Bi-Bi mechanism: NAD+ is added first, then prostaglandin; then 15-keto-rostaglandin is released, then NADH.

Two plaque-pure phage lambda clones designated as λhtX-l and λhtX-2 that hybridized to unfractionated bovine liver tRNA were isolated from a human X chromosome-specific library. The λDNAs were characterized by restriction mapping and Southern blot hybridization techniques. The human DNA segment in λhtX-l contains five or more presumptive tRNA genes and at least one Alu family member. The 19-kilobase human DNA insert in λhtX-2 contains two or more presumptive tRNA genes and at least three Alu family members. Another human genomic clone designated λhVKV7 hybridized to mammalian valine tRNA IAC. The clone was characterized by physical mapping and Southern blot hybridization techniques. The 18.5-kilobase human DNA fragment in λhVKV7 contains a cluster of three tRNA genes and at least nine Alu family members.

This work examines the synthesis of poly(ADP-ribose) in normal and SV40-transformed monolayer cultures of 3T3 cells as a function of growth and DNA fragmentation. A review of the relevant literature is given in the introduction of this work. Poly(ADP-ribose) synthesis has been implicated in transcription, replication, repair, differentiation and regulation of cell growth. The results of this study suggest that poly(ADP-ribose) synthesis is involved in some aspect of cell-growth control and DNA repair.

Stopped-flow experiments in which the NAD-malic enzyme was preincubated with different reactants at near saturating substrate concentrations suggest a slow isomerization of the E:NAD:Mg complex. The lag is eliminated by preincubation with Mg˙² and malate suggesting that the formation of E:Mg:Malate either bypasses or speeds up the slow isomerization step. Circular dichroic spectral studies of the secondary structural changes of the native enzyme in the presence and absence of substrates supports the existence of conformational changes with NAD˙ and malate. Thus, a slow conformational change of the E:NAD:Mg complex is likely one of the rate-limiting steps in the pre-steady state.