Overexpression of plastidic protoporphyrinogen IX oxidase leads to resistance to the diphenyl-ether herbicide acifluorfen

The use of herbicides to control undesirable vegetation has become a universal practice. For the broad application of herbicides the risk of damage to crop plants has to be limited. We introduced a gene into the genome of tobacco (Nicotiana tabacum) plants encoding the plastid-located protoporphyrinogen oxidase of Arabidopsis, the last enzyme of the common tetrapyrrole biosynthetic pathway, under the control of the cauliflower mosaic virus 35S promoter. The transformants were screened for low protoporphyrin IX accumulation upon treatment with the diphenyl ether-type herbicide acifluorfen. Leaf disc incubation and foliar spraying with acifluorfen indicated the lower susceptibility of the transformants against the herbicide. The resistance to acifluorfen is conferred by overexpression of the plastidic isoform of protoporphyrinogen oxidase. The in vitro activity of this enzyme extracted from plastids of selected transgenic lines was at least five times higher than the control activity. Herbicide treatment that is normally inhibitory to protoporphyrinogen IX oxidase did not significantly impair the catalytic reaction in transgenic plants and, therefore, did not cause photodynamic damage in leaves. Therefore, overproduction of protoporphyrinogen oxidase neutralizes the herbicidal action, prevents the accumulation of the substrate protoporphyrinogen IX, and consequently abolishes the light-dependent phytotoxicity of acifluorfen.     

Modeling of substrate-binding region of the active site of monoamine oxidase A

The mold of the substrate-binding region of the active site of monoamine oxidase A (MAO A) was designed using data of the enzyme interaction with reversible competitive inhibitors and the analysis of their three-dimensional structures. The superposition of ligands in biologically active conformations allowed determination of the shape and dimension of the active site cavity accommodating these compounds. The correctness of this approach was validated by the analysis of HIV protease interaction with its inhibitors using three-dimensional structures of HIV protease-inhibitor complexes. The mold of the substrate/inhibitor-binding site can be used for searching for new ligands in molecular databases and the development of a new generation of MAO inhibitors using lead structures that have not been employed for this purpose yet.     

Correct identification of the chloroplastic protoporphyrinogen IX oxidase N-terminus places the biochemical data in frame

CTLA-4 gene constructs were designed to express CTLA-4 exclusively in the endoplasmic reticulum (ER). Four different CTLA-4 gene constructs were transfected into HEK 293 (human embryonic kidney) and A20 (Balb/c mouse B lymphoma) cells. All constructs contained an ER retention signal and coded for CTLA-4 expression in the ER. One of the constructs, which contained the membrane part of CTLA-4, coded for an expression both on the cell surface and in the ER. Three of the expressed CTLA-4 types (including the ER-membrane-expressed form) caused a reduced surface expression of B7 in the A20 cells. Only constructs which allow dimerization of CTLA-4 showed this effect. It is assumed that intracellular CTLA-4 bound B7 and inhibited therefore the transport of B7 to the surface. The binding obviously caused also an enhanced degradation of the complexes because both proteins showed a low concentration in the transfected cell lines. CTLA-4-transfected and B7-reduced A20 cells showed a diminished costimulating activity upon T cells. This was demonstrated by a reduced proliferation of T cells from ovalbumin-immunized Balb/c mice, incubated with ovalbumin peptide-primed CTLA-4-transfected A20 cells.     

Subcellular localization and light-regulated expression of protoporphyrinogen IX oxidase and ferrochelatase in Chlamydomonas reinhardtii

Protoporphyrinogen IX oxidase (PPO) catalyzes the last common step in chlorophyll and heme synthesis, and ferrochelatase (FeC) catalyzes the last step of the heme synthesis pathway. In plants, each of these two enzymes is encoded by two or more genes, and the enzymes have been reported to be located in the chloroplasts or in the mitochondria. We report that in the green alga Chlamydomonas reinhardtii, PPO and FeC are each encoded by a single gene. Phylogenetic analysis indicates that C. reinhardtii PPO and FeC are most closely related to plant counterparts that are located only in chloroplasts. Immunoblotting results suggest that C. reinhardtii PPO and FeC are targeted exclusively to the chloroplast, where they are associated with membranes. These results indicate that cellular needs for heme in this photosynthetic eukaryote can be met by heme that is synthesized in the chloroplast. It is proposed that the multiplicity of genes for PPO and FeC in higher plants could be related to differential expression in differently developing tissues rather than to targeting of different gene products to different organelles. The FeC content is higher in C. reinhardtii cells growing in continuous light than in cells growing in the dark, whereas the content of PPO does not significantly differ in light- and dark-grown cells. In cells synchronized to a light/dark cycle, the level of neither enzyme varied significantly with the phase of the cycle. These results indicate that heme synthesis is not directly regulated by the levels of PPO and FeC in C. reinhardtii.     

Resistance to protoporphyrinogen oxidase-inhibiting compound S23142 from overproduction of mitochondrial protoporphyrinogen oxidase by gene amplification in photomixotrophic tobacco cells

Tobacco YZI-IS cells exhibit a 150-fold greater resistance to the protoporphyrinogen oxidase (Protox)-inhibiting compound, S23142, from wild-type tobacco cells. To investigate the mechanism for this S23142 resistance, the protein level, enzymatic activity, and sensitivity to S23142 in two Protox isoenzymes (plastidal and mitochondrial forms) were examined. The level of mitochondrial Protox protein was greater, and its activity 5-times higher, in YZI-IS cells than in wild-type cells. Furthermore, the apparent IC50 value of S23142 was about 20 nM, which is 20-fold higher than that observed in wild-type cells. In contrast, no differences were found in the plastidal Protox protein level, activity or its inhibition by S23142 between YZI-1S and wild-type cells. A southern blot analysis revealed that the mitochondrial Protox gene had been significantly amplified in the YZI-1S cells. These results suggest that the S23142 resistance of YZI-1S cells was due to the overproduction of mitochondrial Protox by gene amplification.     

Functional studies of mutations in the human protoporphyrinogen oxidase gene in variegate porphyria.

The autosomal dominant disorder, variegate porphyria (VP), results from mutations in the protoporphyrinogen oxidase (PPOX) gene. We have investigated the effects of 22 disease-associated missense mutations in this gene on enzyme activity. Mutants were generated in the expression plasmid pHPPOX by site-directed mutagenesis. They were screened for PPOX activity by complementation of the Escherischia coli strain SAS38X which lacks PPOX activity. Ten mutants (G40E, L85P, G232R, de1281H, V282D, L295P, V335G, S350P, L444P, G453V) had no detectable PPOX activity. PPOX activity of the remaining 12 mutants (L15F, R38P, L73P, V84G, D143V, R152C, L154P, V158M, R168H, A172V, V290L, G453R) ranged from less than 1% to 9.2% of wild-type activity. Our findings show that all 22 mutations substantially impair or abolish PPOX activity in a prokaryotic expression system and add to the evidence that they cause VP.     

Heterologous complementation systems verify the mosaic distribution of three distinct protoporphyrinogen IX oxidase in the cyanobacterial phylum

Journal of Plant Research - The pathways for synthesizing tetrapyrroles, including heme and chlorophyll, are well-conserved among organisms, despite the divergence of several enzymes in these...     

Crystal structure of protoporphyrinogen IX oxidase: a key enzyme in haem and chlorophyll biosynthesis

Protoporphyrinogen IX oxidase (PPO), the last common enzyme of haem and chlorophyll biosynthesis, catalyses the oxidation of protoporphyrinogen IX to protoporphyrin IX. The membrane-embedded flavoprotein is the target of a large class of herbicides. In humans, a defect in PPO is responsible for the dominantly inherited disease variegate porphyria. Here we present the crystal structure of mitochondrial PPO from tobacco complexed with a phenyl-pyrazol inhibitor. PPO forms a loosely associated dimer and folds into an FAD-binding domain of the p-hydroxybenzoate-hydrolase fold and a substrate-binding domain that enclose a narrow active site cavity beneath the FAD and an alpha-helical membrane-binding domain. The active site architecture suggests a specific substrate-binding mode compatible with the unusual six-electron oxidation. The membrane-binding domains can be docked onto the dimeric structure of human ferrochelatase, the next enzyme in haem biosynthesis, embedded in the opposite side of the membrane. This modelled transmembrane complex provides a structural explanation for the uncoupling of haem biosynthesis observed in variegate porphyria patients and in plants after inhibiting PPO.     

Characterization of HemY-type protoporphyrinogen IX oxidase genes from cyanobacteria and their functioning in transgenic Arabidopsis

Plant Molecular Biology - SlPIN8 is expressed specifically within tomato pollen, and that it is involved in tomato pollen development and intracellular auxin homeostasis. The auxin (IAA) transport...     

The enzymic conversion of protoporphyrinogen IX to protoporphyrin IX. Protoporphyrinogen oxidase activity in mitochondrial extracts of Saccharomyces cerevisiae.

The oxidation of protoporphyrinogen IX to protoporphyrin IX in yeast cells is enzyme-dependent. The enzyme, protoporphyrinogen oxidase, associated with purified mitochondria isolated from Saccharomyces cerevisiae was solubilized by sonic treatment in the presence of detergent and partially purified. The molecular weight of the enzyme was 180,000 plus or minus 18,000. The purified preparation could be stored at -20 degrees in the presence of 20% glycerol for several months without loss of activity. Enzyme activity was destroyed by heating above 40 degrees and by proteolytic digestion and irreversible inactivation occurred outside the pH range of 4.0 to 9.5. The pH optimum of the enzymic reaction was 7.45 and the value of the Michaelis constant was approximately 4.8 muM. Protoporphyrinogen oxidase did not catalyse the oxidation of coproporphyrinogen I or III or uroporphyrinogen I or III and the rate of enzymic oxidation of mesoporphyrinogen IX was less than 20% of that observed with protoporphyrinogen IX. The presence of thiol groups in the enzyme system was indicated but no metal ion or other cofactor requirement was demonstrated. Enzyme activity was insensitive to cyanide, 2,4-dinitrophenol, and azide whereas it was inhibited in the presence of Cu-2+ or Co-2+ ions, high ionic strength, heme, or hemin.     

Functional analysis of the hemK gene product involvement in protoporphyrinogen oxidase activity in yeast

The Escherichia coli hemK gene has been described as being involved in protoporphyrinogen oxidase activity; however, there is no biochemical evidence for this. In the context of characterizing the mechanisms of protoporphyrinogen oxidation in the yeast Saccharomyces cerevisiae, we investigated the yeast homolog of HemK, which is encoded by the ORF YNL063w, to find out whether it has any protoporphyrinogen oxidase activity and/or whether it modulates protoporphyrinogen oxidase activity. Phenotype analysis and enzyme activity measurements indicated that the yeast HemK homolog is not involved in protoporphyrinogen oxidase activity. Complementation assays in which the yeast HemK homolog is overproduced do not restore wild-type phenotypes in a yeast strain with deficient protoporphyrinogen oxidase activity. Protein sequence analysis of HemK-related proteins revealed consensus motif for S-adenosyl-methionine-dependent methyltransferase.     

Characteristics of murine protoporphyrinogen oxidase.

Protoporphyrinogen oxidase (EC 1.3.3.4) (PPO) is the penultimate enzyme of the heme biosynthetic pathway. Mouse PPO has been purified in low yield and kinetically characterized by this laboratory previously. A new more rapid purification procedure is described herein, and with this protein we detect a noncovalently bound flavin moiety. This flavin is present at approximately stoichiometric amounts in the purified enzyme and has been identified by its fluorescence spectrum and high performance liquid chromatography as flavin mononucleotide (FMN). Fluorescence quenching studies on the flavin yielded a Stern-Volmer quenching constant of 12.08 M-1 for iodide and 1.1 M-1 for acrylamide. Quenching of enzyme tryptophan fluorescence resulted in quenching constants of 6 M-1 and 10 M-1 for iodide and acrylamide, respectively. Plasma scans performed on purified enzyme preparations did not reveal the presence of stoichiometric amounts of protein-bound metal ions, and we were unable to detect any protein-associated pyrroloquinoline quinone (PQQ). Data from circular dichroism studies predict a secondary structure of the native protein consisting of 30.5% alpha helix, 40.5% beta sheet, 13.7% turn, and 15.3% random coil. Denaturation of PPO with urea resulted in a biphasic curve when ellipticity is plotted against urea concentration, typical of amphipathic proteins.     

The mitochondrial location of protoporphyrinogen oxidase

Using the digitonin method and subsequent fractionation of rat liver mitochondria, protoporphyrinogen oxidase (penultimate enzyme in the heme biosynthesis pathway) was found to be closely associated with the mitochondrial inner membrane fraction. Chemical treatment with non-specific probes (trypsin and diazobenzene sulfonate) of either intact or inverted mitoplasts, indicated that protoporphyrinogen oxidase was anchored within the lipid bilayer of the inner membrane. Protoporphyrinogen had an equal access to the active site of the enzyme from both sides of the inner membrane and its transformation to protoporphyrin did not appear to be energy-dependent. Studies of protoporphyrinogen synthesis from exogenously added coproporphyrinogen in either intact or hypoosmotically treated mitochondria underlined the importance of the peculiar submitochondrial location of coproporphyrinogen oxidase and protoporphyrinogen oxidase for the transfer of substrates to the inner membrane.     

Mitochondrial targeting of human protoporphyrinogen oxidase

Variegate porphyria is an autosomal dominant disorder of heme metabolism resulting from a deficiency in protoporphyrinogen oxidase, an enzyme located on the inner mitochondrial membrane. This study examined the effect of three South African VP-causing mutations (H20P, R59W, R168C) on mitochondrial targeting. Only H20P did not target, and of eight protoporphyrinogen oxidase-GFP chimeric fusion proteins created, N-terminal residues 1-17 were found to be the minimal protoporphyrinogen oxidase sequence required for efficient mitochondrial targeting. Removal of this N-terminal sequence displayed mitochondrial localization, suggesting internal mitochondrial targeting signals. In addition, six constructs were engineered to assess the effect of charge and helicity on mitochondrial targeting of the protein. Of those engineered, only the PPOX20/H20P-GFP construct abolished mitochondrial targeting, presumably through disruption of the protoporphyrinogen oxidase alpha-helix. Based on our results we propose a mechanism for protoporphyrinogen oxidase targeting to the mitochondrion.     

Recombinant maize protoporphyrinogen IX oxidase expressed in Escherichia coli forms complexes with GroEL and DnaK chaperones

The clone corresponding to maize plastidic protoporphyrinogen IX oxidase (PPO) has been isolated by functional complementation and inserted into a pET16b vector for expression in Escherichia coli. Recombinant PPO was purified by standard affinity chromatography using a metal chelating resin. Two contaminants copurified with recombinant PPO and were identified as GroEL and DnaK. Since chaperone binding to hydrophobic regions of the protein is regulated by ATP availability, an ATP washing step was introduced prior to elution of the recombinant protein from an affinity column. This washing step selectively removed both chaperones and allowed the recovery of pure PPO. Coexpression of PPO and GroELS resulted in a sixfold increase of soluble PPO yield, suggesting that bacterial chaperones could be limiting during the folding of the heterologous protein. However, a portion of PPO was still found in the insoluble fraction. Buffer containing the GroEL and DnaK enabled resuspension of PPO from the insoluble fraction but failed to enhance refolding of the denaturated protein. Attempts to increase the amount of soluble PPO using a thioredoxin-PPO fusion protein were not successful. Initial characterization of the recombinant PPO found that it possessed a high V(max), an elevated affinity for substrate, and an elevated sensitivity to PPO inhibitor herbicides compared to previous reports.     

EPR studies of spin-labeled bovine plasma amine oxidase: the nature of the substrate-binding site.

The carbonyl cofactor of bovine plasma amine oxidase (EC 1.4.3.6), recently shown to be 6-hydroxydopa (also known as topa), has been spin labeled to the extent of one label per enzyme dimer molecule, using 4-amino-2,2,6,6-tetramethylpiperidine-N-oxyl (4-amino-TEMPO) and 4-hydrazino-TEMPO followed by reduction with borohydride. By studying the EPR spectra of the labeled enzyme, it has been deduced that there is no magnetic interaction between the copper and the spin label, and that the spin label is at least 1.3 nm distant from the copper(II) ion in the resting enzyme. The bound label is strongly immobilized, is in a sterically constricted environment, and is not accessible to small anions. Removal of the copper does not alter the EPR spectrum of the label. The results are similar to results for porcine plasma amine oxidase, and show that the copper is not close to, and does not directly interact with, the topa-bound substrate.