Molecular characterization and chromosomal localization of cytochrome P450 genes involved in the biosynthesis of cyclic hydroxamic acids in hexaploid wheat.

The cyclic hydroxamic acids, 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA) and 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), are defensive secondary metabolites found in gramineous plants including wheat, maize and rye. cDNAs for five cytochromes P450 (P450s) involved in DIBOA biosynthesis (CYP71C6, CYP71C7v2, CYP71C8v2, CYP71C9v1 and CYP71C9v2) were isolated from seedlings of hexaploid wheat [( Triticum aestivum L. cv. Chinese Spring (2n=6x=42, genomes AABBDD)] by RT-PCR and screening of a cDNA library. CYP71C9v1 and CYP71C9v2 are 97% identical to each other in amino acid and nucleotide sequences. The cloned P450 species showed 76-79% identity at the amino acid level to the corresponding maize P450 species CYP71C1-C4, which are also required for DIBOA biosynthesis. The wheat P450 cDNAs were heterologously expressed in the yeast ( Saccharomyces cerevisiae) strain AH22. Microsome fractions from yeast cells expressing these P450 species catalyzed the same reactions as their maize orthologs. The chromosomes carrying the cyp71C6- C9v1 orthologs were identified by Southern hybridization using aneuploid lines of Chinese Spring wheat. The cyp71C9v1 orthologs were located on the chromosomes of wheat homoeologous group-4. The orthologs of the other P450 genes, cyp71C7v2, cyp71C6 and cyp71C8v2, were located on group-5 chromosomes. The same P450 genes were also present in the three ancestral diploid species of hexaploid wheat, T. monococcum (AA), Aegilops speltoides [BB (approximately SS)] and Ae. squarrosa (DD).

CYP78A1 preferentially expressed in developing inflorescences of Zea mays encoded a cytochrome P450-dependent lauric acid 12-monooxygenase.

Cytochrome P450 (P450 or CYP) monooxygenases play an important role in the oxidation of a number of lipophilic substrates including secondary metabolites in higher plants. Larkin reported that CYP78A1 was preferentially expressed in developing inflorescences of Zea mays (Larkin, Plant Mol. Biol. 25: 343-353, 1994). However, the enzymatic function of CYP78A1 hasn't been clarified yet. To characterized the enzymatic activity of CYP78A1, in this study, CYP78A1 cDNA and tobacco or yeast NADPH-cytochrome P450 oxidoreductase (P450 reductase) was expressed in the yeast Saccharomyces cerevisiae AH22 cells under the control of alcohol dehydrogenase promoter I and terminator. The reduced CO-difference spectrum of a microsomal fraction prepared from the transformed yeast cells expressing CYP78A1 and yeast P450 reductase showed a peak at 449 nm. Based on the spectrum, the content of a P450 molecule was estimated to be 45 pmol P450 equivalent/ mg of protein in the microsomal fraction. The recombinant yeast microsomes containing CYP78A1 and yeast P450 reductase were found to catalyze 12-monooxygenation of lauric acid. Based on these results, CYP78A1 preferentially expressed in developing inflorescences of Zea mays appeared to have participated in the monooxygenation of fatty acids.

Inhibitory effects of 1,4-naphthoquinone derivatives on rat cytochrome P4501A1-dependent monooxygenase activity in recombinant yeast microsomes.

We reported previously that various naphthoquinone derivatives inhibited cytochrome P450-dependent monooxygenase of liver and placenta microsomes [Muto, N. et al. (1987) Biochem. Biophys. Res. Commun. 146, 487-494]. To understand the complex inhibitory behaviors that were observed, it is desirable to study the relationship between structure and inhibitory activity of naphthoquinones in a simplified system containing a single P450 species. In the present study, the inhibitory effects of six derivatives of 1,4-naphthoquinone (hereafter referred to as NQ) on rat cytochrome P4501A1-dependent 7-ethoxycoumarin O-deethylation were examined using yeast microsomes containing overexpressed rat P4501A1. Of these, 2-methyl-5-hydroxy-NQ, 2-methyl-NQ, 2-hydroxy-NQ, and NQ showed competitive inhibition, whereas 5,8-dihydroxy-NQ and 5-hydroxy-NQ showed noncompetitive inhibition. Judging from the inhibitor constant (K(i)), the binding affinity of the four competitive inhibitors for the substrate-binding pocket of P4501A1 is in the order: 2-CH(3)-5-OH-NQ > 2-CH(3)-NQ > NQ >> 2-OH-NQ. On binding with P4501A1, 2-CH(3)-5-OH-NQ, 2-CH(3)-NQ, and NQ induced distinct Type II, Type I, and reverse Type I spectra, respectively. These results indicate that methyl and hydroxyl groups introduced into NQ have unique effects on their binding mode and binding affinity.

Green-tissue-specific expression of a reconstructed cry1C gene encoding the active fragment of Bacillus thuringiensis delta-endotoxin in haploid tobacco plants conferring resistance to Spodoptera litura.

The DNA sequence of a truncated cry1C gene encoding the active fragment of Bacillus thuringiensis (Bt) delta-endotoxin was fully reconstructed by introduction of silent mutations. Each of the truncated wild type and the synthetic genes encoding the active fragment of the protoxin was introduced into haploid tobacco plants under the control of the rbcS promoter. To facilitate selection of transgenic tobacco plants with high insecticidal activity, a fusion gene encoding both rat CYP1A1 cytochrome P450 and yeast NADPH-P450 oxidoreductase was cotransformed with the wild type cry1C gene. The synthetic gene elevated the levels of Cry1C protein and the mRNA in transgenic tobacco plants as well as mortality in Spodoptera litura larvae. The Cry1C protein was accumulated mainly in the leaf tissues of the transgenic tobacco plants. The results reported here imply that the green-tissue-specific expression of the synthetic cry1C gene is useful for the control of S. litura which was rather resistant to the other types of Bt toxins.

Encoding of a cytochrome P450-dependent lauric acid monooxygenase by CYP703A1 specifically expressed in the floral buds of petunia hybrida.

The cDNA clone of novel cytochrome P450 CYP703A1 from petunia floral buds was isolated by RT-PCR. The nucleotide sequences of this cDNA clone contained the open reading frame that has been predicted to encode polypeptides consisting of 539 amino acid residues. A significantly high level of the transcript of the cyp703A1 gene was found in the early stage of petunia flower buds, but not in the leaves, stems and roots. The 1041bp 5'-flanking sequences of the cyp703A1 gene contained the conserved motifs of ATHB-1, AGAMOUS, MYB.Ph3, P and SBF-1 binding boxes. CYP703A1 cDNA was expressed in yeast Saccharomyces cerevisiae AH22 cells under the control of an alcohol dehydrogenase I promoter and terminator. The recombinant yeast microsomes containing the CYP703A1 hemoprotein were found to metabolize lauric acid. Based on these results, CYP703A1 was specifically expressed in the early stage of flower development and appeared to participate in the monooxygenation of fatty acids.

Molecular cloning and expression in Saccharomyces cerevisiae of tobacco NADPH-cytochrome P450 oxidoreductase cDNA.

We obtained information on the full length tobacco NADPH-cytochrome P450 oxidoreductase (P450 reductase) by a combination of the cDNA clone pCTR1 and the genomic DNA clone pGTR1. The deduced primary structure consisting of 713 amino acid residues contained sequences corresponding to FMN, FAD, and NADPH-binding regions. Based on this information, we prepared the full-length cDNA pFTR of tobacco P450 reductase by RT-PCR and expressed it in the yeast Saccharomyces cerevisiae. The transformed yeast cells carrying pFTR produced the corresponding mRNA and protein, and had increased cytochrome c reductase activity in the microsomes. An in vitro reconstitution system of the yeast microsomal fractions expressed tobacco P450 reductase and rat P450 1A1 showed an increased 7-ethoxycoumarin O-deethylase activity. These results indicated that tobacco P450 reductase expressed in the yeast microsomes coupled with rat P450 1A1 resulting in an increased monooxygenase activity.

Molecular cloning and sequence analysis of catalase cDNA from green pepper seedlings elicited with arachidonic acid.

We isolated a cDNA encoding catalase from green pepper seedlings elicited with arachidonic acid, based on the amino acid sequences of the purified protein. The nucleotide sequence of the isolated cDNA contained a single open reading frame predicted to encode 492 amino acid residues with a calculated molecular mass of 56439.0 daltons. The deduced amino acid sequence contained the amino acid sequences found by sequencing of the peptides. The total deduced amino acid sequence showed high similarity with those of the other plant catalases reported so far and was found to possess the peroxisomal targeting sequence conserved among plant catalases. Transcription of the catalase gene in green pepper seedlings was found to be induced by treatment with arachidonic acid.

Hypersensitivity to mosquito bites conceals clonal lymphoproliferation of Epstein-Barr viral DNA-positive natural killer cells.

In order to clarify the relationship between Epstein-Barr (EB) virus and hypersensitivity to mosquito bites (HMB), and to search for the mechanism which induces EB virus-associated lymphoproliferative diseases, we investigated patients with HMB, using hematological, immunological and virological techniques. Among 5 cases of HMB, CD56+ cells had proliferated and CD3+ cells were diminished in 4 cases. Although anti-EB virus antibody titers were not consistent with chronic active EB virus infection, EB viral DNA was detected in the peripheral blood mononuclear cells in all 5 cases. Moreover, EB viral DNA-positive cells had proliferated monoclonally in 4 cases, and biclonally in 1 case. It was proved that most of the EB viral DNA existed in natural killer (NK) cells through polymerase chain reaction analysis. These findings suggest that the basis of HMB may be clonal lymphoproliferation of EB viral DNA-positive NK cells and this hematological abnormality may induce the characteristic symptoms of HMB. In some cases, the proliferating NK cells can metamorphose into leukemic cells, and hemophagocytic syndrome, which has been assumed to be a complication of HMB, may then occur.

Purification and immunochemical characteristics of NADPH-cytochrome P-450 oxidoreductase from tobacco cultured cells.

NADPH-cytochrome P-450 oxidoreductase (EC 1.6.2.4) was purified from the microsomal fraction of tobacco (Nicotiana tabacum) BY2 cells by chromatography on two anion-exchange columns and 2',5' ADP-Sepharose 4B column. The purified enzyme showed a single protein band with a molecular weight of 79 kDa on SDS-PAGE and exhibited a typical flavoprotein redox spectrum, indicating the presence of an equimolar quantity of FAD and FMN. This enzyme followed Michaelis-Menten Kinetics with Km values of 24 microM for NADPH and 16 microM for cytochrome c. An in vitro reconstituted system of the purified reductase with a partially purified tobacco cytochrome P-450 preparation showed the cinnamic acid 4-hydroxylase activity at the rate of 14 pmol min-1 nmol-1 P-450 protein and with a purified rabbit P-4502C14 catalyzed N-demethylation of aminopyrine at the rate of 6 pmol min-1 nmol-1 P-450 protein. Polyclonal antibodies raised against the purified reductase reacted with tobacco reductase but not with yeast reductase on Western blot analysis. Anti-yeast reductase antibodies did not react with the tobacco reductase. This result indicate that the tobacco reductase was immunochemically different from the yeast reductase. The anti-tobacco reductase antibodies totally inhibited the tobacco reductase activity, but not the yeast reductase. Also, Western blot analyses using the anti-tobacco reductase antibodies revealed that leaves, roots and shoots of Nicotiana tabacum plants contained an equal amount of the reductase protein. From these results, it was suggested that there are different antibody binding sites, which certainly participate in enzyme activity, between tobacco and yeast reductase.

Analysis of DNA bend structure of promoter regulatory regions of xylene-metabolizing genes on the Pseudomonas TOL plasmid.

The transcription of both the upper operon (OP1) coding for m-xylene-degrading enzymes and the positive regulatory gene xylS on the TOL plasmid depends on sigma 54-RNA polymerase and requires the activator protein XylR that binds to the cis-acting upstream regulatory sequence of each promoter. For transcription of OP1 in Escherichia coli, integration host factor (IHF) is also required. IHF binds to DNA between the upstream regulatory sequence and the promoter sequence of OP1. We showed that IHF induced a DNA bend in the promoter regulatory region of OP1 upon its binding, supporting the DNA-loop model for the activation of OP1 transcription. In contrast to OP1, the transcriptional activation of xylS does not require IHF. In the absence of IHF, the promoter regulatory region of xylS promoter was shown to have a weak but significant intrinsic DNA bend, which may be involved in stabilizing the DNA-loop structure for the activation of xylS transcription. When IHF was highly produced in E. coli, the xylS transcription was repressed. We found two weak binding sites for IHF, each overlapping with the promoter sequence and the upstream regulatory sequence. The IHF binding to these sites might result in repression of xylS expression by overproduced IHF. Evidence is presented that binding of IHF to these sites induces a DNA bend.

Determination of the subunit contact region of aspartase.

The subunits of tetrameric enzyme aspartase from Escherichia coli and Pseudomonas fluorescens were incapable of forming hybrid tetramers, suggesting that the subunit contact regions of these two enzymes are not conserved in spite of significant homology between the total sequences of the enzymes. To locate the subunit contact region, we modified cysteine residues of the intermediate species formed during the assembly of the subunits of aspartase from E. coli. Subunits modified with N-ethylmaleimide were unable to assemble into tetramers. Further experiments showed that Cys-88 was the primary residue that was modified. The sequence flanking Cys-88 is quite different from that in the the enzyme from P. fluorescens, suggesting that this region participates in the mutual recognition of subunits.

Analysis of an upstream regulatory sequence required for activation of the regulatory gene xylS in xylene metabolism directed by the TOL plasmid of Pseudomonas putida.

Transcription from the promoter of a positive regulatory gene, xylS, on the TOL plasmid of Pseudomonas putida is activated by another positive regulator, XylR, in the presence of m-xylene and is dependent on RNA polymerase containing the NtrA protein (sigma 54). Deletion analysis of the upstream region of the xylS gene revealed an upstream regulatory sequence (URS), located between 145 and 188 bp upstream from the transcription start site. The URS is active in either orientation and can be placed 3.9 kb further upstream without loss of activity. Dependence of activation on helical periodicity was observed in the region between the URS and the promoter of the xylS gene, suggesting DNA loop formation between these two sites, which are located about 100 bp apart. The expression of xylR was autogenously repressed by XylR protein. This autogenous repression is decreased in an NtrA- background, irrespective of the presence of the xylS promoter in cis, indicating that NtrA protein, or NtrA-containing RNA polymerase that is not bound to the xylS promoter, is involved in the binding of XylR protein to the URS.

Activation of aspartase by site-directed mutagenesis.

To elucidate the role of sulfhydryl groups in the enzymatic reaction of the aspartase from Escherichia coli, we used site-directed mutagenesis which showed that the enzyme was activated by replacement of Cys-430 with a tryptophan. This mutation produced functional alterations without appreciable structural change: The kcat values became 3-fold at pH 6.0; the Hill coefficient values became higher under both pH conditions; the dependence of enzyme activity on divalent metal ions increased; and hydroxylamine, a good substrate for the wild-type enzyme, proved a poor substrate for the mutant.

Preparation of active hybrid enzymes composed of the native and chemically inactivated aspartase subunits from Escherichia coli.

The hybridization of the native and chemically inactivated aspartase from Escherichia coli was studied. Preparations of the tetrameric enzyme obtained by mixing the native and N-ethylmaleimide (NEM)-inactivated aspartase in 4 M guanidine-HCl followed by 51-fold dilution at room temperature retained catalytic activity. Affinity chromatography on AF-Red TOYO-PEARL separated several active components in the hybridized preparations, and the presence of [14C]NEM-inactivated subunits in the active hybrids was demonstrated. The addition of the native aspartase to Sepharose-bound NEM-inactivated enzyme in 4 M guanidine-HCl resulted in the formation of an immobilized enzyme with enzyme activity. The specific activity of the various hybrids, composed of unmodified and [14C]NEM-inactivated subunits, was roughly proportional to the number of unmodified subunits in each tetramer. Furthermore, when reversible denaturation was conducted on mixtures of the native and NEM-inactivated enzyme at various proportions, the enzyme activity recovered was proportional to the amount of the native enzyme added. These results strongly suggest that each subunit makes an independent contribution to the overall enzyme activity regardless of the presence of other subunits in the same molecule. The theoretical and practical implications of this work are discussed.

DMPO spin trapping in the presence of Fe ion.

Aminoxyl radical formation from DMPO in the presence of Fe ion was studied to clarify the ambiguous ESR signals obtained by spin trapping with DMPO. It was found that when DMPO was used in a Fenton system, a Fe-DMPO complex was formed immediately. This complex was subsequently attacked by oxidative species originating from H2O2 and thus oxidative degradation of DMPO was induced in the Fenton system. On the other hand, in the case of M4PO, the degradation was found to be very slow, indicating that the 3 position of DMPO was favorably attacked by the oxidative species. Some of the degradation products are probably aminoxyl radicals. This series of the degradation products are probably aminoxyl radicals. This series of reactions may compete with spin trapping and make it difficult to analyze ESR spectra obtained in the presence of Fe ion.

Characterization of intermediate species during the molecular assembly of aspartase.

Molecular assembly of aspartase (L-aspartate ammonia-lyase, EC 4.3.1.1) from Escherichia coli was studied during the reversible denaturation. Although previous studies [Tokushige, M., Eguchi, G., and Hirata, F. (1977) Biochem. Biophys. Acta 480, 479-488] were unable to identify intermediate species during the course of reversible denaturation of aspartase, temperature-controlled HPLC and cross-linking with dimethyl suberimidate of the renaturation products showed that monomeric, dimeric and trimeric species occupied over 80% of the total oligomeric molecules below 13 degrees C; unlike the tetramer, these intermediates were without the activity. The degree of active tetramer formation was a linear function of the restoration of the activity below 18 degrees C, while above 23 degrees C, the activity regain was less than 70% restoration of tetrameric molecules. Upon examination by fluorescence spectroscopy, structural changes during reconstitution exhibited such complex kinetics that the rapid formation of structured oligomers proceeds first with a half-time of less than 10 sec, followed by slow subunit association. These results strongly suggest that the tetramer formation is an essential prerequisite, though not sufficient for the active enzyme.

An artifact in the ESR spectrum obtained by spin trapping with DMPO.

In order to overcome a common problem in spin trapping with high concentrations of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) where ESR spectra are obtained which include an unidentified set of lines composed of a triplet of doublets, commercial DMPO was analyzed for its impurities by high-performance liquid chromatography, mass spectrometry, and nuclear magnetic resonance spectroscopy. It has been determined that this undesirable ESR spectrum is due to an impurity included in the spin trap. This compound has been assigned to the hydroxylamine which is a DMPO-derivative having an epoxy ring located at the 2 and 3 positions.

Reversed-phase ion-pair chromatography of oligodeoxyribonucleotides.

The separation of large oligodeoxyribonucleotides, synthesized chemically and subsequently deblocked, was carried out by reversed-phase ion-pair chromatography (RP-IPC) with a linear gradient of acetonitrile concentration. It was found that tetrabutylammonium phosphate is suitable as an ion-pairing reagent and produces a linear relationship between the base numbers of the samples and their elution volumes. It was also verified that various factors effective in reversed-phase chromatography, such as temperature, end-capping of the columns, differences in the type of C18 alkylating reagents and in the base silica, and the pore size of the base silica have little effect on the resulting separation by RP-IPC.