Molecular Analysis and Heterologous Expression of an Inducible Cytochrome P-450 Protein from Periwinkle (Catharanthus roseus L.)

Cytochrome P450 Enzymes as Key Drivers of Alkaloid Chemical Diversification in Plants

Plants produce more than 20,000 nitrogen-containing heterocyclic metabolites called alkaloids. These chemicals serve numerous eco-physiological functions in the plants as well as medicines and psychedelic drugs for human for thousands of years, with the anti-cancer agent vinblastine and the painkiller morphine as the best-known examples. Cytochrome P450 monooxygenases (P450s) play a key role in generating the structural variety that underlies this functional diversity of alkaloids. Most alkaloid molecules are heavily oxygenated thanks to P450 enzymes' activities. Moreover, the formation and re-arrangement of alkaloid scaffolds such as ring formation, expansion, and breakage that contribute to their structural diversity and bioactivity are mainly catalyzed by P450s. The fast-expanding genomics and transcriptomics databases of plants have accelerated the investigation of alkaloid metabolism and many players behind the complexity and uniqueness of alkaloid biosynthetic pathways. Here we discuss recent discoveries of P450s involved in the chemical diversification of alkaloids and how these inform our approaches in understanding plant evolution and producing plant-derived drugs.

Discovery and modification of cytochrome P450 for plant natural products biosynthesis

Cytochrome P450s are widespread in nature and play key roles in the diversification and functional modification of plant natural products. Over the last few years, there has been remarkable progress in plant P450s identification with the rapid development of sequencing technology, "omics" analysis and synthetic biology. However, challenges still persist in respect of crystal structure, heterologous expression and enzyme engineering. Here, we reviewed several research hotspots of P450 enzymes involved in the biosynthesis of plant natural products, including P450 databases, gene mining, heterologous expression and protein engineering.

Plant cytochrome P450s: nomenclature and involvement in natural product biosynthesis

Cytochrome P450s constitute the largest family of enzymatic proteins in plants acting on various endogenous and xenobiotic molecules. They are monooxygenases that insert one oxygen atom into inert hydrophobic molecules to make them more reactive and hydro-soluble. Besides for physiological functions, the extremely versatile cytochrome P450 biocatalysts are highly demanded in the fields of biotechnology, medicine, and phytoremediation. The nature of reactions catalyzed by P450s is irreversible, which makes these enzymes attractions in the evolution of plant metabolic pathways. P450s are prime targets in metabolic engineering approaches for improving plant defense against insects and pathogens and for production of secondary metabolites such as the anti-neoplastic drugs taxol or indole alkaloids. The emerging examples of P450 involvement in natural product synthesis in traditional medicinal plant species are becoming increasingly interesting, as they provide new alternatives to modern medicines. In view of the divergent roles of P450s, we review their classification and nomenclature, functions and evolution, role in biosynthesis of secondary metabolites, and use as tools in pharmacology.

Biosynthetic pathway of terpenoid indole alkaloids in Catharanthus roseus

Catharanthus roseus is one of the most extensively investigated medicinal plants, which can produce more than 130 alkaloids, including the powerful antitumor drugs vinblastine and vincristine. Here we review the recent advances in the biosynthetic pathway of terpenoid indole alkaloids (TIAs) in C. roseus, and the identification and characterization of the corresponding enzymes involved in this pathway. Strictosidine is the central intermediate in the biosynthesis of different TIAs, which is formed by the condensation of secologanin and tryptamine. Secologanin is derived from terpenoid (isoprenoid) biosynthetic pathway, while tryptamine is derived from indole biosynthetic pathway. Then various specific end products are produced by different routes during downstream process. Although many genes and corresponding enzymes have been characterized in this pathway, our knowledge on the whole TIA biosynthetic pathway still remains largely unknown up to date. Full elucidation of TIA biosynthetic pathway is an important prerequisite to understand the regulation of the TIA biosynthesis in the medicinal plant and to produce valuable TIAs by synthetic biological technology.

Research progress relating to the role of cytochrome P450 in the biosynthesis of terpenoids in medicinal plants

Terpenoids are an extensive and diverse group of plant secondary metabolites. To date, they have been applied in many fields including industry, medicine and health. The wide variety of terpenoid compounds cannot arise solely from simple cyclisations of a precursor molecule or from a single-step reaction; their structural diversity depends on the modification of many specific chemical groups, rearrangements of their skeletal structures and on the post-modification reactions. Most of the post-modification enzymes that catalyse these reactions are cytochrome P450 monooxygenases. Therefore, the discovery and identification of plant P450 genes plays a vital role in the exploration of terpenoid biosynthesis pathways. This review summarises recent research progress relating to the function of plant cytochrome P450 enzymes, describes P450 genes that have been cloned from full-length cDNA and identifies the function of P450 enzymes in the terpenoid biosynthesis pathways of several medicinal plants.

Transcriptome analysis reveals putative genes involved in iridoid biosynthesis in Rehmannia glutinosa

Rehmannia glutinosa, one of the most widely used herbal medicines in the Orient, is rich in biologically active iridoids. Despite their medicinal importance, no molecular information about the iridoid biosynthesis in this plant is presently available. To explore the transcriptome of R. glutinosa and investigate genes involved in iridoid biosynthesis, we used massively parallel pyrosequencing on the 454 GS FLX Titanium platform to generate a substantial EST dataset. Based on sequence similarity searches against the public sequence databases, the sequences were first annotated and then subjected to Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) based analysis. Bioinformatic analysis indicated that the 454 assembly contained a set of genes putatively involved in iridoid biosynthesis. Significantly, homologues of the secoiridoid pathway genes that were only identified in terpenoid indole alkaloid producing plants were also identified, whose presence implied that route II iridoids and route I iridoids share common enzyme steps in the early stage of biosynthesis. The gene expression patterns of four prenyltransferase transcripts were analyzed using qRT-PCR, which shed light on their putative functions in tissues of R. glutinosa. The data explored in this study will provide valuable information for further studies concerning iridoid biosynthesis.

Methylation of sulfhydryl groups: a new function for a family of small molecule plant O-methyltransferases

In plants, type I and II S-adenosyl-l-methionine-dependent O-methyltransferases (OMTs) catalyze most hydroxyl group methylations of small molecules. A homology-based RT-PCR strategy using Catharanthus roseus (Madagascar periwinkle) RNA previously identified six new type I plant OMT family members. We now describe the molecular and biochemical characterization of a seventh protein. It shares 56-58% identity with caffeic acid OMTs (COMTs), but it failed to methylate COMT substrates, and had no activity with flavonoids. However, the in vitro incubations revealed unusually high background levels without added substrates. A search for the responsible component revealed that the enzyme methylated dithiothreitol (DTT), the reducing agent added for enzyme stabilization. Unexpectedly, product analysis revealed that the methylation occurred on a sulfhydryl moiety, not on a hydroxyl group. Analysis of 34 compounds indicated a broad substrate range, with a preference for small hydrophobic molecules. Benzene thiol (Km 220 microm) and furfuryl thiol (Km 60 microm) were the best substrates (6-7-fold better than DTT). Small isosteric hydrophobic substrates with hydroxyl groups, like phenol and guaiacol, were also methylated, but the activities were at least 5-fold lower than with thiols. The enzyme was named C. roseus S-methyltransferase 1 (CrSMT1). Models based on the COMT crystal structure suggest that S-methylation is mechanistically identical to O-methylation. CrSMT1 so far is the only recognized example of an S-methyltransferase in this protein family. Its properties indicate that a few changes in key residues are sufficient to convert an OMT into a S-methyltransferase (SMT). Future functional investigations of plant methyltransferases should consider the possibility that the enzymes may direct methylation at sulfhydryl groups.

Cytochrome P450 Enzymes in Biosyntheses of Some Plant Secondary Metabolites

Secologanin, a secoiridoid glucoside, is a pivotal terpenoid intermediate in the biosynthesis of biologically active monoterpenoid indole alkaloids such as reserpine, ajmaline, and vinblastine which are biosynthesized via strictosidine, an alkaloidal glucoside, formed from secologanin and tryptamine. In secologanin biosynthesis, the oxidative cleavage process of loganin to secologanin and the hydroxylation of 7-deoxyloganin to loganin have remained unknown enzymologically and mechanistically. Cornoside is a unique glucoside with 4-hydroxy-2,5-cyclohexadien-1-one (benzoquinol) ring and is widespread in families such as Cornaceae, Oleaceae, and Scrophulariaceae but its biosynthesis, especially the oxidative process, remain to be investigated. Shikonin is a red naphthazarin pigment derived from the roots of Lithospermum erythorhizon and produced biotechnologically by cell cultures. Its biosynthesis including the production regulation mechanism has been investigated in detail. However, the naphthazarin ring formation process, probably starting with the hydroxylation of the side chain of geranylhydroquinone, a key intermediate at the late stage of shikonin biosynthesis, remained unknown. In the present review, cytochrome P450 monooxygenases involved in the biosyntheses of three structurally and biosynthetically interesting compounds, secologanin, cornoside, and shikonin, a described together with the results of previous and recent biosynthetic studies. The biosyntheses of related compounds are also discussed.

Expression of a ripening-related cytochrome P450 cDNA in Cavendish banana (Musa acuminata cv. Williams)

As part of a study to understand the molecular basis of fruit ripening, this study reports the isolation and characterization of a banana cytochrome P450 (P450) cDNA, designated as MAP450-1, which was associated with fruit ripening of banana. MAP450-1 encoded a single polypeptide of 507 amino acid residues that shared an overall identity of 27-45% with that of several plant P450s, among which MAP450-1 was most related phylogenetically to the avocado P450 CYP71A1. The polypeptide that possessed residue domains conserved in all P450s was classified as CYP71N1. Expression of CYP71N1 varied greatly between banana organs. Transcripts were detected only in peel and pulp of the ripening fruit and not in unripe fruit tissues at all developmental stages or other organs (root, leaf, ovary and flower). During ripening, transcripts were barely detectable in pre-climacteric and climacteric fruits but, as ripening progressed, they began to accumulate and reached a maximum in post-climacteric fruits. CYP71N1 expression in pre-climacteric fruit could be upregulated by exogenous application of ethylene (1-5 ppm) and treatment of overripe fruit with exogenous sucrose (50-300 mM) but not glucose downregulated the expression. These results indicate that P450s may not play a role in fruit development and its expression is associated with ripening, which may be regulated, in part, by ethylene and/or sucrose, at the transcript level.

Predicting the substrates of cloned plant O-methyltransferases

Plant O-methyltransferases (OMTs) have important roles in secondary metabolite biosynthesis. Sequencing projects and homology-based cloning strategies yield sequences for proteins with similarities to known OMTs, but the identification of the physiological substrates is not trivial. We investigated with a cDNA cloned from Catharanthus roseus the possibilities for predicting the substrates of OMTs, using the information from previous work and two newly identified motifs that were based on information from the crystal structures of two plant OMTs. The results, confirmed by functional analysis of the recombinant protein, indicated that a careful analysis of the deduced protein sequence can provide clues for predicting the substrates of cloned OMTs.

Geraniol 10-hydroxylase, a cytochrome P450 enzyme involved in terpenoid indole alkaloid biosynthesis

Geraniol 10-hydroxylase (G10H) is a cytochrome P450 monooxygenase involved in the biosynthesis of iridoid monoterpenoids and several classes of monoterpenoid alkaloids found in a diverse range of plant species. Catharanthus roseus (Madagascar periwinkle) contains monoterpenoid indole alkaloids, several of which are pharmaceutically important. Vinblastine and vincristine, for example, find widespread use as anti-cancer drugs. G10H is thought to play a key regulatory role in terpenoid indole alkaloid biosynthesis. We purified G10H from C. roseus cells. Using degenerate PCR primers based on amino acid sequence information we cloned the corresponding cDNA. The encoded CYP76B6 protein has G10H activity when expressed in C. roseus and yeast cells. The stress hormone methyljasmonate strongly induced G10h gene expression coordinately with other terpenoid indole alkaloid biosynthesis genes in a C. roseus cell culture.

Iridoid biosynthesis in staphylinid rove beetles (Coleoptera: Staphylinidae, Philonthinae)

The biosynthesis of chrysomelidial and plagiodial was studied in the rove beetle subtribe Philonthina (Staphylinidae). Glandular homogenates were found to convert synthetic (2E,6E)-[trideuteromethyl-5,5-(2)H(5)]octa-2,6-diene-1,8-diol (10) into nor-chrysomelidial (14) and nor-plagiodial (13). The overall transformation requires; i) oxidation of the substrate at C(1) and C(8), ii) cyclization of the resulting dialdehyde to nor-plagiodial followed by iii) isomerization to give nor-chrysomelidial. The oxidase requires molecular oxygen as a cofactor and operates with removal of the pro-R hydrogen from C(1) and C(8) of synthetic (1R,8R,2E,6E)-[1,8-(2)H(2)]-2,6-dimethyl-octa-2,6-diene-1,8-diol (15), producing a dialdehyde along with H(2)O(2). Unlike enzymes from iridoid-producing leaf beetle larvae, the Philonthus enzyme is able to oxidize saturated substrates such as citronellol. Crude protein extracts prepared from Philonthus glands by ammonium sulfate precipitation, were found to produce hydrogen peroxide at a rate of 0.085+/-0.003 ng H(2)O(2) (ng protein)(-1) hr(-1) with nerol as an oxidase substrate. The cyclase operates with opposite stereochemistry to the enzyme(s) from Phaedon cochleariae and other herbivorous leaf beetles, specifically removing the C(5)-H(R) hydrogen atom from (4R,5S,2E,6E)-[4,5-(2)H(2)]-2-methyl-octa-2,6-diene-1,8-diol (17). These findings have enabled us to construct a detailed account of iridoid biosynthesis in rove beetles, which resembles the biosynthetic route in leaf beetle larvae, but exhibits distinct stereochemical differences.

Engineering the plant cell factory for secondary metabolite production

Plant secondary metabolism is very important for traits such as flower color, flavor of food, and resistance against pests and diseases. Moreover, it is the source of many fine chemicals such as drugs, dyes, flavors, and fragrances. It is thus of interest to be able to engineer the secondary metabolite production of the plant cell factory, e.g. to produce more of a fine chemical, to produce less of a toxic compound, or even to make new compounds, Engineering of plant secondary metabolism is feasible nowadays, but it requires knowledge of the biosynthetic pathways involved. To increase secondary metabolite production different strategies can be followed, such as overcoming rate limiting steps, reducing flux through competitive pathways, reducing catabolism and overexpression of regulatory genes. For this purpose genes of plant origin can be overexpressed, but also microbial genes have been used successfully. Overexpression of plant genes in microorganisms is another approach, which might be of interest for bioconversion of readily available precursors into valuable fine chemicals. Several examples will be given to illustrate these various approaches. The constraints of metabolic engineering of the plant cell factory will also be discussed. Our limited knowledge of secondary metabolite pathways and the genes involved is one of the main bottlenecks.

Characterization of maize cytochrome P450 monooxygenases induced in response to safeners and bacterial pathogens

Plants use a diverse array of cytochrome P450 monooxygenases in their biosynthetic and detoxification pathways. To determine the extent to which various maize P450s are induced in response to chemical inducers, such as naphthalic anhydride (NA), triasulfuron (T), phenobarbital, and bacterial pathogens (Erwinia stuartii, Acidovorax avenae), we have analyzed the response patterns of seven P450 transcripts after treatment of seedlings with these inducers. Each of these P450 transcripts has distinct developmental, tissue-specific, and chemical cues regulating their expression even when they encode P450s within the same biosynthetic pathway. Most notably, the CYP71C1 and CYP71C3 transcripts, encoding P450s in the DIMBOA biosynthetic pathway, are induced to the same level in response to wounding and NA treatment of younger seedlings and differentially in response to NA/T treatment of younger seedlings and NA and NA/T treatment of older seedlings. NA and T induce expression of both CYP92A1 and CYP72A5 transcripts in older seedling shoots, whereas phenobarbital induces CYP92A1 expression in older seedling shoots and highly induces CYP72A5 expression in young and older seedling roots. Expressed sequence tag (EST) 6c06b11 transcripts, encoding an undefined P450 activity, are highly induced in seedling shoots infected with bacterial pathogens.

Indole alkaloid biosynthesis in Catharanthus roseus: new enzyme activities and identification of cytochrome P450 CYP72A1 as secologanin synthase

The molecular characterization of CYP72A1 from Catharanthus roseus (Madagascar periwinkle) was described nearly a decade ago, but the enzyme function remained unknown. We now show by in situ hybridization and immunohistochemistry that the expression in immature leaves is epidermis-specific. It thus follows the pattern previously established for early enzymes in the pathway to indole alkaloids, suggesting that CYP72A1 may be involved in their biosynthesis. The early reactions in that pathway, i.e. from geraniol to strictosidine, contain several candidates for P450 activities. We investigated in this work two reactions, the conversion of 7-deoxyloganin to loganin (deoxyloganin 7-hydroxylase, DL7H) and the oxidative ring cleavage converting loganin into secologanin (secologanin synthase, SLS). The action of DL7H has not been demonstrated in vitro previously, and SLS has only recently been identified as P450 activity in one other plant. We show for the first time that both enzyme activities are present in microsomes from C. roseus cell cultures. We then tested whether CYP72A1 expressed in E. coli as a translational fusion with the C. roseus P450 reductase (P450Red) has one or both of these activities. The results show that CYP72A1 converts loganin into secologanin.

Plant methionine synthase: new insights into properties and expression

We investigated the enzyme methionine synthase (MSY) in Catharanthus roseus. The properties were characterized with purified protein isolated either from plant cell cultures or after heterologous expression in Escherichia coli. The protein was a monomer and accepted both the triglutamate (CH3-H4PteGlu3, apparent Km = 80 microM) and the monoglutamate (CH3-H4PteGlu1, apparent Km = 350 microM) of methyl-5,6,7,8-tetrahydropteroate as methyl donor, with a ratio of approximately 90:1 in favor of the triglutamate. Both activities required inorganic phosphate, but with different kinetics, and both were dependent on reducing agents. The activity required zinc, as shown by depletion and reconstitution experiments. Mg2+ had no effect on the activity. Two MSY isoforms purified from parsley cell cultures revealed the same properties as the C. roseus enzyme, however, the parsley proteins had no detectable activity with the monoglutamate substrate. The second part of the work compared the expression of the three enzymes of the methyl cycle (MSY, S-adenosyl-L-methionine synthetase, S-adenosyl-L-homocysteine hydrolase). In cell cultures, all three enzymes were present under all conditions investigated, with small changes at the protein level and more pronounced changes at the RNA level. Studies with seedlings revealed a low expression of all three enzymes in cotyledons, when compared to hypocotyls and radiculas. Immunohistochemical experiments indicated that MSY expression in cotyledons is cell-type specific, with the strongest signals detected in the upper epidermis.

Light-induced cytochrome P450-dependent enzyme in indole alkaloid biosynthesis: tabersonine 16-hydroxylase

Vinblastine and vincristine are two medically important bisindole alkaloids from Catharanthus roseus (Madagascar periwinkle). Attempts at production in cell cultures failed because a part of the complex pathway was not active, i.e. from tabersonine to vindoline. It starts with tabersonine 16-hydroxylase (T16H), a cytochrome P450-dependent enzyme. We now show that T16H is induced in the suspension culture by light and we report the cloning of the cDNA. The enzyme was expressed in Escherichia coli as translational fusion with the P450 reductase from C. roseus, and the reaction product was identified by mass spectrometry. The protein (CYP71D12) shares 47-52% identity with other members of the CYP71D subfamily with unknown function. The induction by light was strongly enhanced by a nutritional downshift (transfer into 8% aqueous sucrose). We discuss the possibility that the entire pathway to bisindoles can be expressed in suspension cultures.

Regulation and functional expression of cinnamate 4-hydroxylase from parsley

A previously isolated parsley (Petroselinum crispum) cDNA with high sequence similarity to cinnamate 4-hydroxylase (C4H) cDNAs from several plant sources was expressed in yeast (Saccharomyces cerevisiae) containing a plant NADPH:cytochrome P450 oxidoreductase and verified as encoding a functional C4H (CYP73A10). Low genomic complexity and the occurrence of a single type of cDNA suggest the existence of only one C4H gene in parsley. The encoded mRNA and protein, in contrast to those of a functionally related NADPH:cytochrome P450 oxidoreductase, were strictly coregulated with phenylalanine ammonia-lyase mRNA and protein, respectively, as demonstrated by coinduction under various conditions and colocalization in situ in cross-sections from several different parsley tissues. These results support the hypothesis that the genes encoding the core reactions of phenylpropanoid metabolism form a tight regulatory unit.

MOLECULAR-GENETIC ANALYSIS OF PLANT CYTOCHROME P450-DEPENDENT MONOOXYGENASES

Cytochrome P450-dependent monooxygenases are a large group of heme-containing enzymes, most of which catalyze NADPH- and O2-dependent hydroxylation reactions. The cloning of plant P450s has been hampered because these membrane-localized proteins are typically present in low abundance and are often unstable to purification. Since the cloning of the first plant P450 gene in 1990, there has been an explosion in the rate at which genes encoding plant P450s have been identified. These successes have largely been the result of advances in purification techniques, as well as the application of alternative methods such as mutant- and PCR-based cloning strategies. The availability of these cloned genes has made possible the analysis of P450 gene regulation and may soon reveal aspects of the evolution of P450s in plants. This new knowledge will significantly improve our understanding of many metabolic pathways and may permit their manipulation in the near future.

Cloning, expression in yeast, and functional characterization of CYP81B1, a plant cytochrome P450 that catalyzes in-chain hydroxylation of fatty acids

Several omega and in-chain fatty acid hydroxylases have been characterized in higher plants. In microsomes from Helianthus tuberosus tuber the omega-2, omega-3, and omega-4 hydroxylation of lauric acid is catalyzed by one or a few closely related aminopyrine- and MnCl2-inducible cytochrome P450(s). To isolate the cDNA and determine the sequences of the(se) enzyme(s), we used antibodies directed against a P450-enriched fraction purified from Mn2+-induced tissues. Screening of a cDNA expression library from aminopyrine-treated tubers led to the identification of a cDNA (CYP81B1) corresponding to a transcript induced by aminopyrine. CYP81B1 was expressed in yeast. A systematic exploration of its function revealed that it specifically catalyzes the hydroxylation of medium chain saturated fatty acids, capric (C10:0), lauric (C12:0), and myristic (C14:0) acids. The same metabolites were obtained with transgenic yeast and plant microsomes, a mixture of omega-1 to omega-5 monohydroxylated products. The three fatty acids were metabolized with high and similar efficiencies, the major position of attack depending on chain length. When lauric acid was the substrate, turnover was 30.7 +/- 1.4 min-1 and Km(app) 788 +/- 400 nM. No metabolism of long chain fatty acids, aromatic molecules, or herbicides was detected. This new fatty acid hydroxylase is typical from higher plants and differs from those already isolated from other living organisms.

Differentially regulated NADPH:cytochrome P450 oxidoreductases in parsley

Two NADPH:cytochrome P450 oxidoreductases (CPRs) from parsley (Petroselinum crispum) were cloned, and the complete proteins were expressed and functionally identified in yeast. The two enzymes, designated CPR1 and CPR2, are 80% identical in amino acid sequence with one another and about 75% identical with CPRs from several other plant species. The mRNA accumulation patterns for CPR1 and CPR2 in fungal elicitor-treated or UV-irradiated cultured parsley cells and in developing or infected parsley plants were compared with those for cinnamate 4-hydroxylase (C4H), one of the most abundant CPR-dependent P450 enzymes in plants. All treatments strongly induced the mRNAs for C4H and CPR1 but not for CPR2, suggesting distinct metabolic roles of CPR1 and CPR2 and a functional relationship between CPR1 and C4H.

Isolation and sequence analysis of a cDNA and a related gene for cytochrome P450 proteins from Solanum chacoense

Inosine-containing degenerate PCR primers corresponding to the heme-binding domain of cytochrome P450 proteins have been synthesized and used for cloning cDNAs by the RT-PCR technique from Solanum chacoense. One clone in which the primer was immediately followed by sequences corresponding to the remaining part of the conserved domain was obtained. A leaf cDNA and a genomic library were constructed from S. chacoense. Clones homologous to the PCR fragment were isolated by plaque hybridization from both libraries (CYPs.ch-1 and CYPs.ch-2, respectively). Based on DNA sequence analysis, the selected clones are 87.6% identical and belong to the CYP71 family. The CYPs.ch genes are present in multiple copies in the S. chacoense as well as in the S. tuberosum genome with some polymorphisms. The CYPs.ch transcripts are slightly induced by methyl jasmonate and abscisic acid in S. chacoense foliage.

Spatially distinct expression of two new cytochrome P450s in leaves of Nepeta racemosa: identification of a trichome-specific isoform

Using a PCR-based approach, two novel cytochrome P450 cDNAs were isolated from a catmint (Nepeta racemosa) leaf cDNA library. The cDNAs (pBSK3C7 and pBSK4C3) were 76.9% identical in their nucleotide sequences, indicating that they are the products of two closely-related genes. A comparison of the sequence of these cDNAs with database sequences indicated that they represent new members of the CYP71 gene family of plant cytochrome P450s. Clone pBSK3C7 contains the full-length coding sequence of a cytochrome P450, whilst pBSK4C3 lacks ca. 6 codons at the 5' end. The cytochromes P450 encoded by these clones were designated CYP71A5 and CYP71A6 (pBSK3C7 and pBSK4C3, respectively). Southern blot analysis indicated that the corresponding genes were present as single copies in the genome of N. racemosa. Northern blot analysis showed that a gene homologous with CYP71A5 was expressed in the related species N. cataria, but no homologue of CYP71A6 was detected in this species. Expression of CYP71A5 in N. racemosa was maximal in flowers, tissues within the apical bud, and young expanded leaves. That of CYP71A6 was maximal in older leaves. Expression of CYP71A5 occurred exclusively in trichomes present on the leaf surfaces, in contrast to that of CYP71A6, which occurred predominantly within the leaf blade tissues.

Xenobiotics: Substrates and inhibitors of the plant cytochrome P450

The ability of a plant cytochrome P450 to bind and metabolise plant endogenous molecules and xenobiotics was investigated. The work was performed on the yeast-expressed CYP73A1, a cinnamate 4-hydroxylase isolated from Helianthus tuberosus. CYP73 controls the general phenylpropanoid pathway and is likely to be one of the most abundant sources of P450 in the biosphere. The enzyme shows a high selectivity toward plant secondary metabolites. Nevertheless, it oxygenates several small and planar xenobiotics with low efficiency, including an herbicide (chlorotoluron). One xenobiotic molecule, 2-naphthoic acid, is hydroxylated with an efficiency comparable to that of the physiological substrate. This reaction was used to devise a fluorimetric test for the rapid measurement of enzyme activity. A series of herbicidal molecules (hydroxybenzonitriles) are shown to bind the active site without being metabolised. These molecules behave as strong competitive inhibitors of CYP73 with a K(i) in the same micromolar range as the K(m) for the physiological substrate. It is proposed that their inhibition of the phenylpropanoid pathway reinforces their other phytotoxic effects at the level of the chloroplasts. All our results indicate a strong reciprocal interaction between plant P450s and xenobiotics.

Three differentially expressed S-adenosylmethionine synthetases from Catharanthus roseus: molecular and functional characterization

We describe the molecular and functional characterization of three closely related S-adenosyl-L-methionine synthetase (SAMS) isoenzymes from Catharanthus roseus (Madagascar periwinkle). The genes are differentially expressed in cell cultures during growth of the culture and after application of various stresses (elicitor, nutritional down-shift, increased NaCl). Seedlings revealed organ-specific expression and differential gene regulation after salt stress. A relationship analysis indicated that plant SAMS group in two main clusters distinguished by characteristic amino acid exchanges at specific positions, and this suggested differences in the enzyme properties or the regulation. SAMS1 and SAMS2 are of type I and SAMS3 is of type II. The properties of the isoenzymes were compared after heterologous expression of the individual enzymes, but no significant differences were detected in a) optima for temperature (37 to 45 degrees C) or pH (7 to 8.3); b) dependence on cations (divalent: Mg2+, Mn2+, Co2+; monovalent: K+, NH4+, Na+); c) K(m)s for ATP and L-methionine; d) inhibition by reaction products (S-adenosyl-L-methionine, PPi, Pi), by the reaction intermediate tripolyphosphate, and by the substrate analogues ethionine and cycloleucine; e) response to metabolites from the methyl cycle (L-homocysteine) or from related pathways (L-ornithine, putrescine, spermidine, spermine); f) native protein size (gel permeation chromatography). The results represent the first characterization of plant SAMS isoenzyme properties with individually expressed proteins. The possibility is discussed that the isoenzyme differences reflect specificities in the association with enzymes that use S-adenosyl-L-methionine.

Xanthophyll biosynthesis. Cloning, expression, functional reconstitution, and regulation of beta-cyclohexenyl carotenoid epoxidase from pepper (Capsicum annuum)

Pepper (Capsicum annuum) beta-cyclohexenyl xanthophyll epoxidase cDNA was cloned and the corresponding enzyme overexpressed and purified from Escherichia coli, for investigation of its catalytic activity. The recombinant protein did not directly accept NADPH for epoxidation of cyclohexenyl carotenoids, nor did it operate according to a peroxygenase-based mechanism. Instead, the reducing power of NADPH was transferred to the epoxidase via reduced ferredoxin as shown by reconstitution of epoxidase activity in the presence of NADPH, ferredoxin oxidoreductase, and ferredoxin. Bacterial rubredoxin could be substituted for ferredoxin. The pepper epoxidase acted specifically on the beta-ring of xanthophylls such as beta-cryptoxanthin, zeaxanthin, and antheraxanthin. The proposed reaction mechanism for epoxidation involves the formation of a transient carbocation. This characteristic allows selective inhibition of the epoxidase activity by different nucleophilic diethylamine derivatives, p-dimethylaminobenzenediazonium fluoroborate and N,N-dimethyl-2-phenylaziridinium. It was also shown that the epoxidase gene was up-regulated during oxidative stress and when chloroplasts undergo differentiation into chromoplasts in pepper fruit.

Novel type of receptor-like protein kinase from a higher plant (Catharanthus roseus). cDNA, gene, intramolecular autophosphorylation, and identification of a threonine important for auto- and substrate phosphorylation

We characterize CrRLK1, a novel type of receptor-like kinase (RLK), from the plant Catharanthus roseus (Madagascar periwinkle). The protein (90.2 kDa) deduced from the complete genomic and cDNA sequences is a RLK by predicting a N-terminal signal peptide, a large extracytoplasmic domain, a membrane-spanning hydrophobic region followed by a transfer-stop signal, and a C-terminal cytoplasmic protein kinase with all 11 conserved subdomains. It is a novel RLK type because the predicted extracytoplasmic region shares no similarity with other RLKs. The autophosphorylation was investigated with affinity-purified proteins expressed in Escherichia coli. The activity was higher with Mn2+ than with Mg2+ and achieved half-maximal rates at 2-2.5 microM ATP. The phosphorylation was predominantly on Thr, less on Ser, and not on Tyr. In contrast to other plant RLK, the kinase used an intra- rather than an intermolecular phosphorylation mechanism. After protein cleavage with formic acid, most of the radioactivity was in a 14.1-kDa peptide located at the end of the kinase domain. Mutagenesis of the four Thr residues in this peptide identified Thr-720 in the subdomain XI as important for autophosphorylation and for phosphorylation of beta-casein. This Thr is conserved in other related kinases, suggesting a subfamily sharing common autophosphorylation mechanisms.

Cloning and expression in Escherichia coli and Saccharomyces cerevisiae of a novel tobacco cytochrome P-450-like cDNA

A cDNA library constructed from poly(A)+ RNA of tobacco BY2 cells treated with 2,4-dichlorophenoxyacetic acid was screened by using a synthetic oligonucleotide corresponding to the heme binding region of avocado CYP71A1. A cloned 2-kb cDNA designated as cTBP contained an open reading frame of 1593 bp encoding a protein of molecular size of 58916. The deduced amino acid sequence included a cysteine residue corresponding to fifth ligand of heme-Fe at 497th. The coding sequence was expressed under the control of tac promoter and rrnB terminator in Escherichia coli to yield 7 to 10 nmol P450 equivalent per litre of the culture in the presence of delta-aminolevulinic acid. The modified coding sequences in which NH2-terminal residues 2-25 were replaced by the NH2-terminal 18 amino acid residues of microsomal bovine CYP17 were also expressed under the control of ADH promoter and terminator in Saccharomyces cerevisiae to yield 29 and 30 pmol of P450 equivalent/mg protein in the microsomal fraction, respectively. On co-expression of each of the modified coding sequences and yeast NADPH-cytochrome P-450 oxidoreductase gene, the yeast microsomes exhibited 7-ethoxycoumarin O-deethylase activity. Based on these results, tobacco cTBP was found to encode a novel P450-like species with a monooxygenese activity related to xenobiotic metabolism.

Characterization of a cDNA encoding a proline-rich 14 kDa protein in developing cortical cells of the roots of bean (Phaseolus vulgaris) seedlings

A cDNA clone, corresponding to mRNAs preferentially expressed in the roots of bean (Phaseolus vulgaris L.) seedlings, was isolated. This clone contains a 381 bp open reading frame encoding a polypeptide of 13.5 kDa, designated PVR5 (Phaseolus vulgaris root 5). The amino acid sequence of this clone is rich in proline (13.5%) and leucine (12.7%) and shares significant amino acid sequence homology with root-specific and proline-rich proteins from monocots (maize and rice), and proline-rich proteins from dicots (carrot, oilseed rape, and Madagascar periwinkle). The precise biological roles of these polypeptides are unknown. PVR5 mRNA accumulation is developmentally regulated within the root, with high levels at the root apex and declining levels at distances further from the root tip. In situ hybridization shows that PVR5 mRNA specifically accumulates in the cortical ground meristem in which maximal cell division occurs. Southern blot analysis suggests that genomic DNA corresponding to PVR5 cDNA is encoded by a single gene or a small gene family.

Differential induction of cytochrome P450-mediated triasulfuron metabolism by naphthalic anhydride and triasulfuron

Cytochrome P450 monooxygenases play paramount roles in the detoxification of herbicides as well as in the synthesis of lignins, flavonoids, and phenolic acids. Biochemical analysis of triasulfuron metabolism in maize (Zea mays) seedlings has demonstrated that the P450(s) responsible for detoxification of this herbicide is induced by naphthalic anhydride (NA), a plant safener, and by triasulfuron, the herbicide itself. Induction studies conducted with seedlings of different ages suggest that two separate response pathways modulate this P-450 activity. Induction by NA is independent of the developmental age of the seedlings up to 6.5 d; induction by triasulfuron is tightly modulated with respect to developmental age in that triasulfuron metabolism can be induced by triasulfuron in young (2.5 d) but not older (6.5 d) seedlings. Induction by NA administered in combination with triasulfuron synergistically enhances triasulfuron metabolism in younger seedlings to levels substantially above that obtained with either herbicide or safener treatment alone. In older seedlings, NA plus triasulfuron treatment induces triasulfuron metabolism to only the level of NA treatment alone, indicating again that the induction cascade responding to triasulfuron is nonfunctional in later development. MnCl2 studies indicate that the triasulfuron insensitivity of older seedlings does not result from a general limitation in the inducibility of this P-450 detoxification system but rather from specific limitations in the triasulfuron-response pathway.

Cinnamate 4-hydroxylase from Catharanthus roseus, and a strategy for the functional expression of plant cytochrome P450 proteins as translational fusions with P450 reductase in Escherichia coli

A PCR-based approach was used to isolate cDNAs for cinnamate 4-hydroxylase (C4H) from Catharanthus roseus cell cultures. The protein shared 75.9% identity with C4H from other plants, and the transcription was induced under various stress conditions. The cloned protein was used to investigate the functional expression of plant P450/P450-reductase fusions in E. coli. Fusions containing a modified N-terminal membrane anchor were located in the membrane and possessed C4H activity without solubilization or addition of other factors. The results indicate that the fusion protein strategy provides a useful tool to analyze the activities encoded in the rapidly increasing number of plant P450 sequences of uncertain or unknown function. We also discuss critical elements of the strategy: the choice of the E. coli host strain, the N-terminal membrane anchor, and the conditions for protein expression.

A cDNA encoding a plant homologue to animal HMG box proteins involved in structure-specific recognition of DNA (SSRP family)

We report a cDNA encoding a 71-kDa protein with a single high-mobility group (HMG) box and two nuclear localization signals from the higher plant Catharanthus roseus (Madagascar periwinkle). The protein had 40% amino acid identity with animal DNA-binding proteins of the SSRP (structure-specific recognition protein) family that recognize bent, unwound DNA structures. Genomic Southern analysis suggested the presence of two genes.

Vitamin-B12-independent methionine synthase from a higher plant (Catharanthus roseus). Molecular characterization, regulation, heterologous expression, and enzyme properties

Methionine synthases catalyze the formation of methionine by the transfer of a methyl group from 5-methyltetrahydrofolate to homocysteine. This reaction is the last step in L-methionine biosynthesis, and it also serves to regenerate the methyl group of S-adenosylmethionine, a cofactor required for biological methylation reactions. We describe the cloning, expression and characterization of a methionine synthase from the higher plant Catharanthus roseus. cDNAs were identified that encoded a protein of 85 kDa sharing 50% identify with the cobalamin-independent methionine synthase from Escherichia coli (MetE) and 41% identity with a partial sequence of a yeast homolog of MetE. The C. roseus protein was expressed at high levels in E. coli. The enzyme accepts the triglutamate form of methyltetrahydrofolate as a methyl donor but not the monoglutamate form, and it does not require S-adenosylmethionine or cobalamin for activity. The properties indicate that the enzyme is a cobalamin-independent methionine synthase (EC 2.1.1.14). In contrast to the E. coli MetE, the plant protein does not require phosphate or magnesium ions for activity. Immunoblots of plants extracts showed that the protein was localized in the cytosol, and was present in a variety of plant species. A nutritional downshift of the C. roseus cell culture revealed a strong, transient transcriptional activation, but no significant increment in the total level of the protein. The availability of the protein and the cDNA now provide tools to investigate the complexities of methionine biosynthesis in plants.

Molecular and enzymatic characterization of two stilbene synthases from Eastern white pine (Pinus strobus). A single Arg/His difference determines the activity and the pH dependence of the enzymes

Pinus strobus (Eastern white pine) contains stilbenes biosynthetically derived from cinnamoyl-CoA (pinosylvin) or dihydrocinnamoyl-CoA (dihydropinosylvin). We screened a P. strobus cDNA library with a stilbene synthase (STS) probe from Pinus sylvestris. The eight isolated cDNAs represented two closely related STS genes with five amino acid differences in the proteins. The enzyme properties were investigated after heterologous expression in Escherichia coli. Both proteins preferred cinnamoyl-CoA against dihydrocinnamoyl-CoA and thus represented pinosylvin synthases. Otherwise they revealed large differences. STS1 had only 3-5% of the activity of STS2, its pH optimum was shifted to lower values (pH 6), and it synthesized with cinnamoyl-CoA a second unknown product. Site-directed mutagenesis demonstrated that a single Arg-to-His exchange in STS1 was responsible for all of the differences. The proton acceptor properties of His are discussed as the reason for the properties of STS1.

Molecular cloning and heterologous expression of a cDNA encoding berbamunine synthase, a C--O phenol-coupling cytochrome P450 from the higher plant Berberis stolonifera

A cDNA encoding a cytochrome P450-dependent oxidase, berbamunine synthase (EC 1.1.3.34; CYP80), from cell suspension cultures of the higher plant Berberis stolonifera Koehne and Wolf (barberry) has been isolated and heterologously expressed in functional form in insect cell culture using a baculovirus-based expression system. This cytochrome P450-dependent enzyme is unusual in that it catalyzes the regio- and stereoselective formation of a C--O phenol couple in bisbenzylisoquinoline alkaloid biosynthesis without concomitant incorporation of activated oxygen into the product. Consistent with the function of an oxidase rather than a monooxygenase, an essential glycine residue in the distal helix, which forms the oxygen-binding pocket in the well-studied bacterial enzyme P-450cam, is replaced by proline at the equivalent position in berbamunine synthase. This oxidase was accumulated in an active form in insect cell microsomes and accepted electrons from the endogenous NADPH-cytochrome P450 reductase. The heterologously expressed enzyme oxidatively couples either two molecules of (R)-N-methylcoclaurine to form the (R,R) dimer guattegaumerine or one molecule each of (R)- and (S)-N-methylcoclaurine to form the (R,S) dimer berbamunine. The ratio of the two bisbenzylisoquinolines formed could be altered by reductase source or by varying the enantiomer composition of the substrates.

Expression of a cytochrome P450 gene family in maize

Maize seedlings, like seedlings of many other plants, are rich in cytochrome P450 (P450) enzyme activity. Four P450 genes (CYPzm1-4), isolated from a seedling-specific cDNA library, are characterized by a transient and seedling-specific expression pattern. The maximum steady state mRNA levels are reached at 3 days in root and at 7 days in shoot tissue, respectively. All four genes belong to one gene family and are closely related to the CYP71 family of plant P450 genes, which includes the enzymes of the ripening avocado fruit (CYP71A1) and eggplant hypocotyls (CYP71A2, A3, A4). The expression of these related P450 genes in monocot and dicot plants indicates that these enzymes play a significant role in plants; however, the in vivo enzyme functions are unknown. The divergence of the four members of the maize gene family is sufficiently high to account for different substrate and/or reaction specificity. Although the general expression pattern of the four genes is identical, the maximum steady-state mRNA levels vary in different maize lines. In situ hybridisation reveals the highest mRNA levels in the coleoptile, the first developed leaflets, the ground tissue of the nodular complex, and in the cortex and pith of the region of cell division in the root. The mapping of the maize CYPzm genes shows that, as in animals, P450 genes of the same family can be clustered. The presence of the CYPzm gene cluster in maize argues for generation of distinct plant P450 gene families by gene duplication.