Cloning, yeast expression, and characterization of the coupling of two distantly related Arabidopsis thaliana NADPH-cytochrome P450 reductases with P450 CYP73A5

Taxoid metabolism: Taxoid 14beta-hydroxylase is a cytochrome P450-dependent monooxygenase

The production of the anticancer drug Taxol in Taxus (yew) cell cultures is often accompanied by the formation of side-route polyoxygenated taxoid metabolites bearing a 14beta-hydroxyl group. The recent acquisition of several new semisynthetic taxoid intermediates enabled the screening of a family of Taxus cytochrome P450 cDNA clones for the 14beta-hydroxylase and additional taxoid oxygenases. The candidate cytochrome P450 clones were functionally expressed in yeast and tested by in vivo feeding of radiolabeled 5alpha-acetoxy-10beta-hydroxy taxadiene and 5alpha,13alpha-dihydroxy taxadiene. One clone efficiently and specifically transformed the 5alpha-acetoxy-10beta-ol, but not the 5alpha,13alpha-diol, to a more polar product with the chromatographic properties of a taxoid triol monoacetate, and the identity of this product was confirmed by spectroscopic means as 5alpha-acetoxy-10beta,14beta-dihydroxy taxadiene. Microsome preparation from the transformed yeast allowed characterization of this new hydroxylase, which was shown to resemble other cytochrome P450 taxoid hydroxylases with pH optimum at 7.5 and a K(m) value for the taxoid substrate of about 50 microM. Because Taxol is unsubstituted at C14, the 14beta-hydroxylase cannot reside on the pathway to the target drug but rather appears to be responsible for diversion of the pathway to 14-hydroxy taxoids that are prominent metabolites of Taxus cell cultures. Manipulation of this hydroxylase gene could permit redirection of the pathway to increase flux toward Taxol and could allow the preparation of 13alpha,14beta-hydroxy taxoids as new therapeutic agents.

Molecular cloning, substrate specificity of the functionally expressed dihydroflavonol 4-reductases from Malus domestica and Pyrus communis cultivars and the consequences for flavonoid metabolism

Treatment with the dioxygenase inhibitor prohexadione-Ca leads to major changes in the flavonoid metabolism of apple (Malus domestica) and pear (Pyrus communis) leaves. Accumulation of unusual 3-deoxyflavonoids is observed, which have been linked to an enhanced resistance toward fire blight. The committed step in this pathway is the reduction of flavanones. Crude extracts from leaves are able to perform this reaction. There was previous evidence that DFR enzymes of certain plants possess additional flavanone 4-reductase (FNR) activity. Such an FNR activity of DFR enzymes is proved here by heterologous expression of the enzymes. The heterologously expressed DFR/FNR enzymes of Malus and Pyrus possess distinct differences in substrate specificities despite only minor differences of the amino acid sequences. Kinetic studies showed that dihydroflavonols generally are the preferred substrates. However, with the observed substrate specificities the occurrence of 3-deoxyflavonoids in vivo after application of prohexadione-Ca can be explained.

CYP79F1 and CYP79F2 have distinct functions in the biosynthesis of aliphatic glucosinolates in Arabidopsis

Cytochromes P450 of the CYP79 family catalyze the conversion of amino acids to oximes in the biosynthesis of glucosinolates, a group of natural plant products known to be involved in plant defense and as a source of flavor compounds, cancer-preventing agents and bioherbicides. We report a detailed biochemical analysis of the substrate specificity and kinetics of CYP79F1 and CYP79F2, two cytochromes P450 involved in the biosynthesis of aliphatic glucosinolates in Arabidopsis thaliana. Using recombinant CYP79F1 and CYP79F2 expressed in Escherichia coli and Saccharomyces cerevisiae, respectively, we show that CYP79F1 metabolizes mono- to hexahomomethionine, resulting in both short- and long-chain aliphatic glucosinolates. In contrast, CYP79F2 exclusively metabolizes long-chain elongated penta- and hexahomomethionines. CYP79F1 and CYP79F2 are spatially and developmentally regulated, with different gene expression patterns. CYP79F2 is highly expressed in hypocotyl and roots, whereas CYP79F1 is strongly expressed in cotyledons, rosette leaves, stems, and siliques. A transposon-tagged CYP79F1 knockout mutant completely lacks short-chain aliphatic glucosinolates, but has an increased level of long-chain aliphatic glucosinolates, especially in leaves and seeds. The level of long-chain aliphatic glucosinolates in a transposon-tagged CYP79F2 knockout mutant is substantially reduced, whereas the level of short-chain aliphatic glucosinolates is not affected. Biochemical characterization of CYP79F1 and CYP79F2, and gene expression analysis, combined with glucosinolate profiling of knockout mutants demonstrate the functional role of these enzymes. This provides valuable insights into the metabolic network leading to the biosynthesis of aliphatic glucosinolates, and into metabolic engineering of altered aliphatic glucosinolate profiles to improve nutritional value and pest resistance.

Probing sesquiterpene hydroxylase activities in a coupled assay with terpene synthases

5-epi-Aristolochene dihydroxylase (EAH) catalyzes unique stereo- and regiospecific hydroxylations of a bicyclic sesquiterpene hydrocarbon to generate capsidiol. To define functional and mechanistic features of the EAH enzyme, the utility of a coupled assay using readily available sesquiterpene synthases and microsomes from yeast overexpressing the EAH enzyme was determined. Capsidiol and deoxycapsidiol biosyntheses were readily measured in coupled assays consisting of 5-epi-aristolochene synthase and EAH as determined by the incorporation of radiolabeled farnesyl diphosphate into thin-layer chromatography-isolated products and verified by gas chromatography-mass spectrometry analysis. The assays were dependent on the amounts of synthase and hydroxylase protein added, the incubation times, and the presence of nicotinamide adenine dinucleotide phosphate. The utility of this coupled assay was extended by examining the relative efficiency of the EAH enzyme to catalyze hydroxylations of different sesquiterpene skeletons generated by other terpene synthases.

The pea gene NA encodes ent-kaurenoic acid oxidase

The gibberellin (GA)-deficient dwarf na mutant in pea (Pisum sativum) has severely reduced internode elongation, reduced root growth, and decreased leaflet size. However, the seeds develop normally. Two genes, PsKAO1 and PsKAO2, encoding cytochrome P450 monooxygenases of the subfamily CYP88A were isolated. Both PsKAO1 and PsKAO2 had ent-kaurenoic acid oxidase (KAO) activity, catalyzing the three steps of the GA biosynthetic pathway from ent-kaurenoic acid to GA(12) when expressed in yeast (Saccharomyces cerevisiae). In addition to the intermediates ent-7alpha-hydroxykaurenoic acid and GA(12)-aldehyde, some additional products of the pea KAO activity were detected, including ent-6alpha,7alpha-dihydroxykaurenoic acid and 7beta-hydroxykaurenolide. The NA gene encodes PsKAO1, because in two independent mutant alleles, na-1 and na-2, PsKAO1 had altered sequences and the five-base deletion in PsKAO1 associated with the na-1 allele cosegregated with the dwarf na phenotype. PsKAO1 was expressed in the stem, apical bud, leaf, pod, and root, organs in which GA levels have previously been shown to be reduced in na plants. PsKAO2 was expressed only in seeds and this may explain the normal seed development and normal GA biosynthesis in seeds of na plants.

CYP79B1 from Sinapis alba converts tryptophan to indole-3-acetaldoxime

The cytochrome P450 CYP79B1 from Sinapis alba has been heterologously expressed in Escherichia coli and shown to catalyze the conversion of tryptophan to indole-3-acetaldoxime. Three expression constructs were made, one expressing the native protein and two expressing proteins with different N-terminal modifications. The native construct gave the highest yield as estimated by enzymatic activity per liter of culture. Spheroplasts of E. coli expressing CYP79B1 were reconstituted with the Arabidopsis thaliana NADPH:cytochrome P450 reductase ATR1 heterologously expressed in E. coli to obtain enzymatic activity. This indicates that the E. coli electron-donating system, flavodoxin/flavodoxin reductase, does not support CYP79B1 activity. Recombinant CYP79B1 has a K(m) for tryptophan of 29+/-2 microM and a V(max) of 36.5+/-0.7nmolh(-1)(mlculture)(-1). The identity at the amino acid level of CYP79B1 is, respectively, 93 and 84% to CYP79B2 and CYP79B3 from A. thaliana, and 96% to CYP79B5 (Accession No. AF453287) from Brassica napus. The CYP79B subfamily of cytochromes P450 is likely to constitute a group of orthologous genes in the biosynthesis of indole glucosinolates.

Functional genomics of P450s

Plant systems utilize a diverse array of cytochrome P450 monooxygenases (P450s) in their biosynthetic and detoxicative pathways. Those P450s in biosynthetic pathways play critical roles in the synthesis of lignins, UV protectants, pigments, defense compounds, fatty acids, hormones, and signaling molecules. Those in catabolic pathways participate in the breakdown of endogenous compounds and toxic compounds encountered in the environment. Because of their roles in this wide diversity of metabolic processes, plant P450 proteins and transcripts can serve as downstream reporters for many different biochemical pathways responding to chemical, developmental, and environmental cues. This review focuses initially on defining P450 biochemistries, nomenclature systems, and the relationships between genes in the extended P450 superfamily that exists in all plant species. Subsequently, it focuses on outlining the many approaches being used to assign function to individual P450 proteins and gene loci. The examples of assigned P450 activities that are spread throughout this review highlight the importance of understanding and utilizing P450 sequences as markers for linking biochemical pathway responses to physiological processes.

Organ-specific expression of brassinosteroid-biosynthetic genes and distribution of endogenous brassinosteroids in Arabidopsis

Brassinosteroids (BRs) are steroidal plant hormones that are essential for growth and development. There is only limited information on where BRs are synthesized and used. We studied the organ specificity of BR biosynthesis in Arabidopsis, using two different approaches: We analyzed the expression of BR-related genes using real-time quantitative reverse transcriptase-polymerase chain reaction, and analyzed endogenous BRs using gas chromatography-mass spectrometry. Before starting this study, we cloned the second BR-6-oxidase (BR6ox2) gene from Arabidopsis and found that the encoded enzyme has the same substrate specificity as the enzyme encoded by the previously isolated 6-oxidase gene (BR6ox1) of Arabidopsis. Endogenous BRs and the expression of BR-related genes were detected in all organs tested. The highest level of endogenous BRs and the highest expression of the BR6ox1, BR6ox2, and DWF4 genes were observed in apical shoots, which contain actively developing tissues. These genes are important in BR biosynthesis because they encode the rate-limiting or farthest downstream enzyme in the BR biosynthesis pathway. The second highest level of endogenous BRs and expression of BR6ox1 and DWF4 were observed in siliques, which contains actively developing embryos and seeds. These findings indicate that BRs are synthesized in all organs tested, but are most actively synthesized in young, actively developing organs. In contrast, synthesis was limited in mature organs. Our observations are consistent with the idea that BRs function as the growth-promoting hormone in plants.

Cloning, functional expression, and subcellular localization of multiple NADPH-cytochrome P450 reductases from hybrid poplar

NADPH:cytochrome P450 reductase (CPR) provides reducing equivalents to diverse cytochrome P450 monooxygenases. We isolated cDNAs for three CPR genes (CPR1, CPR2, and CPR3) from hybrid poplar (Populus trichocarpa x Populus deltoides). Deduced CPR2 and CPR3 amino acid sequences were 91% identical, but encoded isoforms divergent from CPR1 (72% identity). CPR1 and CPR2 were co-expressed together with the P450 enzyme cinnamate-4-hydroxylase (C4H) in yeast (Saccharomyces cerevisiae). Microsomes isolated from strains expressing CPR1/C4H or CPR2/C4H enhanced C4H activities approximately 10-fold relative to the C4H-only control strain, and catalyzed NADPH-dependent cytochrome c reduction. The divergent CPR isoforms (CPR1 and CPR2/3) contained entirely different N-terminal sequences, which are conserved in other plant CPRs and are diagnostic for two distinct classes of CPRs within the angiosperms. C-terminal green fluorescent protein fusions to CPR1 and CPR2 were constructed and expressed in both yeast and Arabidopsis. The fusion proteins expressed in yeast retained the ability to support C4H activity and, thus, were catalytically active. Both CPR::green fluorescent protein fusion proteins were strictly localized to the endoplasmic reticulum in transgenic Arabidopsis. The lack of localization of either isoform to chloroplasts, where P450s are known to be present, suggests that an alternative P450 reduction system may be operative in this organelle. Transcripts for the three poplar CPR genes were present ubiquitously in all tissues examined, but CPR2 showed highest expression in young leaf tissue.

Heterologous expression of dihydroflavonol 4-reductases from various plants

Dihydroflavonol 4-reductases (DFR) catalyze the stereospecific reduction of dihydroflavonols to the respective flavan 3,4-diols (leucoanthocyanidins) and might also be involved in the reduction of flavanones to flavan-4-ols, which are important intermediates in the 3-deoxyflavonoid pathway. Several cDNA clones encoding DFR have been isolated from different plant species. Despite the important function of these enzymes in the flavonoid pathway, attempts at heterologous expression of cDNA clones in Escherichia coli have failed so far. Here, three well known heterologous expression systems for plant-derived genes were tested to obtain the functional protein of DFR from Gerbera hybrids. Successful synthesis of an active DFR enzyme was achieved in eukaryotic cells, using either baker's yeast (Saccharomyces cerevisiae) or tobacco protoplasts (Nicotiana tabacum), transformed with expression vectors containing the open reading frame of Gerbera DFR. These expression systems provide useful and powerful tools for rapid biochemical characterization, in particular the substrate specificity, of the increasing number of cloned DFR sequences. Furthermore, this tool allows the stereospecific synthesis of (14)C-labeled leucoanthocyanidins in high quality and quantity, which is a prerequisite for detailed biochemical investigation of the less understood enzymatic reactions located downstream of DFR in anthocyanin, catechin and proanthocyanidin biosynthesis.

Loss-of-function of a rice brassinosteroid biosynthetic enzyme, C-6 oxidase, prevents the organized arrangement and polar elongation of cells in the leaves and stem

Molecular genetic and physiological studies on brassinosteroid (BR)-related mutants of dicot plants have revealed that BRs play important roles in normal plant growth and development. However, little is known about the function of BR in monocots (grasses), except for the phenotypic analysis of a rice mutant partially insensitive to BR signaling. To investigate the function of BR in monocots, we identified and characterized BR-deficient mutants of rice, BR-deficient dwarf1 (brd1). The brd1 mutants showed a range of abnormalities in organ development and growth, the most striking of which were defects in the elongation of the stem and leaves. Light microscopic observations revealed that this abnormality was primarily owing to a failure in the organization and polar elongation of the leaf and stem cells. The accumulation profile of BR compounds in the brd1 mutants suggested that these plants may be deficient in the activity of BR C-6 oxidase. Therefore, we cloned a rice gene, OsDWARF, which has a high sequence similarity to the tomato C-6 oxidase gene, DWARF. Introduction of the wild-type OsDWARF gene into brd1 rescued the abnormal phenotype of the mutants. The OsDWARF gene was expressed at a low level in all of the examined tissues, with preferential expression in the leaf sheath, and the expression was negatively regulated by brassinolide treatment. On the basis of these findings, we discuss the biological function of BRs in rice plants.

Arabidopsis CYP98A3 mediating aromatic 3-hydroxylation. Developmental regulation of the gene, and expression in yeast

The general phenylpropanoid pathways generate a wide array of aromatic secondary metabolites that range from monolignols, which are ubiquitous in all plants, to sinapine, which is confined to crucifer seeds. The biosynthesis of these compounds involves hydroxylated and methoxylated cinnamyl acid, aldehyde, or alcohol intermediates. Of the three enzymes originally proposed to hydroxylate the 4-, 3-, and 5-positions of the aromatic ring, cinnamate 4-hydroxylase (C4H), which converts trans-cinnamic acid to p-coumaric acid, is the best characterized and is also the archetypal plant P450 monooxygenase. Ferulic acid 5-hydroxylase (F5H), a P450 that catalyzes 5-hydroxylation, has also been studied, but the presumptive 3-hydroxylase converting p-coumarate to caffeate has been elusive. We have found that Arabidopsis CYP98A3, also a P450, could hydroxylate p-coumaric acid to caffeic acid in vivo when expressed in yeast (Saccharomyces cerevisiae) cells, albeit very slowly. CYP98A3 transcript was found in Arabidopsis stem and silique, resembling both C4H and F5H in this respect. CYP98A3 showed further resemblance to C4H in being highly active in root, but differed from F5H in this regard. In transgenic Arabidopsis, the promoters of CYP98A3 and C4H showed wound inducibility and a comparable developmental regulation throughout the life cycle, except in seeds, where the CYP98A3 promoter construct was inactive while remaining active in silique walls. Within stem and root tissue, the gene product and the promoter activity of CYP98A3 were most abundant in lignifying cells. Collectively, these studies show involvement of CYP98A3 in the general phenylpropanoid metabolism, and suggest a downstream function for CYP98A3 relative to the broader and upstream role of C4H.

Signal transduction during oxidative stress

As an unfortunate consequence of aerobic life, active oxygen species (AOS) are formed by partial reduction of molecular oxygen. Plants possess a complex battery of enzymatic and non-enzymatic antioxidants that can protect cells from oxidative damage by scavenging AOS. It is becoming evident that AOS, which are generated during pathogen attack and abiotic stress situations, are recognized by plants as a signal for triggering defence responses. An overview of the literature is presented on the signalling role of AOS in plant defence responses, cell death, and development. Special attention is given to AOS and redox-regulated gene expression and the role of kinases and phosphatases in redox signal transduction.

Cloning, heterologous expression, and functional characterization of 5-epi-aristolochene-1,3-dihydroxylase from tobacco (Nicotiana tabacum)

Capsidiol is a bicyclic, dihydroxylated sesquiterpene produced by several solanaceous species in response to a variety of environmental stimuli. It is the primary antimicrobial compound produced by Nicotiana tabacum in response to fungal elicitation, and it is formed via the isoprenoid pathway from 5-epi-aristolochene. Much of the biosynthetic pathway for the formation of this compound has been elucidated, except for the enzyme(s) responsible for the conversion of 5-epi-aristolochene to its dihydroxylated form, capsidiol. Biochemical evidence from previous studies with N. tabacum (Whitehead, I. M., Threlfall, D. R., and Ewing, D. F., 1989, Phytochemistry 28, 775-779) and Capsicum annuum Hoshino, T., Yamaura, T., Imaishi, H., Chida, M., Yoshizawa, Y., Higashi, K., Ohkawa, H., Mizutani, J., 1995, Phytochemistry 38, 609-613. suggested that the oxidation of 5-epi-aristolochene to capsidiol was mediated by at least one elicitor-inducible cytochrome P450 hydroxylase. In extending these observations, we developed an in vivo assay for 5-epi-aristolochene hydroxylase activity and used it to demonstrate a dose-dependent inhibition of activity by ancymidol and ketoconazole, two well characterized inhibitors of cytochrome P450 enzymes. Using degenerate oligonucleotide primers designed to the well conserved domains found within most P450 enzymes, including the heme binding domain, cDNA fragments representing four distinct P450 families (CYP71, CYP73, CYP82, and CYP92) were amplified from a cDNA library prepared against mRNA from elicitor-treated cells using PCR. The PCR fragments were subsequently used to isolate full-length cDNAs (CYP71D20 and D21, CYP73A27 and A28, CYP82E1 and CYP92A5), and these in turn were used to demonstrate that the corresponding mRNAs were all induced in elicitor-treated cells, albeit with different induction patterns. Representative, full-length cDNAs for each of the P450s were engineered into a yeast expression system, and the recombinant yeast assessed for functional expression of P450 protein by measuring the CO difference spectra of the yeast microsomes. Only microsomal preparations from yeast expressing the CYP71D20 and CYP92A5 cDNAs exhibited significant CO difference absorbance spectra at 450 nm and were thus tested for their ability to hydroxylate 5-epi-aristolochene and 1-deoxycapsidiol, a putative mono-hydroxylated intermediate in capsidiol biosynthesis. Interestingly, the CYP71D20-encoded enzyme activity was capable of converting both 5-epi-aristolochene and 1-deoxycapsidiol to capsidiol in vitro, consistent with the notion that this P450 enzyme catalyzes both hydroxylations of its hydrocarbon substrate.

Functional analysis of the LACERATA gene of Arabidopsis provides evidence for different roles of fatty acid omega -hydroxylation in development

We describe lacerata (lcr) mutants of Arabidopsis, which display various developmental abnormalities, including postgenital organ fusions, and report cloning of the LCR gene by using the maize transposon Enhancer/Suppressor-mutator (En/Spm). The pleiotropic mutant phenotype could be rescued by genetic complementation of lcr mutants with the wild-type LCR gene. The LCR gene encodes a cytochrome P450 monooxygenase, CYP86A8, which catalyzes omega-hydroxylation of fatty acids ranging from C12 to C18:1, as demonstrated by expression of the gene in yeast. Although palmitic and oleic acids were efficient substrates for LCR, 9,10-epoxystearate was not metabolized. Taken together with previous studies, our findings indicate that LCR-dependent omega-hydroxylation of fatty acids could be implicated in the biosynthesis of cutin in the epidermis and in preventing postgenital organ fusions. Strikingly, the same pathway seems to control trichome differentiation, the establishment of apical dominance, and senescence in plants.

Localization of CYP86B1 in the outer envelope of chloroplasts

CYP86B1 was cloned from a cDNA library and the protein expressed in E. coli. The protein gave the expected carbon monoxide difference spectrum. Using in vitro import assays with isolated pea chloroplasts, CYP86B1 was shown to be associated with the outer chloroplastic envelope membrane. This study provides the first direct evidence for a chloroplast-localized cytochrome P450-dependent monooxygenase.

A conservative amino acid substitution alters the regiospecificity of CYP94A2, a fatty acid hydroxylase from the plant Vicia sativa

Fatty acid omega-hydroxylation is involved in the biosynthesis of the plant cuticle, formation of plant defense signaling molecules, and possibly in the rapid catabolism of free fatty acids liberated under stress conditions. CYP94A2 is a cytochrome P450-dependent medium-chain fatty acid hydroxylase that was recently isolated from Vicia sativa. Contrary to CYP94A1 and CYP86A1, two other fatty acid hydroxylases previously characterized in V. sativa and Arabidopsis thaliana, CYP94A2 is not a strict omega-hydroxylase, but exhibits chain-length-dependent regioselectivity of oxidative attack. Sequence alignments of CYP94A2 with CYP94A1 and molecular modeling studies suggested that F494, located in SRS-6 (substrate recognition site) was involved in substrate recognition and positioning. Indeed, a conservative amino acid substitution at that position markedly altered the regiospecificity of CYP94A2. The observed shift from omega toward omega-1 hydroxylation was prominent with lauric acid as substrate and declined with increasing fatty acid chain length.

Demonstration that menthofuran synthase of mint (Mentha) is a cytochrome P450 monooxygenase: cloning, functional expression, and characterization of the responsible gene

(+)-Menthofuran is an undesirable monoterpenoid component of peppermint (Mentha x piperita) essential oil that is derived from the alpha,beta-unsaturated ketone (+)-pulegone. Microsomal preparations, from the oil gland secretory cells of a high (+)-menthofuran-producing chemotype of Mentha pulegium, transform (+)-pulegone to (+)-menthofuran in the presence of NADPH and molecular oxygen, implying that menthofuran is synthesized by a mechanism analogous to that of mammalian liver cytochrome P450s involving the hydroxylation of the syn-methyl group of (+)-pulegone, spontaneous intramolecular cyclization to the hemiketal, and dehydration to the furan. An abundant cytochrome P450 clone from a peppermint oil gland cell cDNA library was functionally expressed in Saccharomyces cerevisiae and Escherichia coli and shown to encode the (+)-menthofuran synthase (i.e., (+)-pulegone-9-hydroxylase). The full-length cDNA contains 1479 nucleotides, and encodes a protein of 493 amino acid residues of molecular weight 55,360, which bears all of the anticipated primary structural elements of a cytochrome P450 and most closely resembles (35% identity) a cytochrome P450 monoterpene hydroxylase, (+)-limonene-3-hydroxylase, from the same source. The availability of this gene permits transgenic manipulation of peppermint to improve the quality of the derived essential oil.

Brassinosteroid-6-oxidases from Arabidopsis and tomato catalyze multiple C-6 oxidations in brassinosteroid biosynthesis

Brassinosteroids (BRs) are steroidal plant hormones that are essential for growth and development. It has been proposed that BRs are synthesized via two parallel pathways, the early and late C-6 oxidation pathways according to the C-6 oxidation status. The tomato (Lycopersicon esculentum) Dwarf gene encodes a cytochrome P450 that has been shown to catalyze the C-6 oxidation of 6-deoxocastasterone to castasterone. We isolated an Arabidopsis ortholog (AtBR6ox gene) of the tomato Dwarf gene. The encoded polypeptide has characteristics of P450s and is classified into the CYP85 family. The AtBR6ox and tomato Dwarf gene were expressed in yeast and the ability of the transformed yeast cells to metabolize 6-deoxo-BRs was tested. Metabolites were analyzed by gas chromatography-mass spectrometry. Both enzymes catalyze multiple steps in BR biosynthesis: 6-deoxoteasterone to teasterone, 3-dehydro-6-deoxoteasterone to 3-dehydroteasterone, 6-deoxotyphasterol to typhasterol, and 6-deoxocastasterone to castasterone. Our results indicate that the AtBR6ox gene and the tomato Dwarf gene encode steroid-6-oxidases and that these enzymes have a broad substrate specificity. This suggests that the BR biosynthetic pathway consists of a metabolic grid rather than two separate parallel pathways.

Metabolism of tyrosine and tryptophan--new genes for old pathways

Tyrosine and tryptophan are precursors for the plant defense compounds dhurrin and indole glucosinolates, respectively. In addition, tryptophan is a precursor for the essential phytohormone indole-3-acetic acid. Recent advances in understanding the biosynthesis of these compounds have come from the characterization of enzymes that catalyze the N-hydroxylation of the precursor amino acid to the oxime intermediate. Furthermore, enzymes catalyzing subsequent biosynthetic steps have also been identified.

Functional characterization and subcellular localization of poplar (Populus trichocarpa x Populus deltoides) cinnamate 4-hydroxylase

Cinnamic acid 4-hydroxylase (C4H), a member of the cytochrome P450 monooxygenase superfamily, plays a central role in phenylpropanoid metabolism and lignin biosynthesis and possibly anchors a phenylpropanoid enzyme complex to the endoplasmic reticulum (ER). A full-length cDNA encoding C4H was isolated from a hybrid poplar (Populus trichocarpa x P. deltoides) young leaf cDNA library. RNA-blot analysis detected C4H transcripts in all organs tested, but the gene was most highly expressed in developing xylem. C4H expression was also strongly induced by elicitor-treatment in poplar cell cultures. To verify the catalytic activity of the putative C4H cDNA, two constructs, C4H and C4H fused to the FLAG epitope (C4H::FLAG), were expressed in yeast. Immunoblot analysis showed that C4H was present in the microsomal fraction and microsomal preparations from strains expressing both enzymes efficiently converted cinnamic acid to p-coumaric acid with high specific activities. To investigate the subcellular localization of C4H in vivo, a chimeric C4H-green fluorescent protein (GFP) gene was engineered and stably expressed in Arabidopsis. Confocal laser microscopy analysis clearly showed that in Arabidopsis the C4H::GFP chimeric enzyme was localized to the ER. When expressed in yeast, the C4H::GFP fusion enzyme was also active but displayed significantly lower specific activity than either C4H or C4H::FLAG in in vitro and in vivo enzyme assays. These data definitively show that C4H is localized to the ER in planta.

The CYP88A cytochrome P450, ent-kaurenoic acid oxidase, catalyzes three steps of the gibberellin biosynthesis pathway

We have shown that ent-kaurenoic acid oxidase, a member of the CYP88A subfamily of cytochrome P450 enzymes, catalyzes the three steps of the gibberellin biosynthetic pathway from ent-kaurenoic acid to GA(12). A gibberellin-responsive barley mutant, grd5, accumulates ent-kaurenoic acid in developing grains. Three independent grd5 mutants contain mutations in a gene encoding a member of the CYP88A subfamily of cytochrome P450 enzymes, defined by the maize Dwarf3 protein. Mutation of the Dwarf3 gene gives rise to a gibberellin-responsive dwarf phenotype, but the lesion in the gibberellin biosynthesis pathway has not been identified. Arabidopsis thaliana has two CYP88A genes, both of which are expressed. Yeast strains expressing cDNAs encoding each of the two Arabidopsis and the barley CYP88A enzymes catalyze the three steps of the GA biosynthesis pathway from ent-kaurenoic acid to GA(12). Sequence comparison suggests that the maize Dwarf3 locus also encodes ent-kaurenoic acid oxidase.

The CYP88A cytochrome P450, ent-kaurenoic acid oxidase, catalyzes three steps of the gibberellin biosynthesis pathway

We have shown that ent-kaurenoic acid oxidase, a member of the CYP88A subfamily of cytochrome P450 enzymes, catalyzes the three steps of the gibberellin biosynthetic pathway from ent-kaurenoic acid to GA(12). A gibberellin-responsive barley mutant, grd5, accumulates ent-kaurenoic acid in developing grains. Three independent grd5 mutants contain mutations in a gene encoding a member of the CYP88A subfamily of cytochrome P450 enzymes, defined by the maize Dwarf3 protein. Mutation of the Dwarf3 gene gives rise to a gibberellin-responsive dwarf phenotype, but the lesion in the gibberellin biosynthesis pathway has not been identified. Arabidopsis thaliana has two CYP88A genes, both of which are expressed. Yeast strains expressing cDNAs encoding each of the two Arabidopsis and the barley CYP88A enzymes catalyze the three steps of the GA biosynthesis pathway from ent-kaurenoic acid to GA(12). Sequence comparison suggests that the maize Dwarf3 locus also encodes ent-kaurenoic acid oxidase.

Flavonoid 6-hydroxylase from soybean (Glycine max L.), a novel plant P-450 monooxygenase

Cytochrome P-450-dependent hydroxylases are typical enzymes for the modification of basic flavonoid skeletons. We show in this study that CYP71D9 cDNA, previously isolated from elicitor-induced soybean (Glycine max L.) cells, codes for a protein with a novel hydroxylase activity. When heterologously expressed in yeast, this protein bound various flavonoids with high affinity (1.6 to 52 microm) and showed typical type I absorption spectra. These flavonoids were hydroxylated at position 6 of both resorcinol- and phloroglucinol-based A-rings. Flavonoid 6-hydroxylase (CYP71D9) catalyzed the conversion of flavanones more efficiently than flavones. Isoflavones were hardly hydroxylated. As soybean produces isoflavonoid constituents possessing 6,7-dihydroxy substitution patterns on ring A, the biosynthetic relationship of flavonoid 6-hydroxylase to isoflavonoid biosynthesis was investigated. Recombinant 2-hydroxyisoflavanone synthase (CYP93C1v2) efficiently used 6,7,4'-trihydroxyflavanone as substrate. For its structural identification, the chemically labile reaction product was converted to 6,7,4'-trihydroxyisoflavone by acid treatment. The structures of the final reaction products for both enzymes were confirmed by NMR and mass spectrometry. Our results strongly support the conclusion that, in soybean, the 6-hydroxylation of the A-ring occurs before the 1,2-aryl migration of the flavonoid B-ring during isoflavanone formation. This is the first identification of a flavonoid 6-hydroxylase cDNA from any plant species.

Fusarium oxysporum fatty-acid subterminal hydroxylase (CYP505) is a membrane-bound eukaryotic counterpart of Bacillus megaterium cytochrome P450BM3

The gene of a fatty-acid hydroxylase of the fungus Fusarium oxysporum (P450foxy) was cloned and expressed in yeast. The putative primary structure revealed the close relationship of P450foxy to the bacterial cytochrome P450BM3, a fused protein of cytochrome P450 and its reductase from Bacillus megaterium. The amino acid sequence identities of the P450 and P450 reductase domains of P450foxy were highest (40.6 and 35.3%, respectively) to the corresponding domains of P450BM3. Recombinant P450foxy expressed in yeast was catalytically and spectrally indistinguishable from the native protein, except most of the recombinant P450foxy was recovered in the soluble fraction of the yeast cells, in marked contrast to native P450foxy, which was exclusively recovered in the membrane fraction of the fungal cells. This difference implies that a post (or co)-translational mechanism functions in the fungal cells to target and bind the protein to the membrane. These results provide conclusive evidence that P450foxy is the eukaryotic counterpart of bacterial P450BM3, which evokes interest in the evolutionary aspects concerning the P450 superfamily along with its reducing systems. P450foxy was classified in the new family, CYP505.

Transgenic tobacco and Arabidopsis plants expressing the two multifunctional sorghum cytochrome P450 enzymes, CYP79A1 and CYP71E1, are cyanogenic and accumulate metabolites derived from intermediates in Dhurrin biosynthesis

Novel cyanogenic plants have been generated by the simultaneous expression of the two multifunctional sorghum (Sorghum bicolor [L.] Moench) cytochrome P450 enzymes CYP79A1 and CYP71E1 in tobacco (Nicotiana tabacum cv Xanthi) and Arabidopsis under the regulation of the constitutive 35S promoter. CYP79A1 and CYP71E1 catalyze the conversion of the parent amino acid tyrosine to p-hydroxymandelonitrile, the aglycone of the cyanogenic glucoside dhurrin. CYP79A1 catalyzes the conversion of tyrosine to p-hydroxyphenylacetaldoxime and CYP71E1, the subsequent conversion to p-hydroxymandelonitrile. p-Hydroxymandelonitrile is labile and dissociates into p-hydroxybenzaldehyde and hydrogen cyanide, the same products released from dhurrin upon cell disruption as a result of pest or herbivore attack. In transgenic plants expressing CYP79A1 as well as CYP71E1, the activity of CYP79A1 is higher than that of CYP71E1, resulting in the accumulation of several p-hydroxyphenylacetaldoxime-derived products in the addition to those derived from p-hydroxymandelonitrile. Transgenic tobacco and Arabidopsis plants expressing only CYP79A1 accumulate the same p-hydroxyphenylacetaldoxime-derived products as transgenic plants expressing both sorghum cytochrome P450 enzymes. In addition, the transgenic CYP79A1 Arabidopsis plants accumulate large amounts of p-hydroxybenzylglucosinolate. In transgenic Arabidopsis expressing CYP71E1, this enzyme and the enzymes of the pre-existing glucosinolate pathway compete for the p-hydroxyphenylacetaldoxime as substrate, resulting in the formation of small amounts of p-hydroxybenzylglucosinolate. Cyanogenic glucosides are phytoanticipins, and the present study demonstrates the feasibility of expressing cyanogenic compounds in new plant species by gene transfer technology to improve pest and disease resistance.

Identification of the Arabidopsis thaliana flavonoid 3'-hydroxylase gene and functional expression of the encoded P450 enzyme

The phenylpropanoid pathway results in the synthesis of thousands of compounds, including flavonoids like flavonols, anthocyanidins and tannins. In Arabidopsis thaliana, the lack of tannins in the seed coat (testa) causes the transparent testa (tt) phenotype. In the present study, we identified the gene responsible for the tt7 mutation. We show that TT7 encodes the enzyme flavonoid 3'-hydroxylase (F3'H), and demonstrate that this P450-dependent monooxygenase has F3'H activity. The availability of the AtF3'H gene and promoter sequence will allow us to study the coregulation of a complete set of flavonol and anthocyanidin biosynthesis genes in A. thaliana, and makes in vitro synthesis of hydroxylated flavonoids more feasible.

Functional expression of regiospecific cytochrome P450 limonene hydroxylases from mint (Mentha spp.) in Escherichia coli and saccharomyces cerevisiae

The oxygenation pattern of the essential oil monoterpenes of commercial mint (Mentha) species is determined by regiospecific cytochrome P450-catalyzed hydroxylation of the common olefinic precursor (-)-4S-limonene. In spearmint (M. spicata), C6-allylic hydroxylation leads to (-)-trans-carveol and thence (-)-carvone, whereas in peppermint (M. x piperita), C3-allylic hydroxylation leads to (-)-trans-isopiperitenol and ultimately (-)-menthol. cDNAs encoding the C6-hydroxylase and C3-hydroxylase from spearmint and peppermint, respectively, were isolated by a combination of reverse genetic and homology-based cloning methods (S. Lupien, F. Karp, M. Wildung, and R. Croteau, Arch. Biochem. Biophys. 368, 181-192, 1999). Although both hydroxylase genes were confirmed by functional expression using the baculovirus-Spodoptera system, too little protein was available by this approach to permit detailed study of the structure-function relationships of these catalysts, especially the substrate binding determinants that underlie the regiochemistry and stereochemistry of the reactions. Therefore, heterologous overexpression systems based on Escherichia coli and Saccharomyces cerevisiae were developed to produce several N-terminally modified versions of the recombinant hydroxylases. Ancillary methods for the solubilization, purification, and reconstitution (with recombinant spearmint cytochrome P450 reductase) of the limonene hydroxylases were also devised, with which substrate binding behavior and precise regiochemistry and stereochemistry of product formation were determined.

Bacterial expression and purification of the Arabidopsis NADPH-cytochrome P450 reductase ATR2

An N-terminally modified form of the Arabidopsis NADPH-cytochrome P450 ATR2 (ATR2mod) was expressed from the tactac promoter in Escherichia coli to obtain high yields of the enzyme. The N-terminal modification eliminates the predicted chloroplast transit peptide of ATR2 allowing for more efficient expression. ATR2mod was purified from membrane extracts using a 2',5'-ADP-agarose affinity column. The specific activity of the purified ATR2mod for cytochrome c reduction was 9.4 micromol min(-1) mg(-1) and the K(m) for cytochrome c reduction was 15 +/- 2 microM. The purified NADPH-cytochrome P450 reductase was able to support function of CYP79B2.

Cloning and characterization of the cytochrome P450 oxidoreductase gene from the zygomycete fungus Cunninghamella

The filamentous fungus Cunninghamella utilizes cytochrome P450 system(s) in the metabolism of a broad range of polyaromatic and aliphatic pollutants and a variety of drugs, but prior attempts at isolation of P450 system components of this fungus have been generally unsuccessful. We report upon the cytochrome P450 oxidoreductase (CPR) gene from two widely studied species, C. elegans and C. echinulata. The C. elegans CPR gene was obtained by screening a genomic library using as probe a PCR amplicon obtained with degenerate primers based on known CPRs. The 2420 bp coding region contained two apparent introns (149 bp and 138 bp). Northern blot analysis showed that the CPR gene is transcriptionally expressed in C. elegans and appears to be inducible by an alkane substrate, n-tetradecane. Phylogenetic comparison of the deduced C. elegans CPR (710 aa) suggested that it is more closely related to animal CPRs (41-42%) than to yeast (38-41%) and plant (35-36%) forms. A 2074 bp sequence containing most of the CPR gene homolog from C. echinulata was also isolated.

Cloning and expression of flavone synthase II from Gerbera hybrids

In Gerbera hybrids, flavone synthesis is controlled by the locus Fns. The responsible enzyme, flavone synthase II, belongs to the NADPH-dependent cytochrome P450 monooxygenases. From two different chemogenetic defined Gerbera lines with the dominant (fns +.) or recessive (fns fns) alleles at the locus Fns, a cytochrome P450 fragment (CypDDd7a) was isolated using a differential display technique with upstream primers based on the conserved heme-binding region of cytochrome P450 proteins. The full-length cDNA (CYP93B2) which contained the open-reading frame and part of the CypDDd7a sequence was isolated via 5'-RACE and end-to-end PCR with gene specific primers. Northern blot analysis of total RNA of Gerbera hybrids indicated that the CYP93B2 gene was only transcribed in lines with the dominant allele fns + and that the transcript levels during flower development are in agreement with the measured enzyme activity of FNS II and flavone accumulation. Microsomes from yeast cells expressing CYP93B2 catalysed the direct formation of [14C]-flavones from the respective [14C]-flavanones. Thus, CYP93B2 was shown to encode flavone synthase II. This is the first report of the isolation and expression of a functional FNS II cDNA clone from any species. The comparison of amino acid sequences revealed that CYP93B2 had 54% identity with the sequence of CYP93B1, which has recently been reported as a (2S)-flavanone 2-hydroxylase of Glycyrrhiza echinata L.

Systematic analysis of S. cerevisiae chromosome VIII genes

To begin genome-wide functional analysis, we analysed the consequences of deleting each of the 265 genes of chromosome VIII of Saccharomyces cerevisiae. For 33% of the deletion strains a growth phenotype could be detected: 18% of the genes are essential for growth on complete glucose medium, and 15% grow significantly more slowly than the wild-type strain or exhibit a conditional phenotype when incubated under one of 20 different growth conditions. Two-thirds of the mutants that exhibit conditional phenotypes are pleiotropic; about one-third of the mutants exhibit only one phenotype. We also measured the level of expression directed by the promoter of each gene. About half of the promoters direct detectable transcription in rich glucose medium, and most of these exhibited only low or medium activity. Only 1% of the genes are expressed at about the same level as ACT1. The number of active promoters increased to 76% upon growth on a non-fermentable carbon source, and to 93% in minimal glucose medium. The majority of promoters fluctuated in strength, depending on the medium.

Metabolic engineering of p-hydroxybenzylglucosinolate in Arabidopsis by expression of the cyanogenic CYP79A1 from Sorghum bicolor

Glucosinolates are natural products in cruciferous plants, including Arabidopsis thaliana. CYP79A1 is the cytochrome P450 catalysing the conversion of tyrosine to p-hydroxyphenylacetaldoxime in the biosynthesis of the cyanogenic glucoside dhurrin in sorghum. Both glucosinolates and cyanogenic glucosides have oximes as intermediates. Expression of CYP79A1 in A. thaliana results in the production of high levels of the tyrosine-derived glucosinolate p-hydroxybenzylglucosinolate, which is not a natural constituent of A. thaliana. This provides further evidence that the enzymes have low substrate specificity with respect to the side chain. The ability of the cyanogenic CYP79A1 to integrate itself into the glucosinolate pathway has important implications for an evolutionary relationship between cyanogenic glucosides and glucosinolates, and for the possibility of genetic engineering of novel glucosinolates.

Expression of chimeric P450 genes encoding flavonoid-3', 5'-hydroxylase in transgenic tobacco and petunia plants(1)

Flavonoid-3',5'-hydroxylase (F3'5'H), a member of the cytochrome P450 family, is the key enzyme in the synthesis of 3', 5'-hydroxylated anthocyanins, which are generally required for blue or purple flowers. A full-length cDNA, TG1, was isolated from prairie gentian by heterologous hybridization with a petunia cDNA, AK14, which encodes F3'5'H. To investigate the in vivo function of TG1 and AK14, they were subcloned into a plant expression vector and expressed under the control of the CaMV35S promoter in transgenic tobacco or petunia, both of which originally lack the enzyme. Transgenic petunia plants had a dramatic change in flower color from pink to magenta with a high content of 3',5'-hydroxylated anthocyanins. In contrast, transgenic tobacco plants had minimal color change with at most 35% 3',5'-hydroxylated anthocyanin content. These results indicate that the products of TG1 and AK14 have F3'5'H activity in planta and that interspecific gene transfer alters anthocyanin pigment synthesis. The difference in apparent F3'5'H activity between tobacco and petunia is discussed.

Cytochrome P450 monooxygenases of DIBOA biosynthesis: specificity and conservation among grasses

DIBOA and DIMBOA are secondary metabolites of grasses which function as natural pesticides. The four maize genes BX2 through BX5 encode cytochrome P450-dependent monooxygenases that catalyse four consecutive reactions in the biosynthesis of these secondary products. Although BX2-BX5 share significant sequence homology, the four enzymes have evolved into specific enzymes each catalysing predominantly only one reaction in the pathway. In addition to these natural reactions, BX3 hydroxylates 1,4-benzoxazin-3-one and BX2 shows pCMA demethylase activity. With respect to DIBOA biosynthesis, identical enzymatic reactions have been found in rye as compared to maize, indicating early evolution of the P450 enzymes in the grasses.

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.

The chemically inducible plant cytochrome P450 CYP76B1 actively metabolizes phenylureas and other xenobiotics

Cytochrome P450s (P450s) constitute one of the major classes of enzymes that are responsible for detoxification of exogenous molecules both in animals and plants. On the basis of its inducibility by exogenous chemicals, we recently isolated a new plant P450, CYP76B1, from Jerusalem artichoke (Helianthus tuberosus) and showed that it was capable of dealkylating a model xenobiotic compound, 7-ethoxycoumarin. In the present paper we show that CYP76B1 is more strongly induced by foreign compounds than other P450s isolated from the same plant, and metabolizes with high efficiency a wide range of xenobiotics, including alkoxycoumarins, alkoxyresorufins, and several herbicides of the class of phenylureas. CYP76B1 catalyzes the double N-dealkylation of phenylureas with turnover rates comparable to those reported for physiological substrates and produces nonphytotoxic compounds. Potential uses for CYP76B1 thus include control of herbicide tolerance and selectivity, as well as soil and groundwater bioremediation.

Molecular characterization and functional expression of dihydroxypterocarpan 6a-hydroxylase, an enzyme specific for pterocarpanoid phytoalexin biosynthesis in soybean (Glycine max L.)

Four cytochrome P450-dependent enzymes, among them dihydroxypterocarpan 6a-hydroxylase (D6aH), are specifically involved in the elicitor-inducible biosynthesis of glyceollins, the phytoalexins of soybean. Here we report that CYP93A1 cDNA, which we isolated previously from elicitor-induced soybean cells, codes for a protein with D6aH activity. Analysis of the catalytic properties of recombinant CYP93A1 expressed in yeast, its NADPH dependency, stereoselectivity and high substrate affinity confirmed that D6aH is the physiological function of CYP93A1. It thus represents the first isoflavonoid-specific CYP to be characterized at the molecular level. In elicitor-treated soybean cells producing phytoalexins, increases in D6aH activity were correlated with elevated transcript levels which indicates that expression of the enzyme is regulated at the level of transcription. Therefore, CYP93A1 cDNA can be used as a specific molecular marker for the inducible defense response against pathogen attack.

Substrate specificity of NO synthases: detailed comparison of L-arginine, homo-L-arginine, their N omega-hydroxy derivatives, and N omega-hydroxynor-L-arginine

A detailed comparison of the oxidation of five compounds closely related to L-arginine (Arg) by purified recombinant neuronal and macrophage NO synthases (NOS I and NOS II) was performed. Homo-L-arginine (homo-Arg) is oxidized by both NOSs in the presence of NADPH with major formation of NO and homo-L-citrulline, with a molar ratio of close to 1, and minor formation of N omega-hydroxyhomo-L-arginine (homo-NOHA). Oxidation of homo-NOHA by the two NOSs also leads to NO and homocitrulline in a 1:1 molar ratio. On the contrary, N omega-hydroxynor-L-arginine (nor-NOHA) is a very poor substrate of NOS I and II, which fails to produce significant amounts of nitrite. The catalytic efficiency of both NOSs markedly decreases in the order Arg > NOHA > homo-Arg > homo-NOHA, as shown by the 20- and 10-fold decrease of kcat/Km observed for NOS I and NOS II, respectively, when comparing Arg to homo-NOHA. The greater loss of catalytic efficiency for homo-Arg, when compared to that for Arg, appears to occur at the first step (N-hydroxylation) of the reaction. In that regard, it is noteworthy that the Vm values for NOHA and homo-NOHA oxidation are very similar (about 1 and 2 micromol of NO min-1 mg of protein-1 for NOS I and II, respectively). In fact, lengthening of the Arg chain by one CH2 leads not only to markedly decreased kcat/Km but also to clear disturbances in NOS functioning. This is shown by a greater accumulation of the N omega-hydroxyguanidine intermediate (homo-NOHA:homocitrulline ratio between 0.2 and 0.4) and an increased consumption of NADPH for NO formation (between 2.0 and 2.6 mol of NADPH consumed for the formation of 1 mol of NO in the case of homo-Arg, instead of 1.5 mol in the case of Arg). Most of the above results could be interpreted by comparing the possible positionings of the various substrates relative to the two NOS active oxygen species which are believed to be responsible for the two steps of the reaction.

A yeast recombinant aquaporin mutant that is not expressed or mistargeted in Xenopus oocyte can be functionally analyzed in reconstituted proteoliposomes

We have recently identified AQPcic (for aquaporin cicadella), an insect aquaporin found in the digestive tract of homopteran insects and involved in the elimination of water ingested in excess with the dietary sap (Le Cahérec, F., Deschamps, S., Delamarche, C., Pellerin, I., Bonnec, G., Guillam, M. T., Gouranton, J., Thomas, D., and Hubert, J. F. (1996) Eur. J. Biochem. 241, 707-715). Like many other aquaporins, AQPcic is inhibited by mercury reagents. In this study, we have demonstrated that residue Cys82 is essential for mercury inhibition. Another mutant version of AQPcic (AQP-C134S), expression of which in Xenopus laevis failed to produce an active molecule, was successfully expressed in Saccharomyces cerevisiae. Using stopped-flow analysis of reconstituted proteoliposomes, we demonstrated that the biological activity and Hg sensitivity of yeast-expressed wild type and mutant type AQPcic was readily assessed. Therefore, we propose that the yeast system is a valid alternative to Xenopus oocytes for studying particular mutants of aquaporin.

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.

Cytochrome P450 enzyme systems in fungi

The involvement of cytochrome P450 enzymes in many complex fungal bioconversion processes has been characterized in recent years. Accordingly, there is now considerable scientific interest in fungal cytochrome P450 enzyme systems. In contrast to S. cerevisiae, where surprisingly few P450 genes have been identified, biochemical data suggest that many fungi possess numerous P450 genes. This review summarizes the current information pertaining to these fungal cytochrome P450 systems, with emphasis on the molecular genetics. The use of molecular techniques to improve cytochrome P450 activities in fungi is also discussed.

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.