High-performance liquid chromatography of gibberellins

Fungal Diversity and Community Composition of Culturable Fungi in Stanhopea trigrina Cast Gibberellin Producers

Stanhopea tigrina is a Mexican endemic orchid reported as a threatened species. The naturally occurring microorganisms present in S. tigrina are unknown. In this work, we analyzed the diversity of endophytic and epiphytic culturable fungi in S. tigrina according to morphological and molecular identification. Using this combined approach, in this study we retrieved a total of 634 fungal isolates that presented filamentous growth, which were grouped in 134 morphotypes that were associated to 63 genera, showing that S. tigrina harbors a rich diversity of both endophytic and epiphytic fungi. Among these, the majority of the isolates corresponded to Ascomycetes, with Trichoderma and Penicillium as the most frequent genera followed by Fusarium and Aspergillus. Non-ascomycetes isolated were associated only to the genus Mucor (Mucoromycota) and Schizophyllum (Basidiomycota). Identified genera showed a differential distribution considering their epiphytic or endophytic origin, the tissue from which they were isolated, and the ability of the orchid to grow on different substrates. To our knowledge, this work constitutes the first study of the mycobiome of S. tigrina. Interestingly, 21 fungal isolates showed the ability to produce gibberellins. Almost half of the isolates were related to the gibberellin-producer genus Penicillium based on morphological and molecular identification. However, the rest of the isolates were related to the following genera, which have not been reported as gibberellin producers so far: Bionectria, Macrophoma, Nectria, Neopestalotiopsis, Talaromyces, Trichoderma, and Diplodia. Taken together, we found that S. tigrina possess a significant fungal diversity that could be a rich source of fungal metabolites with the potential to develop biotechnological approaches oriented to revert the threatened state of this orchid in the near future.

A sensing role of the glutamine synthetase in the nitrogen regulation network in Fusarium fujikuroi

In the plant pathogenic ascomycete Fusarium fujikuroi the synthesis of several economically important secondary metabolites (SM) depends on the nitrogen status of the cells. Of these SMs, gibberellin and bikaverin synthesis is subject to nitrogen catabolite repression (NCR) and is therefore only executed under nitrogen starvation conditions. How the signal of available nitrogen quantity and quality is sensed and transmitted to transcription factors is largely unknown. Earlier work revealed an essential regulatory role of the glutamine synthetase (GS) in the nitrogen regulation network and secondary metabolism as its deletion resulted in total loss of SM gene expression. Here we present extensive gene regulation studies of the wild type, the Δgln1 mutant and complementation strains of the gln1 deletion mutant expressing heterologous GS-encoding genes of prokaryotic and eukaryotic origin or 14 different F. fujikuroi gln1 copies with site-directed mutations. All strains were grown under different nitrogen conditions and characterized regarding growth, expression of NCR-responsive genes and biosynthesis of SM. We provide evidence for distinct roles of the GS in sensing and transducing the signals to NCR-responsive genes. Three site directed mutations partially restored secondary metabolism and GS-dependent gene expression, but not glutamine formation, demonstrating for the first time that the catalytic and regulatory roles of GS can be separated. The distinct mutant phenotypes show that the GS (1) participates in NH₄⁺-sensing and transducing the signal towards NCR-responsive transcription factors and their subsequent target genes; (2) affects carbon catabolism and (3) activates the expression of a distinct set of non-NCR GS-dependent genes. These novel insights into the regulatory role of the GS provide fascinating perspectives for elucidating regulatory roles of GS proteins of different organism in general.

Agrobacterium mediated transient gene silencing (AMTS) in Stevia rebaudiana: insights into steviol glycoside biosynthesis pathway

BACKGROUND

Steviol glycoside biosynthesis pathway has emerged as bifurcation from ent-kaurenoic acid, substrate of methyl erythritol phosphate pathway that also leads to gibberellin biosynthesis. However, the genetic regulation of steviol glycoside biosynthesis has not been studied. So, in present study RNA interference (RNAi) based Agrobacterium mediated transient gene silencing (AMTS) approach was followed. SrKA13H and three SrUGTs (SrUGT85C2, SrUGT74G1 and SrUGT76G1) genes encoding ent-kaurenoic acid-13 hydroxylase and three UDP glycosyltransferases of steviol glycoside biosynthesis pathway were silenced in Stevia rebaudiana to understand its molecular mechanism and association with gibberellins.

METHODOLOGY/PRINCIPAL FINDINGS

RNAi mediated AMTS of SrKA13H and three SrUGTs has significantly reduced the expression of targeted endogenous genes as well as total steviol glycoside accumulation. While gibberellins (GA3) content was significantly enhanced on AMTS of SrUGT85C2 and SrKA13H. Silencing of SrKA13H and SrUGT85C2 was found to block the metabolite flux of steviol glycoside pathway and shifted it towards GA3 biosynthesis. Further, molecular docking of three SrUGT proteins has documented highest affinity of SrUGT76G1 for the substrates of alternate pathways synthesizing steviol glycosides. This could be a plausible reason for maximum reduction in steviol glycoside content on silencing of SrUGT76G1 than other genes.

CONCLUSIONS

SrKA13H and SrUGT85C2 were identified as regulatory genes influencing carbon flux between steviol glycoside and gibberellin biosynthesis. This study has also documented the existence of alternate steviol glycoside biosynthesis route.

Quantitative determination of gibberellins by high performance liquid chromatography from various gibberellins producing Fusarium strains

High performance liquid chromatographic (HPLC) method was developed for analysis of seven gibberellins, i.e., GA3, GA4, GA7, GA3 methyl ester, GA7 methyl ester 3,13 diacetate, GA7 methyl ester, and fusaric acid, using an isocratic system. Method was used for estimation of gibberellins from different Fusarium strains. Gibberellins were extracted from 28 strains of Fusarium, out of which six strains of Fusarium were isolated from soil of different parts of India and 22 strains were procured from the Indian Type Culture Collection, Indian Agricultural Research Institute, New Delhi. Extracts were analyzed for qualitative and quantitative estimation of gibberellins by thin layer chromatography and HPLC, respectively. On the basis of quantitative analysis of produced gibberellins by HPLC, they were categorized as low, moderate, and high gibberellin producing strain. For the first time, Fusarium solani was also reported as high GA3 producing strain.

Gibberellin biosynthesis and gibberellin oxidase activities in Fusarium sacchari, Fusarium konzum and Fusarium subglutinans strains

Several isolates of three Fusarium species associated with the Gibberella fujikuroi species complex were characterized for their ability to synthesize gibberellins (GAs): Fusarium sacchari (mating population B), Fusarium konzum (mating population I) and Fusarium subglutinans (mating population E). Of these, F. sacchari is phylogenetically related to Fusarium fujikuroi and is grouped in the Asian clade of the complex, while F. konzum and F. subglutinans are only distantly related to Fusarium fujikuroi and belong to the American clade. Variability was found between the different F. sacchari strains tested. Five isolates (B-12756; B-1732, B-7610, B-1721 and B-1797) were active in GA biosynthesis and accumulated GA(3) in the culture fluid (2.76-28.4 microg/mL), while two others (B-3828 and B-1725) were inactive. GA(3) levels in strain B-12756 increased by 2.9 times upon complementation with ggs2 and cps-ks genes from F. fujikuroi. Of six F. konzum isolates tested, three (I-10653; I-11616; I-11893) synthesized GAs, mainly GA(1), at a low level (less than 0.1 microg/mL). Non-producing F. konzum strains contained no GA oxidase activities as found for the two F. subglutinans strains tested. These results indicate that the ability to produce GAs is present in other species of the G. fujikuroi complex beside F. fujikuroi, but might differ significantly in different isolates of the same species.

Enhanced production of gibberellin A4 (GA4) by a mutant of Gibberella fujikuroi in wheat gluten medium

Mutants of Gibberella fujikuroi with different colony characteristics, morphology and pigmentation were generated by exposure to UV radiation. A mutant, Mor-189, was selected based on its short filament length, relatively high gibberellin A(4) (GA(4)) and gibberellin A(3) (GA(3)) production, as well as its lack of pigmentation. Production of GA(4) by Mor-189 was studied using different inorganic and organic nitrogen sources, carbon sources and by maintaining the pH of the fermentation medium using calcium carbonate. Analysis of GA(4) and GA(3) was done by reversed-phase high-performance liquid chromatography and LC-MS. The mutants of G. fujikuroi produced more GA(4) when the pH of the medium was maintained above 5. During shake flask studies, the mutant Mor-189 produced 210 mg l(-1) GA(4) in media containing wheat gluten as the nitrogen source and glucose as the carbon source. Fed-batch fermentation in a 14 l agitated fermenter was performed to evaluate the applicability of the mutant Mor-189 for the production of GA(4). In 7-day fed-batch fermentation, 600 mg l(-1) GA(4) were obtained in the culture filtrate. The concentration of GA(4) and GA(3) combined was 713 mg l(-1), of which GA(4) accounted for 84% of the total gibberellin. These values are substantially higher than those published previously. The present study indicated that, along with maintenance of pH and controlled glucose feeding, the use of wheat gluten as the sole nitrogen source considerably enhances GA(4) production by the mutant Mor-189.

Isolation and characterization of the gibberellin biosynthetic gene cluster in Sphaceloma manihoticola

Gibberellins (GAs) are tetracyclic diterpenoid phytohormones that were first identified as secondary metabolites of the fungus Fusarium fujikuroi (teleomorph, Gibberella fujikuroi). GAs were also found in the cassava pathogen Sphaceloma manihoticola, but the spectrum of GAs differed from that in F. fujikuroi. In contrast to F. fujikuroi, the GA biosynthetic pathway has not been studied in detail in S. manihoticola, and none of the GA biosynthetic genes have been cloned from the species. Here, we present the identification of the GA biosynthetic gene cluster from S. manihoticola consisting of five genes encoding a bifunctional ent-copalyl/ent-kaurene synthase (CPS/KS), a pathway-specific geranylgeranyl diphosphate synthase (GGS2), and three cytochrome P450 monooxygenases. The functions of all of the genes were analyzed either by a gene replacement approach or by complementing the corresponding F. fujikuroi mutants. The cluster organization and gene functions are similar to those in F. fujikuroi. However, the two border genes in the Fusarium cluster encoding the GA(4) desaturase (DES) and the 13-hydroxylase (P450-3) are absent in the S. manihoticola GA gene cluster, consistent with the spectrum of GAs produced by this fungus. The close similarity between the two GA gene clusters, the identical gene functions, and the conserved intron positions suggest a common evolutionary origin despite the distant relatedness of the two fungi.

The White Collar protein WcoA of Fusarium fujikuroi is not essential for photocarotenogenesis, but is involved in the regulation of secondary metabolism and conidiation

The fungal proteins of the White Collar photoreceptor family, represented by WC-1 from Neurospora crassa, mediate the control by light of different biochemical and developmental processes, such as carotenogenesis or sporulation. Carotenoid biosynthesis is induced by light in the gibberellin-producing fungus Fusarium fujikuroi. In an attempt to identify the photoreceptor for this response, we cloned the only WC-1-like gene present in the available Fusarium genomes, that we called wcoA. The predicted WcoA polypeptide is highly similar to WC-1 and contains the relevant functional domains of this protein. In contrast to the Neurospora counterpart, wcoA expression is not affected by light. Unexpectedly, targeted wcoA disruptant strains maintain the light-induced carotenogenesis. Furthermore, the wcoA mutants show a drastic reduction of fusarin production in the light, and produce less gibberellins and more bikaverins than the parental strain under nitrogen-limiting conditions. The changes in the production of the different products indicate a key regulatory role for WcoA in secondary metabolism of this fungus. Additionally, the mutants are severely affected in conidiation rates under different culture conditions, indicating a more general regulatory role for this protein.

Restoration of gibberellin production in Fusarium proliferatum by functional complementation of enzymatic blocks

Nine biological species, or mating populations (MPs), denoted by letters A to I, and at least 29 anamorphic Fusarium species have been identified within the Gibberella fujikuroi species complex. Members of this species complex are the only species of the genus Fusarium that contain the gibberellin (GA) biosynthetic gene cluster or at least parts of it. However, the ability of fusaria to produce GAs is so far restricted to Fusarium fujikuroi, although at least six other MPs contain all the genes of the GA biosynthetic gene cluster. Members of Fusarium proliferatum, the closest related species, have lost the ability to produce GAs as a result of the accumulation of several mutations in the coding and 5' noncoding regions of genes P450-4 and P450-1, both encoding cytochrome P450 monooxygenases, resulting in metabolic blocks at the early stages of GA biosynthesis. In this study, we have determined additional enzymatic blocks at the first specific steps in the GA biosynthesis pathway of F. proliferatum: the synthesis of geranylgeranyl diphosphate and the synthesis of ent-kaurene. Complementation of these enzymatic blocks by transferring the corresponding genes from GA-producing F. fujikuroi to F. proliferatum resulted in the restoration of GA production. We discuss the reasons for Fusarium species outside the G. fujikuroi species complex having no GA biosynthetic genes, whereas species distantly related to Fusarium, e.g., Sphaceloma spp. and Phaeosphaeria spp., produce GAs.

Distribution of gibberellin biosynthetic genes and gibberellin production in the Gibberella fujikuroi species complex

Gibberella fujikuroi is a species-rich monophyletic complex of at least nine sexually fertile biological species (mating populations, MP-A to MP-I) and more than 30 anamorphs in the genus Fusarium. They produce a variety of secondary metabolites, such as fumonisins, fusaproliferin, moniliformin, beauvericin, fusaric acid, and gibberellins (GAs), a group of plant hormones. In this study, we examined for the first time all nine sexually fertile species (MPs) and additional anamorphs within and outside the G. fujikuroi species complex for the presence of GA biosynthetic genes. So far, the ability to produce GAs was described only for Fusarium fujikuroi (G. fujikuroi MP-C), which contains seven clustered genes in the genome all participating in GA biosynthesis. We show that six other MPs (MPs B, D, E, F, G, and I) and most of the anamorphs within the species complex also contain the entire gene cluster, except for F. verticillioides (MP-A), and F. circinatum (MP-H), containing only parts of it. Despite the presence of the entire gene cluster in most of the species within the G. fujikuroi species complex, expression of GA biosynthetic genes and GA production were detected only in F. fujikuroi (MP-C) and one isolate of F. konzum (MP-I). We used two new molecular marker genes, P450-4 from the GA gene cluster, and cpr, encoding the highly conserved NADPH cytochrome P450 reductase to study phylogenetic relationships within the G. fujikuroi species complex. The molecular phylogenetic studies for both genes have revealed good agreement with phylogenetic trees inferred from other genes. Furthermore, we discuss the role and evolutionary origin of the GA biosynthetic gene cluster.

Determination of gibberellic acid in fermentation broth and commercial products by micellar electrokinetic chromatography

Micellar electrokinetic chromatography (MEKC) was developed as a method for quantitative determination of gibberellic acid (GA3) in fermentation broth and commercial products, using 25 mM disodium tetraborate as a buffer at pH 9.2 and 100 mM sodium dodecyl sulfate as a micellar phase. The baseline resolution (Rs of GA3 from other compounds in fermentation broth was achieved with Rs > 2.5. The addition of methanol or acetonitrile in the MEKC buffer did not give a better resolution. Advantages of this MEKC method include high accuracy and precision and no sample preparation except for dilution and filtration.

Deletion of the Gibberella fujikuroi glutamine synthetase gene has significant impact on transcriptional control of primary and secondary metabolism

In Gibberella fujikuroi, the gibberellin (GA) and bikaverin biosynthesis are under control of nitrogen metabolite repression. However, the signalling components acting upstream of AREA are still unknown. We investigated the role of glutamine synthetase (GS) both as an enzyme and as a possible regulator in the nitrogen regulation system. We cloned and replaced the GS-encoding gene, glnA-Gf. The mutants grow with a phenotype different from the wild type in the presence of glutamine. They were unable to express nitrogen-repressed GA and bikaverin biosynthetic genes even under nitrogen starvation conditions. Complementation with the glnA-Gf wild-type copy fully restored GS activity, the expression of secondary metabolism genes, and the production of GAs and the red pigment, bikaverin. In order to find more target genes of GS, differential cDNA-screening and differential hybridization of macroarrays were performed using cDNA from the wild type and DeltaglnA mutant as probes. Several genes were dramatically up- or downregulated in the mutant. Among them are genes involved in N- and C-catabolism, and in transcriptional and translation control. Some of these genes are also under AREA control. Treatment with the GS inhibitor l-methionine sulphoximine resulted in similar expression patterns as in the glnA mutant with ammonium as nitrogen source, whereas glutamine can overcome the up- or downregulation of most but not all of the target genes. These findings suggest that not only glutamine, but also GS itself might play an important role in nitrogen metabolite repression.

Characterization of the final two genes of the gibberellin biosynthesis gene cluster of Gibberella fujikuroi: des and P450-3 encode GA4 desaturase and the 13-hydroxylase, respectively

Recently, six genes of the gibberellin (GA) biosynthesis gene cluster in Gibberella fujikuroi were cloned and the functions of five of these genes were determined. Here we describe the function of the sixth gene, P450-3, and the cloning and functional analysis of a seventh gene, orf3, located at the left border of the gene cluster. We have thereby defined the complete GA biosynthesis gene cluster in this fungus. The predicted amino acid sequence of orf3 revealed no close homology to known proteins. High performance liquid chromatography and gas chromatography-mass spectrometry analyses of the culture fluid of knock-out mutants identified GA1 and GA4, rather than GA3 and GA7, as the major C19-GA products, suggesting that orf3 encodes the GA4 1,2-desaturase. This was confirmed by transformation of the SG139 mutant, which lacks the GA biosynthesis gene cluster, with the desaturase gene renamed des. The transformants converted GA4 to GA7, and also metabolized GA9 (3-deoxyGA4) to GA120 (1,2-didehydroGA9), but the 2alpha-hydroxylated compound GA40 was the major product in this case. We demonstrate also by gene disruption that P450-3, one of the four cytochrome P450 monooxygenase genes in the GA gene cluster, encodes the 13-hydroxylase, which catalyzes the conversion of GA7 to GA3, in the last step of the pathway. This enzyme also catalyzes the 13-hydroxylation of GA4 to GA1. Disruption of the des gene in an UV-induced P450-3 mutant produced a double mutant lacking both desaturase and 13-hydroxylase activities that accumulated high amounts of the commercially important GA4. The des and P450-3 genes differ in their regulation by nitrogen metabolite repression. In common with the other five GA biosynthesis genes, expression of the desaturase gene is repressed by high amounts of nitrogen in the culture medium, whereas P450-3 is the only gene in the cluster not repressed by nitrogen.

AREA directly mediates nitrogen regulation of gibberellin biosynthesis in Gibberella fujikuroi, but its activity is not affected by NMR

AREA (NIT2) is a general transcription factor involved in derepression of numerous genes responsible for nitrogen utilization in Gibberella fujikuroi and many other fungi. We have previously shown that the deletion of areA-GF resulted in mutants with significantly reduced gibberellin (GA) production. Here we demonstrate that the expression level of six of the seven GA biosynthesis genes is drastically reduced in mutants lacking areA. Furthermore, we show that, despite the fact that GAs are nitrogen-free diterpenoid compounds, which are not obviously involved in nitrogen metabolism, AREA binds directly to the promoters of the six N-regulated genes. The binding of AREA was analysed in more detail using the promoter of one of the GA-biosynthesis genes encoding the ent-kaurene oxidase (P450-4). Deletion/mutation analysis of the P450-4 promoter fused to the Escherichia coli uidA gene, which encodes beta-glucuronidase, allowed the in vivo identification of functional GATA motifs. We have also analysed the nmr gene of G. fujikuroi (nmr-GF) which has high similarity to the Neurospora crassa nmr-1 and Aspergillus nidulans nmrA genes, both involved in nitrogen metabolite repression. In contrast to our expectation, deletion of nmr-GF did not result in significant derepression of the GA biosynthesis genes in the presence of ammonium, glutamine or glutamate. Overexpression of the nmr-GF gene fused to the strong promoter of the G. fujikuroi glutamine synthetase (gs) gene revealed only a very slight repression of the nitrate reductase (niaD) gene, resulting in weak resistance to chlorate. Surprisingly, this effect was only observed in the presence of high amounts of glutamate; cultivation on ammonium failed to induce any resistance to chlorate. Despite the limited effect of gene replacement and overexpression of nmr-GF on the nitrogen metabolism of G. fujikuroi itself, the gene fully restored nitrogen metabolite repression in A. nidulans and N. crassa nmr mutants. Therefore, we postulate that, in contrast to A. nidulans and N. crassa, NMR does not function independently as the main modulator of AREA in G. fujikuroi.

The polyketide synthase gene pks4 from Gibberella fujikuroi encodes a key enzyme in the biosynthesis of the red pigment bikaverin

The ascomycete Gibberella fujikuroi mating population C (MP-C) is well known for the production of gibberellins, but also produces many other secondary metabolites, including the red polyketide pigment bikaverin. Here, we used a differential display method to clone a polyketide synthase gene pks4 responsible for the first step of bikaverin biosynthesis. Sequence analysis indicated that pks4 encoded a 2009-amino acid polypeptide consisting of four functional domains: beta-ketoacyl synthase (KS), acyltransferase (AT), acyl carrier (ACP), and thioesterase (TE). Disruption of pks4 resulted in the loss of both pks4 transcripts and bikaverin biosynthesis in G. fujikuroi cultures. Expression of pks4 is strongly repressed by high amounts of ammonium and basic pH. Unexpectedly, pks4 was overexpressed in mutants of the regulatory gene, areA, which is responsible for the activation of nitrogen assimilation genes. Three additional polyketide synthase genes have been cloned from G. fujikuroi MP-C by heterologous hybridization. The presence of these four PKS genes demonstrates the diversity of polyketide biosynthetic pathways in this fungus.

The gibberellin 20-oxidase of Gibberella fujikuroi is a multifunctional monooxygenase

The genes for gibberellin (GA) biosynthesis are clustered in the fungus Gibberella fujikuroi. In addition to genes encoding a GA-specific geranylgeranyl diphosphate synthase and a bifunctional ent-copalyl diphosphate/ent-kaurene synthase, the cluster contains four cytochrome P450 monooxygenase genes (P450-1, -2, -3, -4). Recently it was shown that P450-4 and P450-1 encode multifunctional enzymes catalyzing the three oxidation steps from ent-kaurene to ent-kaurenoic acid and the four oxidation steps from ent-kaurenoic acid to GA14, respectively. Here we describe the functional analysis of the P450-2 gene by gene disruption and by expressing the gene in a mutant that lacks the entire GA biosynthesis gene cluster. Mutants in which P450-2 is inactivated by the insertion of a large piece of DNA accumulated GA14 and lacked biosynthetically more advanced metabolites, indicating that the gene encodes a 20-oxidase. This was confirmed by incubating lines containing P450-2 in the absence of the other GA biosynthesis genes with isotopically labeled substrates. The P450-2 gene product oxidized the 3beta-hydroxylated intermediate, GA14, and its non-hydroxylated analogue GA12 to GA4 and GA9, respectively. Expression of P450-2 is repressed by high amounts of nitrogen in the culture medium but is not affected by the presence of biosynthetically advanced GAs, i.e. there is no evidence for feedback regulation. The fact that the GA 20-oxidase is a cytochrome P450 monooxygenase in G. fujikuroi and not a 2-oxoglutarate-dependent dioxygenase as in plants, together with the significant differences in regulation of gene expression, are further evidence for independent evolution of the GA biosynthetic pathways in plants and fungi.

The P450-4 gene of Gibberella fujikuroi encodes ent-kaurene oxidase in the gibberellin biosynthesis pathway

At least five genes of the gibberellin (GA) biosynthesis pathway are clustered on chromosome 4 of Gibberella fujikuroi; these genes encode the bifunctional ent-copalyl diphosphate synthase/ent-kaurene synthase, a GA-specific geranylgeranyl diphosphate synthase, and three cytochrome P450 monooxygenases. We now describe a fourth cytochrome P450 monooxygenase gene (P450-4). Gas chromatography-mass spectrometry analysis of extracts of mycelia and culture fluid of a P450-4 knockout mutant identified ent-kaurene as the only intermediate of the GA pathway. Incubations with radiolabeled precursors showed that the metabolism of ent-kaurene, ent-kaurenol, and ent-kaurenal was blocked in the transformants, whereas ent-kaurenoic acid was metabolized efficiently to GA(4). The GA-deficient mutant strain SG139, which lacks the 30-kb GA biosynthesis gene cluster, converted ent-kaurene to ent-kaurenoic acid after transformation with P450-4. The B1-41a mutant, described as blocked between ent-kaurenal and ent-kaurenoic acid, was fully complemented by P450-4. There is a single nucleotide difference between the sequence of the B1-41a and wild-type P450-4 alleles at the 3' consensus sequence of intron 2 in the mutant, resulting in reduced levels of active protein due to a splicing defect in the mutant. These data suggest that P450-4 encodes a multifunctional ent-kaurene oxidase catalyzing all three oxidation steps between ent-kaurene and ent-kaurenoic acid.

The P450-1 gene of Gibberella fujikuroi encodes a multifunctional enzyme in gibberellin biosynthesis

Recent studies have shown that the genes of the gibberellin (GA) biosynthesis pathway in the fungus Gibberella fujikuroi are organized in a cluster of at least seven genes. P450-1 is one of four cytochrome P450 monooxygenase genes in this cluster. Disruption of the P450-1 gene in the GA-producing wild-type strain IMI 58289 led to total loss of GA production. Analysis of the P450-1-disrupted mutants indicated that GA biosynthesis was blocked immediately after ent-kaurenoic acid. The function of the P450-1 gene product was investigated further by inserting the gene into mutants of G. fujikuroi that lack the entire GA gene cluster; the gene was highly expressed under GA production conditions in the absence of the other GA-biosynthesis genes. Cultures of transformants containing P450-1 converted ent-[(14)C]kaurenoic acid efficiently into [(14)C]GA(14), indicating that P450-1 catalyzes four sequential steps in the GA-biosynthetic pathway: 7beta-hydroxylation, contraction of ring B by oxidation at C-6, 3beta-hydroxylation, and oxidation at C-7. The GA precursors ent-7alpha-hydroxy[(14)C]kaurenoic acid, [(14)C]GA(12)-aldehyde, and [(14)C]GA(12) were also converted to [(14)C]GA(14). In addition, there is an indication that P450-1 may also be involved in the formation of the kaurenolides and fujenoic acids, which are by-products of GA biosynthesis in G. fujikuroi. Thus, P450-1 displays remarkable multifunctionality and may be responsible for the formation of 12 products.

Gibberellin biosynthetic pathway in Gibberella fujikuroi: evidence for a gene cluster

Differential screening of a Gibberella fujikuroi cDNA library was used to successfully clone and identify genes involved in the pathway of gibberellin biosynthesis. Several cDNA clones that hybridized preferentially to a cDNA probe prepared from mycelium induced for gibberellin production were isolated and characterized. The deduced amino acid sequences of two (identical) clones contained the conserved heme-binding motif of cytochrome P450 monooxygenases (FXXGXXXCXG). One of these cDNA fragments was used as a homologous probe for the screening of a genomic library. A hybridizing 6.7-kb genomic SalI fragment was cloned into pUC19. The sequencing of this clone revealed that a second cytochrome P450 monooxygenase gene was closely linked to the first one. Since at least four cytochrome P450 monooxygenase-catalyzed steps are involved in the synthesis of gibberellins, chromosome walking was performed to find a further gene of this family or other genes involved in gibberellin pathway. Next to the two P450 monooxygenase genes, a putative geranylgeranyl diphosphate synthase gene, the copalyl diphosphate synthase gene, which is the first specific gene of the gibberellin pathway, and a third P450 monooxygenase gene were identified. These results suggest that at least some of the genes involved in the biosynthesis of gibberellins are closely linked in a gene cluster in G. fujikuroi, as has been recently found for other "dispensable" pathways in fungi.