ent-Kaurene Synthase from the Fungus Phaeosphaeria sp. L487 cDNA ISOLATION, CHARACTERIZATION, AND BACTERIAL EXPRESSION OF A BIFUNCTIONAL DITERPENE CYCLASE IN FUNGAL GIBBERELLIN BIOSYNTHESIS*

ent-Kaurene is the first cyclic diterpene intermediate of gibberellin biosynthesis in both plants and fungi. In plants, ent-kaurene is synthesized from geranylgeranyl diphosphate via copalyl diphosphate in a two-step cyclization catalyzed by copalyl diphosphate synthase and ent-kaurene synthase. A cell-free system of the fungus Phaeosphaeria sp. L487 converted labeled geranylgeranyl diphosphate to ent-kaurene. A cDNA fragment, which possibly encodes copalyl diphosphate synthase, was isolated by reverse transcription-polymerase chain reaction using degenerate primers based on the consensus motifs of plant enzymes. Translation of a full-length cDNA sequence isolated from the fungal cDNA library revealed an open reading frame for a 106-kDa polypeptide. The deduced amino acid sequence shared 24 and 21% identity with maize copalyl diphosphate synthase and pumpkin ent-kaurene synthase, respectively. A fusion protein produced by expression of the cDNA in Escherichia coli catalyzed the two-step cyclization of geranylgeranyl diphosphate to ent-kaurene. Amo-1618 completely inhibited the copalyl diphosphate synthase activity of the enzyme at 10 M, whereas it did not inhibit the ent-kaurene synthase activity even at 10 M. These results indicate that the fungus has a bifunctional diterpene cyclase that can convert geranylgeranyl diphosphate into ent-kaurene. They may be separate catalytic sites for the two cyclization reactions.

Gibberellins (GAs) 1 are one of an important group of phytohormones regulating many aspects of plant growth and development. Some fungal species produce GAs as secondary metabolites (1). Gibberella fujikuroi is a rice pathogenic fungus producing high amount of GAs. The fermentation and biosyn-thesis of GA in G. fujikuroi were well characterized, since some GAs are produced industrially using the fungus (2,3).
GA is unequivocally synthesized from ent-kaurene in both fungi and plants (4,5). ent-Kaurene is a tetracyclic diterpene hydrocarbon formed from geranylgeranyl diphosphate (GGDP) via copalyl diphosphate (CDP). The pathway of ent-kaurene biosynthesis was first confirmed using cell-free systems from G. fujikuroi (6). The two-step cyclization was thought to involve two different enzymes: copalyl diphosphate synthase (CPS, formerly ent-kaurene synthase A) and ent-kaurene synthase (KS, formerly ent-kaurene synthase B). These enzymes were partially purified from Fusarium moniliforme, an anamorph of G. fujikuroi. (7). It was suggested that CPS and KS in the fungus might be tightly associated, because the two activities could not be separated. On the other hand, the two activities were successfully separated from a plant enzyme preparation from Marah macrocarpus by chromatographic methods (8). The two enzymes are possibly localized in plastids (9). Quite recently, genes encoding both enzymes have been cloned from plants: CPS from Arabidopsis (10), maize (11) and pea (12), and KS from pumpkin (13). Several other GA biosynthetic enzymes have been cloned from plants as well (14). In contrast, no GA biosynthetic enzymes have been cloned from fungi.
GA biosynthesis in another GA-producing fungus, Phaeosphaeria sp. L487 has been characterized recently (15). This fungus produces significant amounts of GA 1 through a pathway similar to that in higher plants: namely, 3␤-hydroxylation of GA 9 and GA 20 occurs to form GA 4 and GA 1 , respectively (15,16). This contrasts to G. fujikuroi, in which 3␤-hydroxylation occurs early in the pathway of GA 12 -aldehyde and GA 9 is not converted to GA 4 . Although ent-kaurene is an intermediate of GA biosynthesis in both fungi and plants (17), we were unable to compare the characteristics of the plant CPS and KS with fungal CPS and KS at molecular levels. Little is known about enzymes and genes involved in ent-kaurene biosynthesis of Phaeosphaeria sp. L487. In addition, none of the diterpene cyclases from fungi have been isolated yet. Therefore, we focused on fungal-type CPS (FCPS) of Phaeosphaeria sp. L487. In this paper we report cDNA cloning and characterization of an enzyme involved in ent-kaurene biosynthesis in the fungus Phaeosphaeria sp. L487.  (18). Amo-1618 was obtained from Serva (Heidelberg).

EXPERIMENTAL PROCEDURES
Preparation of Cell-free Extracts-Eleven-day-old mycelium of Phaeosphaeria sp. L487 grown as described (15) was harvested by filtration and homogenized in potassium phosphate buffer (50 mM, pH 8.0) containing dithiothreitol (2 mM), pepstatin A (0.1 mM), and (p-amidinophenyl) methanesulfonyl fluoride hydrochloride (10 mM) using Dyno-Mill (Willy A. Bachofen, Basel, Switzerland). The supernatant after centrif-* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
The nucleotide sequence(s) reported in this paper has been submitted to the GenBank TM /EBI Data Bank with accession number(s) AB003395.
Polymerase Chain Reaction-Two degenerate oligonucleotides were synthesized based on the common amino acid sequences of plant CPSs (12,13,14). The degenerate primers used for the amplification were 5Ј-GCITA(T/C)GA(T/C)ACIGCITGGGT-3Ј and 5Ј-(A/G)AAIGCCATIG-CIGT(A/G)TC(A/G)TC-3Ј. PCR was performed in a 50-l reaction volume using Expand HF buffer with 1.5 mM MgCl 2 , 0.25 mM dNTP, 8 M of each primer, 1.75 units of the enzyme (Expand HF, Boehringer Mannheim) and 1 l of double-stranded cDNA solution as described below with the following program: denaturation at 94°C for 1 min, annealing at 46°C for 1 min, extension at 72°C for 2 min (35 cycles), then final extension at 72°C for 7 min. PCR products were directly cloned into pCRII vector (Invitrogen).
cDNA Library Construction and Screening-Double-stranded cDNA was synthesized from 4.9 g of poly(A) ϩ RNA by reverse transcription with Superscript II (Life Technologies, Inc.) followed by DNA polymerase (Takara) and RNase H treatments. The cDNA, ligated with EcoRI/ NotI adaptor (Pharmacia Biotech Inc.), was size fractionated using a SizeSep 400 spun column (Pharmacia) and ligated to EcoRI-digested ZAP II (Stratagene). The phage was packaged in vitro with Gigapack III Gold (Stratagene) and transfected into E. coli XL1-Blue. Hybond N ϩ membrane (Amersham Corp.) replicas were screened with the PCR product, which had been labeled by random priming (rediprime DNA labeling system, Amersham). Hybridization was performed at 68°C for 16 h in Rapid-hyb buffer (Amersham) containing salmon sperm DNA (200 g/ml). The filters were washed with 2 ϫ SSC at 68°C for 15 min, with 1 ϫ SSC, 0.1% SDS at 68°C for 30 min then with 0.1 ϫ SSC, 0.1% SDS at 68°C for 30 min. The positive plaques were purified by three more rounds of screening as described above. The cDNA was in vivo excised into pBluescript II SK(Ϫ).
Sequencing of cDNA-To determine the nucleotide sequence of FCPS cDNA, deletion clones were prepared from the pBluescript-inserted cDNA with ExoIII/mung bean nuclease deletion kit (Stratagene) according to manufacturer's instruction. FCPS cDNA and its deletion clones were sequenced using the dye primer cycle sequencing method (PRISM 377, Applied Biosystems).
To align the FCPS sequence with other sequences, the Genetics Computer Group programs (University of Wisconsin) were used. Homology search was carried out using the BLAST program. To make phylogenetic tree, PHYLIP programs (version 3.5) were used (J. Felsenstein, University of Washington, Seattle).
Isolation of the 5Ј Region of the cDNA-The sequence of 5Ј end of the FCPS cDNA was determined using Marathon cDNA amplification kit (CLONTECH). The first strand cDNA was reverse-transcribed from poly(A) ϩ RNA (0.5 g) using a gene specific primer 1: 5Ј-CCAGCTAC-CGTCCTCGCCTTG-3Ј (complementary to nucleotides 202-222). Double-stranded cDNA with adaptors was prepared according to the manufacturer's instruction and subjected to a PCR using adaptor primer 1: (5Ј-CCATCCTAATACGACTCACTATAGGGC-3Ј and a gene-specific primer 2: 5Ј-GGCCGCCCAGGCTGTGTCGTAGAT-3Ј (complementary to nucleotides 106 -129). The PCR-amplified DNA fragment (ϳ300 bp) was directly cloned into the pCR II vector.
RNA Blot Analysis-Poly(A) ϩ RNA (0.3 g), prepared from 6-day-old mycelium of Phaeosphaeria, was size-fractionated with a 0.8% agarose gel containing formaldehyde and blotted on to a Hybond N ϩ membrane. The membrane was hybridized using 32 P-labeled probe from PCR products (873 bp) by random priming at 68°C and finally washed with 0.1 ϫ SSC containing 0.1% SDS at 68°C for 30 min.
Expression of Protein and Preparation of Fungal CPS-The cDNA inserted in pBluescript was digested with NotI. The digested DNA fragment was ligated to NotI-digested pGEX 4T-3 (Pharmacia). E. coli JM109 cells harboring the ligated plasmid, pGEX-FCPS, was grown in Luria-Bertani medium containing ampicillin (50 g/ml) and 0.2% glucose at 37°C. When A 600 reached 0.7, expression of the glutathione S-transferase (GST) fusion protein was induced by addition of isopropyl-1-thio-␤-D-galactoside (IPTG) to a final concentration of 1 mM. The cells were collected after 24-h induction at 20°C and washed with 50 mM phosphate buffer, pH 8.0. Cells were resuspended in a lysis buffer (50 mM potassium phosphate buffer, pH 8.0, 10% glycerol, 2 mM dithiothreitol, 5 mM MgCl 2 ) and sonicated on ice. After centrifugation at 15,000 ϫ g for 25 min, the supernatant was used as a crude soluble fraction. The protein concentration of the solution was estimated with the Bio-Rad protein standard assay as described above. The soluble protein was analyzed by SDS-polyacrylamide gel electrophoresis with Coomassie Brilliant Blue staining (21). The GST-fusion protein was detected by Western blot analysis using GST detection module (Pharmacia).

Conversion of [ 3 H]CDP, [ 3 H]GGDP, and Unlabeled GGDP-CPS
and KS activities of the cell-free system and the recombinant protein were measured as described previously (15,18). In the case of the cell-free system, each substrate was incubated in 100 l of the S-200 enzyme solution containing Mg 2ϩ (5 mM) and uniconazole (1 mM), as ent-kaurene oxidase inhibitor (22), at 30°C for 30 min. A ent-kaurene fraction after incubation was extracted with n-hexane and separated on silica gel TLC with n-hexane. The radioactivity of the ent-kaurene region on the TLC plate (R F 0.6 -1.0) was measured by liquid scintillation counting of the silica gel.
To identify metabolites by gas chromatograph-mass spectrometry (GC-MS), GGDP (2 g) was incubated in 100 l of enzyme solution containing Mg 2ϩ (5 mM) and uniconazole (1 mM) at 30°C for 1 h. The incubation was stopped by heating (65°C) for 5 min. Six units of bacterial alkaline phosphatase (Takara) was added into the reaction mixture and incubated overnight at 37°C. After incubation, 100 l of acetone was added to terminate the hydrolysis reaction. The mixture was extracted three times with 100 l of n-hexane. The n-hexane extracts were concentrated under gentle nitrogen flow and injected into a 15-m fused silica capillary column (DB-1, J & W Scientific, Folsom, CA) coupled to an ion-trap GC-MS (GCQ, Thermo Quest, San Jose, CA).  Table I). The denatured enzyme by heating was used as negative controls. The activities were lost by adding 20 mM EDTA. This effect of EDTA indicated that the enzyme requires a divalent metal ion such as Mg 2ϩ , for the cyclization, as has been reported for other plant terpene cyclases (23,24). To confirm the production of ent-kaurene from GGDP by the enzyme, the n-hexane extract from the reaction with unlabeled GGDP was analyzed by full-scan GC-MS. The product was detected at 6:02 after injection: m/z 272 ([M] ϩ ; relative intensity, 32%); m/z 257 (relative intensity, 100%), m/z 229 (relative intensity, 97%), and m/z 213 (relative intensity, 93%). This compound was identified as ent-kaurene by comparing with full mass spectrum of the authentic compound. These results demonstrated that the presence of both CPS and KS activities in mycelia of Phaeosphaeria sp. L487. These activities were found exclusively in the S-200 fraction, suggesting that the enzymes for ent-kaurene biosynthesis are soluble. This is also the case for in G. fujikuroi and higher plant enzymes (17).

CPS and KS Activities in a Cell
Isolation and Characterization of cDNA Clone-To design ) was added to the soluble enzyme preparation (S-200, 1.0 g l Ϫ1 , 100 l total), which was then incubated at 30°C for 30 min. The ent-kaurene fraction from a n-hexane extract was separated by silica gel TLC and analyzed by liquid scintillation counting.  291 amino acid residues that showed significant similarity to sequences of CPS (10 -12), KS (13), and abietadiene synthase (AS) (25). RNA blot analysis was performed using poly(A) ϩ RNA prepared from 6-day-old mycelium of Phaeosphaeria to detect a 3-kilobase transcript that hybridized with the PCR product (data not shown).
Using this PCR product as a hybridization probe, 2.5 ϫ 10 5 plaques from a cDNA library constructed in ZAP II were screened. Four positive clones were isolated and further purified. One of the clones, pFCPS, was further characterized. Sequence analysis revealed that the 3,034 bp pFCPS cDNA insert contained an open reading frame of 2,838 bp encoding 946 amino acid residues. To determine the 5Ј end of the FCPS cDNA, 5Ј-rapid amplification of cDNA ends experiments were performed. The 5Ј-end of the FCPS transcript was mapped at 24 bp upstream from the 5Ј end of pFCPS. Since there was no ATG codon in the extended sequence, we concluded that the 2,838-bp open reading frame encodes FCPS. The complete sequence of FCPS cDNA is shown in Fig. 1.
The predicted amino acid sequence of FCPS has significant homology with those of diterpene cyclases from plants; 20% identity with AS (grand fir, Ref. 25), 20% with casbene synthase (castor bean, Ref. 26), 24% with maize CPS (11), 22% with pumpkin KS (13), and 21% with taxadiene synthase (pacific yew, Ref. 27). The FCPS sequence is not closely related (ϳ19% identity) to any of the microbial sesquiterpene cyclases (28 -30). Sequence alignment with plant diterpene cyclases revealed that the motifs, YDTAW, QXXDGSW, and DVDDTA, were conserved in the FCPS sequence (Fig. 2). The electronrich aromatic residue of the QXXDGSW motif is proposed to stabilize intermediate cation during the cyclization process (31). FCPS contains aspartate-and aspartate/glutamate-rich motifs (DDVLD: amino acids 132-136 and DEVIDEVVD: amino acids 683-691, Fig. 2). These functions have been strongly suggested to mediate substrate binding by chelation of the divalent metal ion (32,33). The transit peptide in plant diterpene cyclases is rich in serine and threonine residues, with a net positive charge (34), is found in the amino-terminal region. However, the amino-terminal region of FCPS was not rich in serine and threonine. The calculated pI of the first 30 amino acids of the FCPS was 4.8 (complete amino acid sequence: calculated pI ϭ 5.2). Therefore, the transit peptide region is not present in FCPS (Fig. 1).
Expression and Functional Analysis of the Recombinant Pro-tein in E. coli-The pFCPS was introduced into pGEX 4T-3 and the GST-FCPS fusion protein was expressed in E. coli after induction with 1 mM IPTG. The IPTG-induced protein was detected in the soluble fraction as a 132-kDa band on SDSpolyacrylamide gel electrophoresis. It was confirmed to be GST-FCPS by Western blot analysis using GST detection module (data not shown). The soluble enzyme fraction containing GST-FCPS was used for enzyme assays. Interestingly, when [ 3 H]GGDP was incubated with the soluble enzyme, radioactivity showing formation of a diterpene hydrocarbon was detected by TLC analysis. Furthermore, when [ 3 H]CDP was incubated with the enzyme, radioactivity associated with the ent-kaurene-like compound was also detected. To confirm production of ent-kaurene from GGDP by the FCPS, full-scan GC-MS analysis was carried out. To identify residual GGDP and CDP as geranylgeraniol and copalol by GC-MS, respectively, the reaction mixture after incubation of unlabeled GGDP with the enzyme was treated with alkaline phosphatase. After the hydrolysis, nonpolar compounds were extracted with n-hexane. After full-scan GC-MS analysis, ent-kaurene was identified. At the same time, copalol was also detected in the reaction mixture (Fig. 3). When pea CPS (PsCPS) fusion protein was incubated under the same condition, conversion of GGDP to CDP, but not to ent-kaurene, occurred (Fig. 3). Furthermore, when pumpkin KS (CmKS) fusion protein was added to PsCPS fusion protein, ent-kaurene was produced (Fig. 3). The KS activity of the FCPS was confirmed using [ 3 H]CDP (Table II). Thus, it is clear that the FCPS fusion protein catalyzes both the cyclization of GGDP to CDP and that of CDP to ent-kaurene (Fig. 4). Hence, here we changed the name of FCPS to fungal-type ent-kaurene synthase or FCPS/KS as a bifunctional enzyme.
Amo-1618 is a quaternary ammonium compound that inhibits CPS activity (7,35). When Amo-1618 was added to the  (Table II). This suggested that sites for CPS and KS activities are distinct in FCPS/KS. DISCUSSION We report the isolation and characterization of a cDNA encoding the fungal ent-kaurene synthase, FCPS/KS, from Phaeosphaeria sp. L487. The genes for the fungal sesquiterpene cyclases, trichodiene synthase from Fusarium sporotrichoides (28) and aristolochene synthase from Penicillium roqueforti, have been characterized (29). However, although many fungal species produce interesting bioactive diterpene compounds, such as the GAs and fusicoccins (36), none of the genes encoding diterpene cyclases have been isolated from fungi. Thus, this is the first report of the cloning of a diterpene cyclase of fungal origin. We initially focused on the cDNA cloning of fungal CPS, since the information about the sequence of CPS was obtained from various plants. The predicted amino acid sequence of FCPS/KS isolated from Phaeosphaeria sp. L487 has homology with the plant CPS and KS. The phylogenetic tree (Fig. 5) indicates that FCPS/KS has a significant evolutional relationship with plant CPS. However, whereas both CPS and KS have a transit peptide in their amino-terminal regions, this is absent in FCPS/KS. FCPS/KS contains neither membrane translocation sequences or membrane spanning regions. Therefore, FCPS/KS is likely to be localized in the cytoplasm, as are the fungal sesquiterpene cyclases (29). This is also consistent with the presence of enzyme activity in the soluble fraction of the cell-free preparation from Phaeosphaeria sp. L487. In plants, all GGDP-derived carbon skeletons are synthesized within plastids and sesquiterpene hydrocarbons within the cytoplasm or endoplasmic reticulum (37).
We revealed that FCPS/KS is a bifunctional cyclase having both activities of CPS and KS in plants. FCPS/KS catalyzes the two-step cyclization of GGDP to ent-kaurene via CDP with a single polypeptide. Amo-1618 inhibits CPS activity in FCPS/KS at low concentrations. In contrast, it does not inhibit the KS activity, even at high concentrations. Similar inhibition characteristics are observed in enzyme preparations of G. fujikuroi and Marah macrocarpus (35). It has been suggested that, in plants, CPS and KS may interact with each other to synthesize ent-kaurene from GGDP (6). This suggests that the catalytic sites for the two cyclizations are separated. Furthermore, the similar modes of action of Amo-1618 on ent-kaurene biosynthesis in fungi and plants indicate that FCPS/KS of Phaeosphaeria sp. L487 and CPS-KS complex of plants have similar structure. At present, the two active sites responsible for CPS and KS activities within the FCPS/KS cannot be determined by sequence comparison. In contrast, AS is also a bifunctional enzyme which catalyzes the cyclization of GGDP to abietadiene via labdadienyl diphosphate (25). The aminoterminal region of AS, containing the DXDDTA motif, resembles CPS, whereas the carboxyl-terminal region, containing the DDXXD motif, resembles KS (Fig. 2). The two active sites within the AS suggested a fusion of elements of CPS-type enzymes and KS-type enzymes (25). Thus, there are some examples where biosynthetic genes are separate in one organism but linked in another (38 -40). FCPS/KS is an attractive model for understanding of structure-activity relationship of the enzymes involved in ent-kaurene biosynthesis between fungi and plants. To understand the catalytic sites of FCPS/KS, more detail studies will be required. Comparison of FCPS/KS with diterpene cyclases from G. fujikuroi and other fungi will be necessary for understanding the evolution of GA biosynthesis in fungi.  Ϫ1 ) was added to the soluble enzyme preparation (3.4 g l Ϫ1 , 100 l total), which was then incubated at 30°C for 30 min. The ent-kaurene fraction was analyzed as described in Table I