β-amyrin, a typical pentacyclic triterpene having an oleanane skeleton, is one of the most commonly occuring triterpenes in nature and is biosynthesized from (3S)-2,3-oxidosqualene. The enzyme, β-amyrin synthase, catalyzing the cyclization of oxidosqualene into β-amyrin, generates five rings and eight asymmetric centers in a single transformation. A homology-based PCR method was attempted to obtain the cDNA of this enzyme from the hairy root of Panax ginseng which produces oleanane saponins together with dammarane-type saponins. Two sets of degenerate oligonucleotide primers were designed at the regions which are highly conserved among known oxidosqualene cyclases (OSCs). Nested PCRs using these primers successfully amplified the core fragment which revealed the presence of two OSC clones PNX and PNY. Specific amplification of each clone by 3′-RACE and 5′-RACE was carried out to obtain the whole sequences. The two clones exhibited 60% amino acid identity to each other. A full-length clone of PNY was ligated into the yeast expression vector pYES2 under the GAL1 promoter to give pOSC PNY . β-amyrin production was observed with the mutant yeast lacking lanosterol synthase, transformed by this plasmid. The sequence of pOSC PNY contains an open reading frame of 2289 nucleotides which codes for 763 amino acids with a predicted molecular mass of 88 kDa. Sequence comparison with other OSCs showed a high level of similarity with lanosterol, cycloartenol and lupeol synthases. The other clone, pOSC PNX , was shown to be cycloartenol synthase by similar expression in yeast. The present studies have revealed that distinct OSC exists for triterpene formation in higher plants, and the high level of similarity with cycloartenol synthase indicates close evolutional relationship between sterol and triterpene biosynthesis.

Chalcone synthases, which biosynthesize chalcones (the starting materials for many flavonoids), have been believed to be specific to plants. However, the rppA gene from the Gram-positive, soil-living filamentous bacterium Streptomyces griseus encodes a 372-amino-acid protein that shows significant similarity to chalcone synthases. Several rppA-like genes are known, but their functions and catalytic properties have not been described. Here we show that a homodimer of RppA catalyses polyketide synthesis: it selects malonyl-coenzyme-A as the starter, carries out four successive extensions and releases the resulting pentaketide to cyclize to 1,3,6,8-tetrahydroxynaphthalene (THN). Site-directed mutagenesis revealed that, as in other chalcone synthases, a cysteine residue is essential for enzyme activity. Disruption of the chromosomal rppA gene in S. griseus abolished melanin production in hyphae, resulting in 'albino' mycelium. THN was readily oxidized to form 2,5,7-trihydroxy-1,4-naphthoquinone (flaviolin), which then randomly polymerized to form various coloured compounds. THN formed by RppA appears to be an intermediate in the biosynthetic pathways for not only melanins but also various secondary metabolites containing a naphthoquinone ring. Therefore, RppA is a chalcone-synthase-related synthase that synthesizes polyketides and is found in the Streptomyces and other bacteria.

Meroterpenoids are hybrid natural products of both terpenoid and polyketide origin. We identified a biosynthetic gene cluster that is responsible for the production of the meroterpenoid pyripyropene in the fungus Aspergillus fumigatus through reconstituted biosynthesis of up to five steps in a heterologous fungal expression system. The cluster revealed a previously unknown terpene cyclase with an unusual sequence and protein primary structure. The wide occurrence of this sequence in other meroterpenoid and indole-diterpene biosynthetic gene clusters indicates the involvement of these enzymes in the biosynthesis of various terpenoid-bearing metabolites produced by fungi and bacteria. In addition, a novel polyketide synthase that incorporated nicotinyl-CoA as the starter unit and a prenyltransferase, similar to that in ubiquinone biosynthesis, was found to be involved in the pyripyropene biosynthesis. The successful production of a pyripyropene analogue illustrates the catalytic versatility of these enzymes for the production of novel analogues with useful biological activities.

Claisen-type cyclization is assumed to be involved in formation of aromatic compounds by some fungal type I PKSs. These PKSs have a quite identical architecture of active site domain organization, beta-ketoacyl synthase, acyltransferase, tandem ACPs and thioesterase (TE) domains. Since the C-terminus region of WA PKS of this type was determined to be involved in Claisen-type cyclization of the second ring of naphthopyrone, we propose that the so far called TE of these PKSs work not just as TE but as Claisen cyclase.

The solanapyrone biosynthetic gene cluster was cloned from Alternaria solani. It consists of six genes-sol1-6-coding for a polyketide synthase, an O-methyltransferase, a dehydrogenase, a transcription factor, a flavin-dependent oxidase, and cytochrome P450. The prosolanapyrone synthase (PSS) encoded by sol1 was expressed in Aspergillus oryzae and its product was identified as desmethylprosolanapyrone I (8). Although PSS is closely related to the PKSs/Diels-Alderases LovB and MlcA of lovastatin and compactin biosynthesis, it did not catalyze cycloaddition. Sol5, encoding a flavin-dependent oxidase (solanapyrone synthase, SPS), was expressed in Pichia pastoris and purified. The purified recombinant SPS showed activity for the formation of (-)-solanapyrone A (1) from achiral prosolanapyrone II (2), establishing that this single enzyme catalyzes both the oxidation and the subsequent cycloaddition reaction, possibly as a Diels-Alder enzyme.

Site-directed mutagenesis was carried out on two triterpene synthases, β-amyrin (PNY) and lupeol (OEW) synthases, to identify the amino acid residues responsible for their product specificity. In addition to sequence comparison among known oxidosqualene cyclases, our previous chimeric studies suggested that 258 MWCYCR 263 sequence of β-amyrin synthase PNY ( 255 MLCYCR 260 sequence of lupeol synthase OEW) would participate in product differentiation. To test this hypothesis, Trp259 (MWCYCR of PNY) was mutated to Leu (PNY W259L mutant). Functional expression in yeast and product analysis revealed that this mutant produced lupeol as a major product together with β-amyrin in 2:1 ratio. Some other minor products including butyrospermol were also produced. On the other hand, Leu256 (MLCYCR of OEW) was mutated to Trp (OEW L256W mutant). This mutant produced exclusively β-amyrin with only minor amount of lupeol, demonstrating that a single mutation had engineered lupeol synthase into β-amyrin synthase. Therefore, Trp259 of β-amyrin synthase was identified to be the residue controlling β-amyrin formation presumably through stabilization of oleanyl cation, while lack of this effect by Leu residue may terminate the reaction at lupenyl cation stage. In further mutation studies, Tyr residue (MWCYCR in PNY and MLCYCR in OEW) conserved in all of the OSCs producing pentacyclic triterpenes was mutated into His which is found in all of those producing tetracyclic carbon skeletons to investigate the role of this Tyr261 of PNY. PNY Y261H mutant produced dammara-18,21-dien-3β-ol (as a 3:5 mixture of E/Z isomer at ∆ 18 ) together with a minor amount of dammara-18(28),21-dien-3β-ol, demonstrating that Tyr261 of β-amyrin synthase plays an important role in producing pentacyclic triterpenes presumably by stabilizing one of the cation intermediates generated after dammarenyl cation.

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Medermycin is a Streptomyces aromatic C-glycoside antibiotic classified in the benzoisochromanequinones (BIQs), which presents several interesting biosynthetic problems concerning polyketide synthase (PKS), post-PKS tailoring and deoxysugar pathways. The biosynthetic gene cluster for medermycin (the med cluster) was cloned from Streptomyces sp. AM-7161. Completeness of the clone was proved by the heterologous expression of a cosmid carrying the entire med cluster in Streptomyces coelicolor CH999 to produce medermycin. The DNA sequence of the cosmid (36 202 bp) revealed 34 complete ORFs, with an incomplete ORF at either end. Functional assignment of the deduced products was made for PKS and biosynthetically related enzymes, tailoring steps including strereochemical control, oxidation, angolosamine pathway, C-glycosylation, and regulation. The med cluster was estimated to be about 30 kb long, covering 29 ORFs. An unusual characteristic of the cluster is the disconnected organization of the minimal PKS genes: med-ORF23 encoding the acyl carrier protein is 20 kb apart from med-ORF1 and med-ORF2 for the two ketosynthase components. Secondly, the six genes (med-ORF14, 15, 16, 17, 18 and 20) for the biosynthesis of the deoxysugar, angolosamine, are all contiguous. Finally, the finding of a glycosyltransferase gene, med-ORF8, suggests a possible involvement of conventional C-glycosylation in medermycin biosynthesis. Comparison among the three complete BIQ gene clusters – med and those for actinorhodin (act) and granaticin (gra) – revealed some common genes whose deduced functions are unavailable from database searches (the ‘unknowns’). An example is med-ORF5, a homologue of actVI-ORF3 and gra-ORF18, which was highlighted by a recent proteomic analysis of S. coelicolor A3(2).