Molecular cloning of columbamine O-methyltransferase from cultured Coptis japonica cells.

To identify all of the O-methyltransferase genes involved in isoquinoline alkaloid biosynthesis in Coptis japonica cells, we sequenced 1014 cDNA clones isolated from high-alkaloid-producing cultured cells of C. japonica. Among them, we found all three reported O-methyltransferases and an O-methyltransferase-like cDNA clone (CJEST64). This cDNA was quite similar to S-adenosyl-l-methionine:coclaurine 6-O-methyltransferase and S-adenosyl-l-methionine:isoflavone 7-O-methyltransferase. As S-adenosyl-l-methionine:columbamine O-methyltransferase, which catalyzes the conversion of columbamine to palmatine, is one of the remaining unelucidated components in isoquinoline alkaloid biosynthesis in C. japonica, we heterologously expressed the protein in Escherichia coli and examined the activity of columbamine O-methyltransferase. The recombinant protein clearly showed O-methylation activity using columbamine, as well as (S)-tetrahydrocolumbamine, (S)-, (R,S)-scoulerine and (R,S)-2,3,9,10-tetrahydroxyprotoberberine as substrates. This result clearly indicated that EST analysis was useful for isolating the candidate gene in a relatively well-characterized biosynthetic pathway. The relationship between the structure and substrate recognition of the O-methyltransferases involved in isoquinoline alkaloid biosynthesis, and a reconsideration of the biosynthetic pathway to palmatine are discussed.     

Molecular cloning and characterization of CYP719, a methylenedioxy bridge-forming enzyme that belongs to a novel P450 family,from cultured Coptis japonica cells

Two cytochrome P450 (P450) cDNAs involved in the biosynthesis of berberine, an antimicrobial benzylisoquinoline alkaloid, were isolated from cultured Coptis japonica cells and characterized. A sequence analysis showed that one C. japonica P450 (designated CYP719) belonged to a novel P450 family. Further, heterologous expression in yeast confirmed that it had the same activity as a methylenedioxy bridge-forming enzyme (canadine synthase), which catalyzes the conversion of (S)-tetrahydrocolumbamine ((S)-THC) to (S)-tetrahydroberberine ((S)-THB, (S)-canadine). The other P450 (designated CYP80B2) showed high homology to California poppy (S)-N-methylcoclaurine-3'-hydroxylase (CYP80B1), which converts (S)-N-methylcoclaurine to (S)-3'-hydroxy-N-methylcoclaurine. Recombinant CYP719 showed typical P450 properties as well as high substrate affinity and specificity for (S)-THC. (S)Scoulerine was not a substrate of CYP719, indicating that some other P450, e.g. (S)-cheilanthifoline synthase, is needed in (S)-stylopine biosynthesis. All of the berberine biosynthetic genes, including CYP719 and CYP80B2, were highly expressed in selected cultured C. japonica cells and moderately expressed in root, which suggests coordinated regulation of the expression of biosynthetic genes.