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Plants belonging to the Apiaceae or Rutaceae accumulate methoxylated psoralens, such as bergapten or xanthotoxin, as the final products of their furanocumarin biosynthesis, and the rate of accumulation depends on environmental and other cues. Distinct O-methyltransferase activities had been reported to methylate bergaptol to bergapten and xanthotoxol to xanthotoxin, from induced cell cultures of Ruta graveolens, Petroselinum crispum and Ammi majus. Bergaptol 5-O-methyltransferase (BMT) cDNA was cloned from dark-grown Ammi majus L. cells treated with a crude fungal elicitor. The translated polypeptide revealed considerable sequence similarity to heterologous caffeic acid 3-O-methyltransferases (COMTs). For homologous comparison, COMT was cloned from A. majus plants and shown a 64% identity with the BMT sequence at the polypeptide level. Functional expression of both enzymes in Escherichia coli revealed that the BMT activity in the bacterial extracts was labile and rapidly lost on purification, whereas theCOMT activity remained stable. Furthermore, the recombinant AmBMT showed narrow substrate specificity for bergaptol, while the AMCOMT accepted 5-hydroxyferulic acid, esculetin and other substrates.
The AmBMT sequence thus represents a novel database accession specific for the biosynthesis of psoralens.