The inducible 9, 10-dihydrophenanthrene pathway: characterization and expression of bibenzyl synthase and S-adenosylhomocysteine hydrolase.

Abstract Tricyclic 9,10-dihydrophenanthrenes originate from phenylpropane derivatives by chain elongation and cyclization according to the polyacetate rule. Bibenzyls are bicyclic intermediates, and O-methylation is a prerequisite for their conversion into dihydrophenanthrenes. cDNA clones encoding bibenzyl synthases and S -adenosylhomocysteine hydrolase of the orchid Phalaenopsis sp. were isolated from a cDNA library representing the stage of elicitor-induced plants. The deduced amino acid sequences of two clones, pBibSy811 and pBibSy212, indicated that we obtained two full-length sequences of bibenzyl synthases characterized by their homology to stilbene synthases previously investigated. That indeed bibenzyl synthase cDNAs rather than a homologous stilbene synthase cDNA or chalcone synthase cDNA have been isolated was demonstrated by expression of two enzymatically active bibenzyl synthase proteins in Escherichia coli . These proteins showed virtually the same selectivity towards m -hydroxyphenylpropionyl-CoA as substrate as the enzyme isolated from orchid plants. In young sterile Phalaenopsis plants, the formation of both bibenzyl synthase mRNAs and S -adenosylhomocysteine hydrolase mRNAs was increased upon elicitation more than 100-fold. The time courses of gene expression exhibited transient profiles, reaching maximum mRNA levels 20 h after onset of fungal infection followed by a rapid decline to 40 h.