THE INDUCIBLE 9,10-DIHYDROPHENANTHRENE PATHWAY - CHARACTERIZATION ANDEXPRESSION OF BIBENZYL SYNTHASE AND S-ADENOSYLHOMOCYSTEINE HYDROLASE

Tricyclic 9,10-dihydrophenanthrenes originate from phenylpropane deriv atives by chain elongation and cyclization according to the polyacetat e rule. Bibenzyls are bicyclic intermediates, and O-methylation is a p rerequisite for their conversion into dihydrophenanthrenes. cDNA clone s encoding bibenzyl synthases and S-adenosylhomocysteine hydrolase of the orchid Phalaenopsis sp. were isolated from a cDNA library represen ting the stage of elicitor-induced plants. The deduced amino acid sequ ences of two clones, pBibSy811 and pBibSy212, indicated that we obtain ed two full-length sequences of bibenzyl synthases characterized by th eir homology to stilbene synthases previously investigated. That indee d bibenzyl synthase cDNAs rather than a homologous stilbene synthase c DNA or chalcone synthase cDNA have been isolated was demonstrated by e xpression of two enzymatically active bibenzyl synthase proteins in Es cherichia coli. These proteins showed virtually the same selectivity t owards m-hydroxyphenylpropionyl-CoA as substrate as the enzyme isolate d from orchid plants. In young sterile Phalaenopsis plants, the format ion of both bibenzyl synthase mRNAs and S-adenosylhomocysteine hydrola se mRNAs was increased upon elicitation more than 100-fold. The time c ourses of gene expression exhibited transient profiles, reaching maxim um mRNA levels 20 h after onset of fungal infection followed by a rapi d decline to 40 h. (C) 1995 Academic Press, Inc.