Genomic organization of plant terpene synthases and molecular evolutionaryimplications

Terpenoids are the largest, most diverse class of plant natural products an d they play numerous functional roles in primary metabolism and in ecologic al interactions. The first committed step in the formation of the various t erpenoid classes is the transformation of the prenyl diphosphate precursors , geranyl diphosphate, farnesyl diphosphate, and geranylgeranyl diphosphate , to the parent structures of each type catalyzed by the respective monoter pene (C-10), sesquiterpene (C-15), and diterpene synthases (C-20). Over 30 cDNAs encoding plant terpenoid synthases involved in primary and secondary metabolism have been cloned and characterized. Here we describe the isolati on and analysis of six genomic clones encoding terpene synthases of conifer s, [(-)-pinene (C-10), ( -)-limonene(C-10), (E)-alpha -bisaboIene (C-15), d elta -selinene (C-15) and abietadiene synthase (C-20) from Abies grandis an d taxadiene synthase (C-20) from Taxus brevifolia], all of which are involv ed in natural products biosynthesis. Genome organization (intron number, si ze, placement and phase, and exon size) of these gymnosperm terpene synthes es was compared to eight previously characterized angiosperm terpene syntha se genes and to six putative terpene synthase genomic sequences from Arabid opsis thaliana. Three distinct classes of terpene synthase genes were disce rned, from which assumed patterns of sequential intron loss and the loss of an unusual internal sequence element suggest that the ancestral terpenoid synthase gene resembled a contemporary conifer diterpene synthase gene in c ontaining at least 12 introns and 13 exons of conserved size. A model prese nted for the evolutionary history of plant terpene synthases suggests that this superfamily of genes responsible for natural products biosynthesis der ived from terpene synthase genes involved in primary metabolism by duplicat ion and divergence in structural and functional specialization. This novel molecular evolutionary approach focused on genes of secondary metabolism ma y have broad implications for the origins of natural products and for plant phylogenetics in general.