A three enzyme system to generate the Strychnos alkaloid scaffold from a central biosynthetic intermediate.

Monoterpene indole alkaloids comprise a diverse family of over 2000 plant-produced natural products. This pathway provides an outstanding example of how nature creates chemical diversity from a single precursor, in this case from the intermediate strictosidine. The enzymes that elicit these seemingly disparate products from strictosidine have hitherto been elusive. Here we show that the concerted action of two enzymes commonly involved in natural product metabolism-an alcohol dehydrogenase and a cytochrome P450-produces unexpected rearrangements in strictosidine when assayed simultaneously. The tetrahydro-ß-carboline of strictosidine aglycone is converted into akuammicine, a Strychnos alkaloid, an elusive biosynthetic transformation that has been investigated for decades. Importantly, akuammicine arises from deformylation of preakuammicine, which is the central biosynthetic precursor for the anti-cancer agents vinblastine and vincristine, as well as other biologically active compounds. This discovery of how these enzymes can function in combination opens a gateway into a rich family of natural products.The biosynthetic pathway of preakuammicine, a monoterpene precursor of the anti-cancer agent vinblastine, has remained largely unexplored. Here, the authors provide transcriptomic and biochemical data to identify two enzymes that, in tandem, convert strictosidine to akuammicine, the stable shunt product of preakuammicine.

ITS2 secondary structure for species circumscription: case study in southern African Strychnos L. (Loganiaceae).

Recently developed computational tools in ITS2 sequence-structure phylogenetics are improving tree robustness by exploitation of the added information content of the secondary structure. Despite this strength, however, their adoption for species-level clarifications in angiosperms has been slow. We investigate the utility of combining ITS2 sequence and secondary structure to separate species of southern African Strychnos, and assess correlation between compensatory base changes (CBCs) and currently recognised species boundaries. Combined phylogenetic analysis of sequence and secondary structure datasets performed better, in terms of robustness and species resolution, than analysis involving primary sequences only, achieving 100 and 88.2 % taxa discriminations respectively. Further, the Strychnos madagascariensis complex is well-resolved by sequence-structure phylogenetic analysis. The 17 Strychnos species corresponded to 14 ITS2 CBC clades. Four of the five taxa in section Densiflorae belong to a single CBC clade, whose members tend to form natural hybrids. Our finding supports the application of ITS2 as a complementary barcoding marker for species identification. It also highlights the potential of comparative studies of ITS2 CBC features among prospective parental pairs in breeding experiments as a rapid proxy for cross compatibility assessment. This could save valuable time in crop improvement. Patterns of CBC evolution and species boundaries in Strychnos suggests a positive correlation. We conclude that the CBC pattern coupled with observed ITS2 sequence paraphyly in section Densiflorae points to a speciation work-in-progress.