Sustainable heterologous production of terpene hydrocarbons in cyanobacteria

Cyanobacteria can be exploited as photosynthetic platforms for heterologous generation of terpene hydrocarbons with industrial application. However, the slow catalytic activity of terpene synthases (k (cat) = 4 s(-1) or slower) makes them noncompetitive for the pool of available substrate, thereby limiting the rate and yield of product generation. Work in this paper applied transformation technologies in Synechocystis for the heterologous production of beta-phellandrene (monoterpene) hydrocarbons. Conditions were defined whereby expression of the beta-phellandrene synthase (PHLS), as a CpcB center dot PHLS fusion protein with the beta-subunit of phycocyanin, accounted for up to 20 % of total cellular protein. Moreover, CpcB center dot PHLS was heterologously co-expressed with enzymes of the mevalonic acid (MVA) pathway and geranyl-diphosphate synthase, increasing carbon flux toward the terpenoid biosynthetic pathway and enhancing substrate availability. These improvements enabled yields of 10 mg of beta-phellandrene per g of dry cell weight generated in the course of a 48-h incubation period, or the equivalent of 1 % beta-phellandrene:biomass (w:w) carbon-partitioning ratio. The work helped to identify prerequisites for the efficient heterologous production of terpene hydrocarbons in cyanobacteria: (i) requirement for overexpression of the heterologous terpene synthase, so as to compensate for the slow catalytic turnover of the enzyme, and (ii) enhanced endogenous carbon partitioning toward the terpenoid biosynthetic pathway, e.g., upon heterologous co-expression of the MVA pathway, thereby supplementing the native metabolic flux toward the universal isopentenyl-diphosphate and dimethylallyl-diphosphate terpenoid precursors. The two prerequisites are shown to be critical determinants of yield in the photosynthetic CO2 to terpene hydrocarbons conversion process.