# Trepenoid Metabolic Engineering in Photosynthetic Microorganisms  
[PERSON]  
1 Australian Institute of Bioengineering and Nanotechnology, The University of Queensland, Brisbane, QLD 4072, Australia 13 CIIR  
The MEP pathway is present in most bacteria, in plastids of photosynthetic organisms, and in some eukaryotic organisms, including pathogens such as the malaria parasite. The MVA pathway appears in archaea and the cytosol of most eukaryotic organisms. Consequently, plants and other eukaryotic organisms--including some algae species--have both pathways present but in different compartments and with distinct roles (e.g. FPP is predominantly supplied by the cytosolic MVA pathway while GPP and GGPP by the chloroplast MEP pathway in plants) [2; 3; 4]. The prenyl pyrophosphates serve as substrates for specialized terpene synthases that dephosphorylate and rearrange and/or cyclize them by using the energy derived from the pyrophosphate hydrolysis to form the terpenoid skeletons. The last part of the biosynthesis, which delivers the vast terpenoid diversity, is the decoration and modification of the previously mentioned skeleton by enzymes such as cytochrome P450s, acetyl- and methyl- transferases, and cleaving enzymes (Figure 1).  
Plant-derived specialized terpenoids are often produced in low quantities or in mixtures of similar compounds (e.g. resins), which makes extraction from natural sources commercially unviable and chemical synthesis very challenging, due to the complexity of the structures of interest [5; 6; 7]. Consequently, metabolic engineers have turned to heterologous hosts for more efficient production. This, however, presents two fundamental challenges: ensuring a sufficiently high pool of isoprenoid precursors; and the efficient expression and tuning of the heterologous terpenoid pathway to achieve economically viable production.  
The industrial workhorses _Escherichia coli_ and _Saccharomyces cerevisiae_ (yeast) have been the primary target hosts. They are well-characterized heterologous systems with a defined terpenoid  
Figure 1: Biosynthesis of terpenoids. The pathways have been conceptually separated into four modules, which is representative of the modularized method many metabolic engineers use to approach isoprenoid pathway engineering. Mevalonate (MVA) and methyl-D-erythribtosphosphate (MEP) pathways lead to IPP (isopentenyl pyrophosphate) and DMAPP (dimethylallyl pyrophosphate) (**module I**). Additions of IPP produce higher-order prenyl phosphates (**module II**), dephosphorylation (often coincident with or followed by bond rearrangement and/or cyclisation) to form specialized terpenoid backbones (**module III**), chemical decorations, and other modifications to yield end products. Note that not all end products undergo decorations of the carbon skeleton.  
background, which facilitates the analytics process and allows the expression of enzymes one by one, or in combinations. This involves the biosynthetic steps of known terpenoids, and even allows the production of new-to-nature compounds [8; 9; 10; 11; 12; 13]. Key examples of pathways engineered in heterologous hosts using synthetic biology approaches include the production of artemisinin acid [14] and taxadiene [15]--precursors of the anti-malarial artemisinin and cancer chemotherapeutic taxol, respectively. There have been many successful engineering studies that have increased the range of produced compounds and our understanding on how to engineer terpenoid pathways [16; 17; 18; 19; 20].  
In this review, we summarize key advances that used metabolic engineering to both rewire the host's primary metabolism to maximize the production of isoprenoid precursors and to enable the production of the desired terpenoid product in tractable, well studied hosts _E. coli_ and yeast. We then explore advances in engineering photosynthetic microorganisms and discuss their potential as alternative production platforms for heterologous terpenoids.