Taxol (paclitaxel) is a potent anticancer drug first isolated from the Taxus brevifolia Pacific yew tree. Currently, cost-efficient production of Taxol and its analogs remains limited. Here, we report a multivariate-modular approach to metabolic-pathway engineering that succeeded in increasing titers of taxadiene-the first committed Taxol intermediate-approximately 1 gram per liter (~15,000-fold) in an engineered Escherichia coli strain. Our approach partitioned the taxadiene metabolic pathway into two modules: a native upstream methylerythritol-phosphate (MEP) pathway forming isopentenyl pyrophosphate and a heterologous downstream terpenoid-forming pathway. Systematic multivariate search identified conditions that optimally balance the two pathway modules so as to maximize the taxadiene production with minimal accumulation of indole, which is an inhibitory compound found here. We also engineered the next step in Taxol biosynthesis, a P450-mediated 5α-oxidation of taxadiene to taxadien-5α-ol. More broadly, the modular pathway engineering approach helped to unlock the potential of the MEP pathway for the engineered production of terpenoid natural products.Taxol (paclitaxel) and its structural analogs are among the most potent and commercially successful anticancer drugs (1). Taxol was first isolated from the bark of the Pacific yew tree (2), and early-stage production methods required sacrificing two to four fully grown trees to secure sufficient dosage for one patient (3). Taxol's structural complexity limited its chemical synthesis to elaborate routes that required 35 to 51 steps, with a highest yield of 0.4% (4-6). A semisynthetic route was later devised in which the biosynthetic intermediate baccatin III, isolated from plant sources, was chemically converted to Taxol (7). Although this approach and subsequent plant cell culture-based production efforts have decreased the need for harvesting the yew tree, production still depends on plant-based processes (8), with

Engineering robust microbes for the biotech industry typically requires high-level, genetically stable expression of heterologous genes and pathways. Although plasmids have been used for this task, fundamental issues concerning their genetic stability have not been adequately addressed. Here we describe chemically inducible chromosomal evolution (CIChE), a plasmid-free, high gene copy expression system for engineering Escherichia coli. CIChE uses E. coli recA homologous recombination to evolve a chromosome with approximately 40 consecutive copies of a recombinant pathway. Pathway copy number is stabilized by recA knockout, and the resulting engineered strain requires no selection markers and is unaffected by plasmid instabilities. Comparison of CIChE-engineered strains with equivalent plasmids revealed that CIChE improved genetic stability approximately tenfold and growth phase-specific productivity approximately fourfold for a strain producing the high metabolic burden-biopolymer poly-3-hydroxybutyrate. We also increased the yield of the nutraceutical lycopene by 60%. CIChE should be applicable in many organisms, as it only requires having targeted genomic integration methods and a recA homolog.

Terpenoids represent a diverse class of molecules that provide a wealth of opportunities to address many human health and societal issues. The expansive array of structures and functionalities that have been evolved in nature provide an excellent pool of molecules for use in human therapeutics. While this class of molecules has members with therapeutic properties including anticancer, antiparasitic, antimicrobial, antiallergenic, antispasmodic, antihyperglycemic, anti-inflammatory, and immunomodulatory properties, supply limitations prevent the large scale use of some molecules. Many of these molecules are only found in ppm levels in nature thus requiring massive harvesting to obtain sufficient amounts of the drug. Synthetic biology and metabolic engineering provide innovative approaches to increase the production of the desired molecule in the native organism, and most importantly, transfer the biosynthetic pathways to other hosts. Microbial systems are well studied, and genetic manipulations allow the optimization of microbial metabolisms for the production of common terpenoid precursors. Using a host of tools, unprecedented advancements in the large scale production of terpenoids have been achieved in recent years. Identification of limiting steps and pathway regulation, coupled with design strategies to minimize terpenoid byproducts wih a high flux to the desired biosynthetic pathways, have yielded greater than 100-fold improvements in the production of a range of terpenoids. This review focuses on the biodiversity of terpenoids, the biosynthetic pathways involved, and engineering efforts to maximize the production through these pathways.