Production of fatty acid-derived fuels and chemicals in cyanobacteria

Photosynthetic microorganisms, such as cyanobacteria, serve as promising hosts for the production of fuels and chemicals. In the last decade, cyanobacteria have been engineered to produce either short chain-length (£C4) or long chain-length (3C16) molecules. The development of microbial biosynthesis of short chain-length alcohols (e.g. ethanol and n-butanol), however, suffers from the high energy requirements in the separation processes. Similarly, longer chain-length molecules (i.e. pentadecane and heptadecane) are usually produced in low titer. In this work, to produce molecules suitable for fuel replacement with higher productivity, we created several synthetic metabolic pathways in cyanobacteria for the production of: (1) 1-octanol, (2) hydrocarbons (e.g. terminal alkenes and fatty alka(e)nes), and (3) fatty acid methyl esters (FAMEs). The systematic engineering of 1-octanol-producing strain resulted in the improvement of the titer from 3.5 mg/L to 520 mg/L. The highest productive pathway for hydrocarbon production was achieved by combining TesA (a thioesterase from Escherichia coli) with UndB (a desaturase-like enzyme from Pseudomonas mendocina) or FAP (a fatty acid photodecarboxylase from Chlorella variabilis). The combination of Tes12 (a thioesterase from Umbellularia californica) and UndB also resulted in the extracellular production of 1-undecene and 1-tridecene. Lastly, the first engineered cyanobacteria for the methanol-free biosynthesis of FAME was demonstrated by combining Tes12 and DmJHAMT (a juvenile hormone acid O-methyltransferase from Drosophila melanogaster). By the use of specific solvent overlay, 1-octanol, 1-undecene, 1-tridecene, and methyl laurate could be captured and recovered from the liquid medium. This study pioneers the creation of synthetic metabolic pathways for 1-octanol, terminal alkenes, fatty alkanes, and fatty acid methyl esters in photosynthetic microorganisms, paving the path for the development of cyanobacteria as a biocatalyst for the production of drop-in fuels from carbon dioxide.Open Acces