Final Report Summary - PHOTOBIOFUEL (Direct photobiological conversion of solar energy to volatile transport fuels) The ultimate goal of the project was to construct strains of cyanobacteria that are capable of propane biosynthesis. Since no such metabolic pathway was known at the start of the project, we initially developed 5 pathways to butanol and propane in an Escherichia coli model system, including a search for required enzymes. As part of this work we have identified unexpected but highly useful substrate promiscuity in carboxylic acid reductases and thioesterases. When these pathways were transferred to cyanobacteria, stable strains could not be obtained. Using the insight and materials developed from the construction of propane pathways, we also implemented pathways to 1-octanol, heptane, and a range of medium-chain length alkanes and alcohols. In addition, we have studied the mechanism for cellular efflux of these molecules and identified (1) a transporter with specificity for medium-chain length products and (2) that even very small molecules also depend on active excretion for their accumulation to be possible. Further engineering of cyanobacteria will be facilitated by understanding of (and methods to overcome) factors that influence genetic (in)stability.
