17-37: Engineering Saccharomyces cerevisiae fermentative and nonfermentative pathways for the production of isobutanol

The rising cost of petroleum derived fuels and concerns about carbon emissions have driven the search for alternative transportation fuels and sources of chemicals currently obtained from petroleum that can be produced from rapidly renewable resources like cellulosic biomass. Isobutanol is a potential liquid transport fuel and it along with its precursor, isobutyraldehyde are valuable platform chemicals. These compounds can both be produced in yeast by the bioconversion of carbohydrate or amino acid precursors. We demonstrate that yeast can be engineered to produce isobutanol either by fermentation of carbohydrate precursors or by bioconversion of amino acids via the Ehrlich pathway. Additionally, we demonstrate that isobutanol tolerant strains of S. cerevisae can be generated, predominantly through modification of the plasma membrane fatty acid composition. Through upregulation of the valine biosynthetic pathway we achieved isobutanol titres approaching 30mg/L. Amplification of the Ehrlich pathway yielded isobutanol titres of greater than 300mg/L. We systematically identified metabolic limitations to isobutanol production through fermentative and nonfermentative routes and demonstrate strain improvements to overcome those limitations. Conclusion: Yeast has potential as a chassis for the production of higher alcohol biofuels; however substantial engineering and strain improvement will be required to achieved economically viable production levels.