|n-butanol production by the different strains. Click to enlarge.|
Researchers at the Joint BioEnergy Institute (JBEI), led by Dr. Jay Keasling at UC Berkeley, have engineered the common industrial yeast Saccharomyces cerevisiae with an n-butanol biosynthetic pathway, resulting in a ten-fold improvement in n-butanol production from one of the strains to 2.5 mg/L. An open access paper on their work was published online 3 December in the journal Microbial Cell Factories.
Butanol has a number of advantages over ethanol for use as a biofuel—it is more hydrophobic; has a higher energy density; can be transported through existing pipeline infrastructure; and can be mixed with gasoline at any ratio.
A variety of Clostridial species can produce n-butanol fermentatively. However, the team notes in their paper, Clostridia are not ideal because of the relative lack of genetic tools to manipulate their metabolism, their slow growth, their intolerance to n-butanol above 1-2% and oxygen, and their production of butyrate, acetone, and ethanol as byproducts.
Recently, two groups have re-constructed the n-butanol pathway from Clostridia and measured n-butanol production in Escherichia coli (~1 mM). We chose Saccharomyces cerevisiae as a host for n-butanol production because it is a genetically tractable, well-characterized organism, the current industrial strain alcohol (ethanol) producer, and it has been previously manipulated to produce other heterologous metabolites. Since n-butanol and ethanol only differ by two carbons, S. cerevisiae may be able to tolerate high concentrations of n-butanol by the same mechanisms it tolerates ethanol.—Steen et al. 2008
The researchers used n-butanol pathway isozymes from a range of organisms, and engineered a number of different strains of S. cerevisiae. Results varied, but the best strain, ESY7, produced 2.5 mg/L n-butanol from 2% galactose as a carbon source.
Comparison of our strain to the native n-butanol producers, Clostridia (~10 g/L), or the recently engineered E. coli strains (~0.5 g/L) provides a goal for future n-butanol titer. Given the results presented above and S. cerevisiae’s other attributes—inherent tolerance to solvents, widespread use for industrial production of ethanol, and ability to withstand oxygen (as opposed to Clostridia)—S. cerevisiae may be an ideal host for industrial n-butanol production. While increases in product titer will certainly be necessary, increases of this magnitude and greater have precedence.—Steen et al. 2008
Keasling has financial interests in Amyris and LS9, both of which are involved in developing advanced renewable hydrocarbon fuels.
The Joint BioEnergy Institute (JBEI), one of three new US Department of Energy Bioenergy Research Centers, is a San Francisco Bay Area scientific partnership led by Lawrence Berkeley National Laboratory (Berkeley Lab) and including the Sandia National Laboratories (Sandia), the University of California (UC) campuses of Berkeley and Davis, the Carnegie Institution for Science and the Lawrence Livermore National Laboratory (LLNL). JBEI’s primary scientific mission is to advance the development of the next generation of biofuels.
Eric J. Steen, Rossana Chan, Nilu Prasad, Samuel Myers, Christopher J. Petzold, Alyssa Redding, Mario Ouellet, Jay D. Keasling (2008) Metabolic engineering of Saccharomyces cerevisiae for the production of n-butanol. Microbial Cell Factories 7:36 doi: doi:10.1186/1475-2859-7-36