Engineering Escherichia coli to Tolerate Ionic Liquids for Biofuel Production
Ionic liquids (IL) are a class of environmentally friendly solvents that are effective at loosening cellulose from lignin in plant biomass. This is an important step in the production of biofuels as it makes cellulose available for breakdown into its component sugars. The sugars are fermented into biofuels by microbes such as Escherichia coli. While most of the IL is recovered from the processed biomass, some remains and can inhibit the growth of E. coli and the enzymes that convert cellulose into biofuel, greatly reducing yields of biofuel product. To address this inhibition, scientists at the U.S. Department of Energy’s Joint BioEnergy Institute (JBEI) looked for genes that might confer tolerance on the E. coli to the ILs. They looked to Enterobacter lignolyticus, a bacterium known to grow in the presence of ILs. First, they moved large parts of the E. lignolyticus genome into E. coli and asked the E. coli to grow in the presence of the IL. Several colonies were found to now tolerate the IL; each colony had two E. lignolyticus genes in common, an efflux pump gene and its regulator. Efflux pumps confer tolerances by transporting toxic compounds out of the cell into the medium. To determine if the tolerance conferring efflux pump could improve biofuel synthesis in the presence of IL, the efflux pump genes were placed together in a strain of E. coli engineered to produce a biofuel precursor, bisabolene. The resulting strain was able to produce more bisabolene in the presence of much greater amounts of IL than the E. coli strain without the efflux pump. An E. coli strain that tolerates ILs and synthesizes bisabolene means that ILs can be used to treat biomass to free cellulose from lignin without negatively impacting subsequent biofuel production. This can reduce biofuel production costs because extra expense is not needed to remove the last amounts of IL from the processed biomass. As cellulosic biofuel production plants come online, such adaption of technological advances like these that will improve their economic viability.
Rüegg, T. L., E.-M. Kim, B. A. Simmons, J. D. Keasling, S. W. Singer, T. S. Lee, and M. P. Thelen. 2014. “An Auto-Inducible Mechanism for Ionic Liquid Resistance in Microbial Biofuel Production,” Nature Communications 5. DOI:10.1038/ncomms4490.