Key Genome for Ethanol-Tolerance in Mutated Clostridium thermocellum

Fuels from cellulosic biomass are one of the leading alternatives in meeting sustainability and energy security challenges associated with fossil fuels. Biological conversion processes involving microbial fermentation (after biomass pretreatment) are among the leading options for producing these cellulosic biofuels. However, in the use of microorganisms for biofuel production, the microorganism's low ethanol tolerance (i.e. the maximum ethanol concentration in the fermentation medium in which the microorganism could still produce ethanol) limits the maximum yield of the bioethanol product during fermentation.

A team of researchers from the Oak Ridge National Laboratory (ORNL), the Thayer School of Engineering, Dartmouth College and the University of Tennessee (United States), recently discovered a single key gene responsible for the high ethanol tolerance in Clostridium thermocellum. In their study, the researchers first resequenced the genome of an ethanol tolerant mutant and compared it with the normal strain of Clostridium thermocellum. Then, mutations in the genome were identified and mutations associated with ethanol metabolism/tolerance were pinpointed. The researchers verified the genome's ethanol-tolerant trait by putting a particular copy of the pinpointed alcohol tolerance gene into a wild type strain. They then determined the transformed organism's capability to grow in elevated levels of ethanol.

From the results of their study, the researchers found that the increased ethanol tolerance of the C. thermocellum is due to a mutated bifunctional acetaldehyde-CoA/alcohol dehydrogenase gene (adhE) and the possible mechanism for the increased ethanol tolerance is the alteration of the NADH cofactor binding specificity in the protein. The simplicity of the genetic basis for the ethanol-tolerant phenotype observed, according to the researchers, is a major breakthrough that could improve cellulosic ethanol production by the utilization of more ethanol-tolerant mutant microbial strains. The full paper is published in the journal, Proceedings of the National Academy of Sciences of the United States of America (PNAS) (URLabove).