Sunday, April 29, 2007

Transcriptomic analysis of cellulose fermentation by Clostridium thermocellum

Babu Raman, Catherine K. McKeown, Steven D. Brown, and Jonathan R. Mielenz. Bioscience Division, Oak Ridge National Laboratory, Bethel Valley Road, Oak Ridge, TN 37931

The bacterium Clostridium thermocellum ferments cellulose directly to ethanol and other metabolic products using its own multi-enzyme cellulase complex, the cellulosome, without the need for external cellulase addition. Elimination of the cellulase production/digestion step consolidates the cellulosic ethanol production process and significantly reduces costs. This study aims to probe the genetic expression of C. thermocellum during cellulose fermentation using microarray technology. We compared global changes in gene expression both during the batch growth cycle (i.e., exponential vs. stationary phase) and during growth on soluble cellobiose as compared to an insoluble crystalline cellulose, Avicel. During exponential growth on Avicel, several genes involved in energy production, and core amino acid, nucleotide, lipid and coenzyme metabolism were significantly up regulated. In comparison, genes involved in signal transduction, chemotaxis and several cellulosomal enzymes were up regulated in stationary phase. A similar trend was observed during growth on cellobiose. In addition, several genes involved in cell motility were up regulated in stationary phase during cellobiose growth. Differential comparison of cellobiose and cellulose fermentations showed increased expression of genes involved in amino acid transport/metabolism and cell motility during exponential cellobiose growth. In comparison, several cellulosome-related genes were up regulated during cellulose fermentation, as would be expected. The implications of these and other findings will be discussed in relation to the potential for this microorganism for consolidated bioprocessing of lignocellulosic biomass to ethanol.