Abstract
Clostridium thermocellum is a thermophilic, anaerobic, cellulolytic bacterium that produces ethanol and acetic acid as major fermentation end products. The effect of growth conditions on gene expression in C. thermocellum ATCC 27405 was studied using cells grown in continuous culture under cellobiose or cellulose limitation over a approximately 10-fold range of dilution rates (0.013 to 0.16 h(-1)). Fermentation product distribution displayed similar patterns in cellobiose- or cellulose-grown cultures, including substantial shifts in the proportion of ethanol and acetic acid with changes in growth rate. Expression of 17 genes involved or potentially involved in cellulose degradation, intracellular phosphorylation, catabolite repression, and fermentation end product formation was quantified by real-time PCR, with normalization to two calibrator genes (recA and the 16S rRNA gene) to determine relative expression. Thirteen genes displayed modest (fivefold or less) differences in expression with growth rate or substrate type: sdbA (cellulosomal scaffoldin-dockerin binding protein), cdp (cellodextrin phosphorylase), cbp (cellobiose phosphorylase), hydA (hydrogenase), ldh (lactate dehydrogenase), ack (acetate kinase), one putative type IV alcohol dehydrogenase, two putative cyclic AMP binding proteins, three putative Hpr-like proteins, and a putative Hpr serine kinase. By contrast, four genes displayed >10-fold-reduced levels of expression when grown on cellobiose at dilution rates of >0.05 h(-1): cipA (cellulosomal scaffolding protein), celS (exoglucanase), manA (mannanase), and a second type IV alcohol dehydrogenase. The data suggest that at least some cellulosomal components are transcriptionally regulated but that differences in expression with growth rate or among substrates do not directly account for observed changes in fermentation end product distribution.