Role of transcription and enzyme activities in redistribution of carbon and electron flux in response to N2 and H2 sparging of open-batch cultures of Clostridium thermocellum ATCC 27405

Growth, end-product synthesis, enzyme activities, and transcription of select genes associated with the "malate shunt," pyruvate catabolism, H-2 synthesis, and ethanol production were studied in the cellulolytic anaerobe, Clostridium thermocellum ATCC 27405, during open-batch fermentation of cellobiose to determine the effect of elevated N-2 and H-2 gas sparging on metabolism using a 14-L fermenter with a 7-L working volume. The metabolic shift from acetate, H-2, and CO2 to ethanol and formate in response to high H-2 versus high N-2 sparging (20 mL s(-1)) was accompanied by (a) a 2-fold increase in nicotinamide adenine dinucleotide (NADH)-dependent alcohol dehydrogenase (Adh) activity, (b) a 10-fold increase in adhE transcription, and (c) a 3-fold decrease in adhZ transcription. A similar, but less pronounced, metabolic shift was also observed when the rate of N-2 sparging was decreased from 20 to 2 mL s(-1), during which (a) NADH-dependent ADH and pyruvate: ferredoxin oxidoreductase (PFOR) activities increased by similar to 1.5-fold, (b) adhY transcription increased 6-fold, and (c) transcription of selected pfor genes increased 2-fold. Here we demonstrate that transcription of genes involved in ethanol metabolism is tightly regulated in response to gas sparging. We discuss the potential impacts of dissolved H-2 on electron carrier (NADH, NADPH, ferredoxin) oxidation and how these electron carriers can redirect carbon and electron flux and regulate adhE transcription.