REGULATION OF METABOLIC SHIFTS IN CLOSTRIDIUM-ACETOBUTYLICUM ATCC-824

Alcohol formation was initiated in continuous cultures of Clostridium acetobutylicum under distinct steady-state conditions: (i) in glucose- limited cultures established at low operating pH with formation of but anol, ethanol and acetone (induction of the solventogenesis) in which cells contained normal levels of NADH and a high level of ATP and buty ric acid; and (ii) by increasing the NADH pressure at neutral pH in gl ucose-limited cultures after addition of Neutral red, or in glucose-gl ycerol or glucose-glycerol-pyruvate grown cultures, with a strictly al cohologenic metabolism (no acetone produced) associated with high leve ls of intracellular NADH and various levels of ATP. These two differen t metabolic shift systems are correlated with the expression of differ ent genes involved in the solvent-forming pathways and the electron fl ow distribution. A high NADH level leading to butanol and ethanol form ation was accompanied by increased activities of the NADH-dependent al cohol and butyraldehyde dehydrogenases, and ferredoxin:NAD(P)(+) reduc tases, and by decreased activities of the NADH:ferredoxin reductase. T his last group of enzymes constitutes the key enzymes regulating elect ron flow, since no change in hydrogenase activity was observed. On the other hand, classical solventogenesis appears to be characterized by high levels of expression of the NADPH-dependent alcohol and butyralde hyde dehydrogenases, and of the two enzymes involved in the acetone-fo rming pathway, while the ferredoxin:NAD(P)(+) reductases were not synt hesized. A decrease of the in vitro hydrogenase activity explains the lower hydrogen generation. In addition, the regulation of the intracel lular pH was different between the alcohologenic culture grown at neut ral pH and the solventogenic cultures grown at low pH. An inversion of the transmembrane pH gradient was observed during the production of a lcohol at neutral pH and was related to a lower in vivo specific rate of hydrogen production while in the cultures grown at low pH the trans membrane pH generation was not linked to the F1F0 ATPase activity.