Regulation of solvent formation in solventogenic Clostridia

Clostridium acetobutylicum, Clostridium saccharobutylicum and Clostridium beijerinckii are solventogenic Clostridia that grow by a butyric acid fermentation on carbohydrates. In late exponential and stationary phase they take up the acids produced and convert them into the solvents butanol and acetone. This metabolism has been used in biotechnology in order to produce solvents from carbohydrates in large scale fermentations starting during world war I, until the 1960ies. Due to a falling price for petrochemical products this process was substituted later on only to be reestablished these days when mineral oil prices are on a sharp rise. To circumvent competition between foodstuff and fuel production, modern solvent fermentation processes plan to use mainly lignocellulosic material such as straw as a substrate.

Our lab works on the regulation of central metabolism of C. acetobutylicum during acidogenic and solventogenic growth. Using DNA microarrays we studied the transcriptional response to the switch in continuous cultures and are currently analyzing the consequences of various mutations for the central metabolism. Very interesting is the role of the alcohol dehydrogenases adhE1 and adhE2 for the switch to solventogenesis. Another focus is solvent formation from xylose, a major component of hemicellulosic material and the role of the etfAB dependant dehydrogenases in C. acetobutylicum. Energy metabolism in solventogenic Clostridia is an important topic as the species differ in a significant aspect: C. acetobutylicum contains no Rnf complex for conserving additional energy from reduced ferredoxin during fermentation while the other species of solventogenic Clostridia do. This seems to have important consequences for the lifestyle of these organisms. To better understand these differences that could have direct implications in solvent formation we are currently establishing genetic systems and methods for clean deletions in C. beijerinckii and C. saccharobutylicum. For the latter organism we are involved in sequencing the whole genome.