Metabolic engineering of Clostridium tyrobutyricum for n-butanol production

Clostridium tyrobutyricum ATCC 25755, a butyric acid producing bacterium, has been engineered to overexpress aldehyde/alcohol dehydrogenase 2 ( adhE2, Genebank no. AF321779) from Clostridium acetobutylicum ATCC 824, which converts butyryl-CoA to butanol, under the control of native thiolase ( thl) promoter. Butanol titer of 1.1 g/L was obtained in C. tyrobutyricum overexpressing adhE2. The effects of inactivating acetate kinase ( ack) and phosphotransbutyrylase ( ptb) genes in the host on butanol production were then studied. A high C4/C2 product ratio of 10.6 (mol/mol) was obtained in ack knockout mutant, whereas a low C4/C2 product ratio of 1.4 (mol/mol) was obtained in ptb knockout mutant, confirming that ack and ptb genes play important roles in controlling metabolic flux distribution in C. tyrobutyricum. The highest butanol titer of 10.0 g/L and butanol yield of 27.0% (w/w, 66% of theoretical yield) were achieved from glucose in the ack knockout mutant overexpressing adhE2. When a more reduced substrate mannitol was used, the butanol titer reached 16.0 g/L with 30.6% (w/w) yield (75% theoretical yield). Moreover, C. tyrobutyricum showed good butanol tolerance, with >80% and ∼60% relative growth rate at 1.0% and 1.5% (v/v) butanol. These results suggest that C. tyrobutyricum is a promising heterologous host for n-butanol production from renewable biomass.