Engineering Clostridium thermocellum for Enhanced Ethanol Production

Clostridium thermocellum is an anaerobic thermophilic bacterium with the ability to produce ethanol. However, its application as biocatalyst for ethanol production is limited since pyruvate ferredoxin oxidoreductase, which directs pyruvate into the ethanol production pathway, has low affinity to the substrate. Hence, researchers from SRM University in India aimed to enhance the ethanol production of C. thermocellum.

The pyruvate carboxylase (pdc) and alcohol dehydrogenase (adh) genes from Zymomonas mobilis were cloned and transformed to Clostridium thermocellum DSM 1313 to generate recombinant CTH-pdc, CTH-adh and CTH-pdc-adh strains that carried heterologous pdc, adh, and both genes, respectively.

Although both pdc and adh were functional in C. thermocellum, the presence of adh severely limited the growth of the recombinant strains, regardless of the presence of the pdc gene. Whereas, the recombinant CTH-pdc strain showed two-fold increase in pyruvate carboxylase activity and ethanol production when compared with the wild type strain.

Pyruvate decarboxylase gene of the homoethanol pathway from Z. mobilis was functional in recombinant C. thermocellum strain and enhanced its ability to produce ethanol. Strain improvement and bioprocess optimizations may further increase the ethanol production from this recombinant strain.