Ferredoxin:NAD+ Oxidoreductase of Thermoanaerobacterium saccharolyticum and its Role in Ethanol Formation

Ferredoxin:NAD+ oxidoreductase (NADH-FNOR) catalyzes the transfer of electrons from reduced ferredoxin to NAD+. This enzyme has been hypothesized to be the main enzyme responsible for ferredoxin oxidization in the NADH-based ethanol pathway in Thermoanaerobacterium saccharolyticum, however, the corresponding gene has not yet been identified. Here, we identified the Tsac_1705 protein as a candidate FNOR based on the homology of its functional domains. We then confirmed its activity in vitro with a ferredoxin-based FNOR assay. To determine its role in metabolism, the tsac_1705 gene was deleted in different strains of T. saccharolyticum. In wild-type T. saccharolyticum, deletion of tsac_1705 resulted in a 75% loss of NADH-FNOR activity which indicated that Tsac_1705 is the main NADH-FNOR in T. saccharolyticum. When both NADH and NADPH-linked FNOR genes were deleted, ethanol titer decreased, and the ratio of ethanol to acetate approached unity, indicative of the absence of FNOR activity. Finally, we tested the effect of heterologous expression of Tsac_1705 in C. thermocellum and found improvements in both the titer and the yield of ethanol.