Hydrogen Formation and Its Regulation in Ruminococcus albus: Involvement of an Electron-Bifurcating [FeFe]-Hydrogenase, of a Non-Electron-Bifurcating [FeFe]-Hydrogenase, and of a Putative Hydrogen-Sensing [FeFe]-Hydrogenase

ABSTRACT Ruminococcus albus 7 has played a key role in the development of the concept of interspecies hydrogen transfer. The rumen bacterium ferments glucose to 1.3 acetate, 0.7 ethanol, 2 CO 2 , and 2.6 H 2 when growing in batch culture and to 2 acetate, 2 CO 2 , and 4 H 2 when growing in continuous culture in syntrophic association with H 2 -consuming microorganisms that keep the H 2 partial pressure low. The organism uses NAD + and ferredoxin for glucose oxidation to acetyl coenzyme A (acetyl-CoA) and CO 2 , NADH for the reduction of acetyl-CoA to ethanol, and NADH and reduced ferredoxin for the reduction of protons to H 2 . Of all the enzymes involved, only the enzyme catalyzing the formation of H 2 from NADH remained unknown. Here, we report that R. albus 7 grown in batch culture on glucose contained, besides a ferredoxin-dependent [FeFe]-hydrogenase (HydA2), a ferredoxin- and NAD-dependent electron-bifurcating [FeFe]-hydrogenase (HydABC) that couples the endergonic formation of H 2 from NADH to the exergonic formation of H 2 from reduced ferredoxin. Interestingly, hydA2 is adjacent to the hydS gene, which is predicted to encode an [FeFe]-hydrogenase with a C-terminal PAS domain. We showed that hydS and hydA2 are part of a larger transcriptional unit also harboring putative genes for a bifunctional acetaldehyde/ethanol dehydrogenase (Aad), serine/threonine protein kinase, serine/threonine protein phosphatase, and a redox-sensing transcriptional repressor. Since HydA2 and Aad are required only when R. albus grows at high H 2 partial pressures, HydS could be a H 2 -sensing [FeFe]-hydrogenase involved in the regulation of their biosynthesis.