Distribution and orientation of hydrogenase in various photosynthetic bacteria

All of the tested photosynthetic bacteria possessed hydrogenase activity. Cell membranes of representatives of the Chromatiaceae, Rhodospirillaceae, and Chlorobiaceae families were found to be impermeable to the oxidized redox dyes methyl viologen and benzyl viologen, whereas the reduced forms could easily penetrate the membranes. This permeability difference made possible the localization of the hydrogenase enzyme. Members of the Chromatiaceae and the Rhodospirillaceae contained a predominantly or exclusively membranebound enzyme. In contrast, the majority of hydrogenase activity was in the cytoplasm ofChlorobium limicola f. thiosulfatophilum. Hydrogenase, or at least its active center, was oriented toward the outer surface of the cell membrane in purple bacteria.