Abstract: MohC is required for expression of uptake hydrogenase in A. vinelandii and inactivation of this gene results in H2 evolution via nitrogenase 3. Since MohC? mutants are tungsten tolerant under nitrogen-fixing conditions, tungsten can be used to select spontaneous tungsten-tolerant mutants of nitrogen-fixing bacteria that are genetically uncharacterized. A major advantage of producing H2 via nitrogenase 3 is that carbon substrates used to culture the nitrogen-fixing bacteria harboring this nitrogenase are derived from plant sources.