Free-living cells as well as bacteroids of Bradyrhizobium japonicum are able to oxidize H2evolved from nitrogenase reaction. Low partial pressure of O2in the presence of H2and low concentration of carbon substrates were considered to be required for the expression of the uptake hydrogenase (Hup) activity in free-living B. japonicum (Maier et al. 1978). However, the Hup activity reported by Maier et al. (1979) was lower than that of bacteroids (McCrae et al. 1978). When we examined the conditions of derepression for higher Hup activity in free-living B. japonicum cells, we observed that the addition of NH4C1 enhanced the Hup activity. Therefore, we decided to investigate the effects of nitrogen sources on the derepression of the Hup activity in free-living B. japonicum.
- Bradyrhizobium japonicum