Oxygen protection of nitrogenase in Frankia sp. HFPArI3

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Abstract
O2 protection of nitrogenase in a cultured Frankia isolate from Alnus rubra (HFPArI3) was studied in vivo. Evidence for a passive gas diffusion barrier in the vesicles was obtained by kinetic analysis of in vivo O2 uptake and acetylene reduction rates in response to substrate concentration. O2 of NH 4 + -grown cells showed an apparent K m O2 of approximately 1μM O2. In N2-fixing cultures a second K m O2 of about 215 μM O2 was observed. Thus, respiration remained unsaturated by O2 at air-saturation levels. In vivo, the apparent K m for acetylene was more than 10-fold greater than reported in vitro values. These data were inter oreted as evidence for a gas diffusion barrier in the vesicles but not vegetative filaments of Frankia sp. HFPArI3.