Physiological characterisation of an Azotobacter vinelandii nifU ‐deletion mutant and its spontaneous Nif + revertants that over‐produce cytochrome bd

A nifU ‐deletion mutant of Azotobacter vinelandii fixed N 2 , but only under low atmospheric O 2 (2 kPa), whereas, under air, it reverted to Nif + at 5×10 −8 . The revertant's O 2 ‐tolerant nitrogenase activity, surprisingly, was not accompanied by an increased respiration rate, although, like cydR mutants, the revertants over‐produce cytochrome bd . The introduction of a cydR mutation into the nifU mutant yielded transformants, of which 100% fixed N 2 in air. This is consistent with the revertant mutations residing in cydR . Inactivation of CydR (a Fnr‐like transcriptional repressor) could lead to the up‐regulation of a process (e.g. IscU activity in i ron‐ s ulfur c luster formation) that substitutes for NifU.