Summary: The cytochrome complex in the obligately aerobic diazotroph is an oxidase, which, has a low affinity for oxygen and is required for respiratory protection of nitrogenase. Mutations caused by insertion of Tn5-B20 upstream of the structural genes () for cytochrome result in over-expression of this oxidase and, for unexplained reasons, inability of the organism to grow microaerobically. Cloning and sequencing of this upstream region revealed a gene, The deduced amino acid sequence of CydR indicates that it is a new member of the Fnr class of regulators and that it represses expression. Refined mapping data for three insertions in are presented. The cloned gene complemented anaerobic growth of mutants and strongly enhanced expression of a fusion in an mutant.


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