THE CYDR GENE-PRODUCT, REQUIRED FOR REGULATION OF CYTOCHROME BD EXPRESSION IN THE OBLIGATE AEROBE AZOTOBACTER-VINELANDII, IS AN FNR-LIKE PROTEIN

The cytochrome bd complex in the obligately aerobic diazotroph Azotoba cter vinelandii is an oxidase, which, in vivo, has a low affinity for oxygen and is required for respiratory protection of nitrogenase. Muta tions caused by insertion of Tn5-B20 upstream of the structural genes (cydAB) for cytochrome bd result in over-expression of this oxidase an d, for unexplained reasons, inability of the organism to grow microaer obically. Cloning and sequencing of this upstream region revealed a ge ne, cydR. The deduced amino acid sequence of CydR indicates that it is a new member of the Fnr class of regulators and that it represses cyd AB expression. Refined mapping data for three insertions in cydR are p resented. The cloned cydR gene complemented anaerobic growth of Escher ichia coil fnr mutants and strongly enhanced expression of a narG-lacZ fusion in an E. coil fnr mutant.