Two clusters containing nitrogen fixation genes have been cloned. The general organization is very similar to that of Azotobacter vinelandii. A cluster carries nitrogenase structural genes (nifHDK), nijTY and nifNE (AJ313205). A second cluster carries nifLA and nifBQ (AJ307878). nifA is located 7 kb upstream of nifB. Inactivation of nifA, nifB, nifH and nifE led to a Nif negative phenotype while a nifY mutant remained Nif' . A second copy of nifY was found upstream of nifH as well as a set of genes sharing identity with the rnfCDGEFH operon (AJ297529), involved in electron transport to nitrogenase in Rhodobacter capsulatus. Different mutants have been constructed and it was found that the rnf cluster is required for nitrogen fixation in P. stutzeri.
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