Summary: In ORS571, which is able to form both root and stem nodules on the tropical plant and to grow in the free-living state at the expense of N, two copies of the gene, which codes for the nitrogenase Fe-protein, were characterized. One copy, , was localized in the operon; the other, , was localized elsewhere. and differed in only six nucleotides and both encoded a polypeptide of 296 amino acids with a single change at position 282, a serine in and a threonine in At position 102, arginine in the sequence Gly-Arg-Gly-Val-Ile-Thr might, as in , be the site of ‘switch-off’ inactivation. Compared with other genes, the highest S was found with and sp. (). A sequence identical () or very similar () to the consensus sequence was found upstream from the initiation codon. Homology with sequences upstream from promoters, which affect -mediated activation, was also found in both cases. From mutants carrying either a single or deletion, or a double deletion, it appeared that both genes were functional and and that no other copy seemed to be present in ORS571. The optimal expression of and of might depend on different physiological conditions.


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