Recently, Fix- mutants of Rhizobium meliloti 41 defective for nifHDK transcription in the bacteroid state have been described. Two of these mutants have been used to identify bacterial genes involved in the regulation of nif gene expression. A nifA::lacZ fusion was introduced into the mutant strains and β-galactosidase activity was assayed in nodule bacteria, as well as in bacteria grown under microaerobic conditions. One of the mutants did not express the nifA gene in symbiosis, suggesting that the gene inactivated by mutation fix-24 is involved in controlling the expression of the nif structural genes via the regulatory gene nifA, The mutation fix-24 also impaired the expression of nifA under microaerobic conditions. These data are in agreement with earlier findings that low oxygen concentration may serve as a signal for nif gene expression in symbiosis. The fix gene marked by the mutation fix-24 might be a positive regulator of nifA expression in R. meliloti 41. The other mutation (fix-25) represented another cluster of fix genes which also affected the expression of nifA. This influence, however, was specific for symbisis. The fix genes (fix-24, fix-25) were localized on the symbiotic megaplasmid pRme41 b. The two genes are 10 kb apart from each other and are located at 200 kb downstream of the nif structural genes in R. meliloti 41.
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