The nifX and orfl mutants were capable of fixing nitrogen in the presence of iron or molybdenum sufficiency. However, when iron was omitted from the medium those mutants failed to reduce acetylene whereas the wild type strain had normal activity. Addition of 0.16 and 16 pg/mL of Fe.EDTA recovered 50% and full acetylene reduction activity respectively of the mutants. Both mutants were fully complemented by the nifNXorflorf2 genes. Equal expression of (3-galactosidase activity in the both mutants with lacZ fusion was observed under low and high iron conditions supporting the suggestion that nifXorfJorf2 are required for nitrogenase activity rather than nif gene expression. The nifX and orfl mutants were tested in molybdenum-free medium. The results show that both mutants had two-fold reduced nitrogenase activity under molybdenum deficiency compared to the wild type strain. The results indicate that NifX and Orfl may be involved in both iron and molybdenum mobilization or incorporation into FeMo-co in H. seropedicae.
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