Direct response of Bradyrhizobium japonicum nifA-mediated nif gene regulation to cellular oxygen status.

The nifA genes of Klebsiella pneumoniae and Bradyrhizobium japonicum were constitutively expressed from the pBR329-derived chloramphenicol resistance promoter. The inserts of these nifA plasmid constructs were devoid of any other intact flanking genes. The nifA genes thus expressed led to a marked activation of a B. japonicum nifD-lacZ fusion under microaerobic conditions. Under aerobic growth conditions, however, activation was mediated only by the K. pneumoniae nifA gene but not by the B. japonicum nifA gene. This selective effect was observed in both the Escherichia coli as well as the B. japonicum backgrounds. Several lines of evidence suggest that in these experiments oxygen adversely affects B. japonicum nifA-dependent nif gene regulation at the post-transcriptional level, probably even at the post-translational level, and that this effect does not require a nifL-like gene. Models are proposed in which oxygen inhibits the B. japonicum NifA protein either directly or indirectly via other cellular components involved in general protein oxidation pathways.