Disease relevance of nifA

• Direct response of Bradyrhizobium japonicum nifA-mediated nif gene regulation to cellular oxygen status [1].
• The nifA genes of Klebsiella pneumoniae and Bradyrhizobium japonicum were constitutively expressed from the pBR329-derived chloramphenicol resistance promoter [1].
• The Azorhizobium caulinodans nitrogen-fixation regulatory gene, nifA, is controlled by the cellular nitrogen and oxygen status [2].
• Identification of a regulatory nifA type gene and physical mapping of cloned new nif regions of Azospirillum brasilense [3].

High impact information on nifA

• The nucleotide sequence of the Azorhizobium caulinodans ORS571 nifA locus was determined and the deduced NifA amino acid sequence compared with that of NifA from other nitrogen-fixing species [2].
• Expression of the fixR-nifA operon in Bradyrhizobium japonicum depends on a new response regulator, RegR [4].
• Similarly, regR mutants showed only about 2% residual nitrogen fixation activity, but unlike nodules induced by nifA mutants, the morphology of those nodules was normal, displaying no signs of necrosis [4].
• NifA is encoded in the fixR-nifA operon, which is expressed at a low level under aerobic conditions and induced approximately fivefold under low-oxygen tension [4].
• A chromosomal deletion of the upstream activator sequence (UAS) led to a symbiotically defective phenotype which was typical of nifA mutants [5].

Biological context of nifA

• The inserts of these nifA plasmid constructs were devoid of any other intact flanking genes [1].
• Basal expression of this operon depends on the response regulator RegR and a DNA element located around position -68 in the fixR-nifA promoter region [6].
• This led to the identification of 11 critical nucleotides within a 17 bp minimal RegR binding site centered at position -64 upstream of the fixR-nifA transcription start site [7].

Regulatory relationships of nifA

• The nifA genes thus expressed led to a marked activation of a B. japonicum nifD-lacZ fusion under microaerobic conditions [1].

References