Functional Bradyrhizobium japonicum NifA expression under a hybrid nptII-nifH promoter in E. coli and Acetobacter diazotrophicus SRT4.
A hybrid promoter consisting of the in tandem fusion of the Tn5 nptII and the Klebsiella pneumoniae nifH promoters was constructed to study the functionality of the nif genes transcriptional activator NifA from Bradyrhizobium japonicum in two different host bacteria. beta-galactosidase experiments in E. coli revealed that the hybrid nptII-nifH promoter can behave as a constitutive or a NifA-inducible promoter depending on the aeration conditions. Expression of the B. japonicum NifA from the hybrid nptII-nifH promoter (plasmid pBPF204) induced "in trans" lacZ transcription from the Azotobacter chroococcum nifH promoter in E. coli and A. diazotrophicus cells grown at low pO2. Similarly, the plasmid pBPF204 increased nitrogenase activity in A. diazotrophicus cells grown under microaerobic conditions. Based on these results, we suggest that the B. japonicum NifA could function as an efficient O2-sensitive transcriptional activator of nif genes in genetically distant diazotrophic bacteria.