Klebsiella pneumoniae nifM gene product is required for stabilization and activation of nitrogenase iron protein in Escherichia coli

A series of plasmids encoding various Klebsiella pneumoniae nif (nitrogen fixation) genes were constructed to determine which were required to produce active iron (Fe) protein in Escherichia coli, a species which does not normally fix nitrogen. The greatest success was achieved with binary plasmid systems that produced nifA regulatory protein under the control of a tac promoter on one plasmid, which then induced synthesis of nifH and nifM proteins from their native promoter sites on a second plasmid. nifH protein, the monomeric subunit of Fe protein, produced in the presence of nifM constituted nearly 10% of the whole cell protein and exhibited the corresponding amount of C2H2-reducing activity in nitrogenase assays conducted in vitro. nifH protein formed in the absence of nifM constituted 4.7% of the whole cell protein and exhibited no detectable activity in assays of whole cell extracts. The plasmid-encoded Fe protein was purified to homogeneity and was found to be indistinguishable from that isolated from derepressed wild type K. pneumoniae, having a similar specific activity, approximately 4 Fe/dimer of 68 kDa, and similar epr features. Although these experiments do not exclude the participation of other E. coli gene products in the maturation of nifH protein, they limit the nif-specific genes required for active Fe protein production to nifA, nifH, and nifM. Since nifA is thought to be the required activator protein involved in nif operon transcription, the simplest explanation for these observations is that nifH codes for the peptide of the Fe protein, while nifM acts to convert this nifH peptide to the functioning Fe protein of nitrogenase. In the absence of nifM, only an inactive nifH polypeptide is produced.