The NifM Protein

The role of the nifM product in the maturation of the Fe-protein component of nitrogenase is obscure. Thus far, the nifM gene has been cloned from Klebsiella pneumoniae, A. vinelandii and A. chroococcum. A comparison of the amino acid sequences deduced from the nucleotide sequences of these genes clearly demonstrates the existence of a very low level of interspecies sequence identity. This lack of sequence identity could be the reason for the inability to identify nifM homologs in various nitrogen-fixing bacteria by using m/M-specific DNA probes. In fact, the confirmation that the nifM DNA cloned from A. vinelandii or A. chroococcum has the same function as that of the nifM gene of K. pneumoniae was established only after demonstrating that the nifM gene from A. vinelandii and A. chroococcum could compliment a point mutation in the nifM DNA of K pneumoniae. However, the carboxyl terminal regions of these nifM products do show significant homology suggesting that the active portion of the nifM product is likely to be located near the C-terminal region of the polypeptide. Characterization of the activities of the nitrogenase components in nifM mutants of K. pneumoniae revealed that such strains are primarily deficient in Fe-protein activity (Roberts, Brill 1981).

The role of different nif genes in the maturation of nitrogenase component proteins was examined by expressing the nifH gene of K. pneumoniae in an E. coli background in combination with these different nif genes. This was accomplished by using a binary plasmid system (Howard et al. 1986). The nifH gene's expression was conducted under anaerobic conditions; and it was observed that its expression in E. coli in the absence of the nifM gene resulted in the accumulation of only very low levels of the Fe-protein polypeptide. This Fe-protein had no detectable activity as determined by an in vitro C2H2-reduction assay. These results suggested that the nifM gene product plays a role in conferring activity and some stability to the Fe-protein. Since isolated Fe-protein does not contain any NifM protein, it is unlikely that NifM is a subunit of the Fe-protein complex. Moreover, it was demonstrated that when the nifH gene was expressed in foreign hosts (E. coli and yeast) in the absence of the nifM gene product, the homodimers of the Fe-protein were still detectable. Therefore, the role of the NifM protein could be to impart activity and stability to the Fe-protein through some sort of catalytic event.

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