The dependence on iron availability of allocation of iron to nitrogenase components in Klebsiella pneumoniae and Escherichia coli.

Nitrogenase contains approximately 38 iron ions/complete unit. Therefore, we sought to identify steps and genes involved in nitrogenase production that are responsive to iron availability. We have characterized nitrogenase production in Klebsiella pneumoniae grown in a range of different iron concentrations. We find significant accumulation (50-75%) and normal synthesis rates of the structural polypeptides, even under conditions in which the observed nitrogenase activities are only 14-28% of those observed in iron-sufficient conditions. Thus, maturation instead of synthesis of the structural polypeptides is primarily responsible for the iron dependence of nitrogenase activity. We have also used a binary plasmid system in Escherichia coli to investigate the contributions of various nitrogen fixation (nif) genes to the iron dependence of nitrogenase production. At least one of the nif genes DKTYENXUSVW can modulate synthesis of the structural polypeptide NIF H in response to iron availability. We speculate that an iron-deficient complex of the product(s) of at least one of these genes may repress structural polypeptide synthesis in iron-depleted K. pneumoniae. Such a system would compensate for the inactivity of NIF L in iron-depleted cultures and ensure balanced production of the structural polypeptides of nitrogenase in accordance with the iron available for their maturation.