Role of a Fur homolog in iron metabolism in Nitrosomonas europaea

Of the three N. europaea fur homologs, only the Fur homolog encoded by gene NE0616 complemented the Escherichia coli H1780 fur mutant. A N. europaea fur:kanP mutant strain was created by insertion of kanamycin-resistance cassette in the promoter region of NE0616 fur homolog. The total cellular iron contents of the fur:kanP mutant strain increased by 1.5-fold compared to wild type when grown in Fe-replete media. Relative to the wild type, the fur:kanP mutant exhibited increased sensitivity to iron at or above 500 μM concentrations. Unlike the wild type, the fur:kanP mutant was capable of utilizing iron-bound ferrioxamine without any lag phase and showed over expression of several outer membrane TonB-dependent receptor proteins irrespective of Fe availability.Our studies have clearly indicated a role in Fe regulation by the Fur protein encoded by N. europaea NE0616 gene. Additional studies are required to fully delineate role of this fur homolog.The molecular basis for the coordinated regulation of iron acquisition systems by iron was first described for Escherichia coli [1]. Several bacteria are now known to regulate their iron acquisition systems via Fur (ferric uptake regulator) [2-5]. Fur is a sequence-specific DNA-binding protein that acts mainly as a negative regulator of transcription in vivo by complexing with ferrous (Fe2+) ion to repress the expression of iron-regulated genes [6]. Fur also activates the expression of many genes by either indirect or direct mechanisms [7]. Mutations in the fur gene resulted in constitutive expression of siderophores and outer membrane Fe3+-siderophore receptors potentially required for iron uptake [8].Nitrosomonas europaea is an aerobic chemolithoautotroph that uses NH3 and CO2 for growth [9]. Mechanisms for iron transport are essential to this bacterium for maintaining the many cytochromes and other heme-binding proteins involved in ammonia metabolism [10,11]. The genome of N. europaea has 4% of its genes dedicated for iron