Iron nutrition and physiological responses to iron stress in Nitrosomonas europaea

Abstract

Nitrosomonas europaea, as an ammonia-oxidizing bacterium, has a high Fe requirement and has 90 genes dedicated to Fe acquisition. Under Fe-limiting conditions (0.2 microM Fe), N. europaea was able to assimilate up to 70% of the available Fe in the medium even though it is unable to produce siderophores. Addition of exogenous siderophores to Fe-limited medium increased growth (final cell mass). Fe-limited cells had lower heme and cellular Fe contents, reduced membrane layers, and lower NH3- and NH2OH-dependent O2 consumption activities than Fe-replete cells. Fe acquisition-related proteins, such as a number of TonB-dependent Fe-siderophore receptors for ferrichrome and enterobactin and diffusion protein OmpC, were expressed to higher levels under Fe limitation, providing biochemical evidence for adaptation of N. europaea to Fe-limited conditions.

Characterization of the ferrioxamine uptake system of Nitrosomonas europaea

“…The standard (Fe-replete) medium contained 10 mM Fe 3+ (FeCl 3 ) complexed with EDTA to prevent Fe from precipitation. Fe-limited medium was made from reagent-grade chemicals, without addition of any Fe salt, and contained 0.2 mM Fe (Wei et al, 2006). All media, buffers and other reagents were made in double-deionized water.…”

“…Iron acquisition by N. europaea is supported by approximately 90 genes that have high similarity to Fe uptake genes in other organisms; these genes include approximately 29 genes with similarity to TonB-dependent outer-membrane (OM) siderophore transporter genes (Chain et al, 2003;Wei et al, 2006). Genomics analysis suggests that N. europaea has the potential to take up a broad spectrum of siderophores (Wei et al, 2006). However, with the exception of genes for citrate synthesis, the N. europaea genome has no predicted genes for siderophore biosynthesis (Chain et al, 2003).…”

“…However, with the exception of genes for citrate synthesis, the N. europaea genome has no predicted genes for siderophore biosynthesis (Chain et al, 2003). Although N. europaea has a greater Fe requirement than Escherichia coli and Pseudomonas species, under Fe-limiting conditions without added siderophores, N. europaea grows moderately well by assimilating most of the Fe present in the culture medium (Wei et al, 2006). However, the cells grown under Fe-limited conditions have lower haem and cellular Fe contents and fewer membrane layers than cells grown under Fe-replete conditions, and they upregulate the production of several Fe siderophore receptors and diffusion proteins (Wei et al, 2006).…”

“…A major challenge to the characterization of the function and regulation of the OM siderophore transporters in N. europaea is the large number of such genes, and their deduced functional redundancy (Chain et al, 2003;Wei et al, 2006). For example, multiple genes are predicted to encode OM ferrichrome (hydroxamate-type) transporters, catechol-type siderophore transporters, and OM transporters for other siderophores.…”

“…However, of the hydroxamatetype siderophore transporters in N. europaea, only NE1097 and NE1088 putatively code for OM ferrioxamine transporters. These two genes are located in one of the two islands on the N. europaea chromosome where Fetransport-related genes are concentrated (Chain et al, 2003;Wei et al, 2006). NE1097 is more similar than NE1088 to the well-characterized foxA genes of other bacteria.…”