Abstract
BackgroundIn the alpha subclass of proteobacteria iron homeostasis is controlled by diverse iron responsive regulators. Caulobacter crescentus, an important freshwater α-proteobacterium, uses the ferric uptake repressor (Fur) for such purpose. However, the impact of the iron availability on the C. crescentus transcriptome and an overall perspective of the regulatory networks involved remain unknown.ResultsIn this work we report the identification of iron-responsive and Fur-regulated genes in C. crescentus using microarray-based global transcriptional analyses. We identified 42 genes that were strongly upregulated both by mutation of fur and by iron limitation condition. Among them, there are genes involved in iron uptake (four TonB-dependent receptor gene clusters, and feoAB), riboflavin biosynthesis and genes encoding hypothetical proteins. Most of these genes are associated with predicted Fur binding sites, implicating them as direct targets of Fur-mediated repression. These data were validated by β-galactosidase and EMSA assays for two operons encoding putative transporters. The role of Fur as a positive regulator is also evident, given that 27 genes were downregulated both by mutation of fur and under low-iron condition. As expected, this group includes many genes involved in energy metabolism, mostly iron-using enzymes. Surprisingly, included in this group are also TonB-dependent receptors genes and the genes fixK, fixT and ftrB encoding an oxygen signaling network required for growth during hypoxia. Bioinformatics analyses suggest that positive regulation by Fur is mainly indirect. In addition to the Fur modulon, iron limitation altered expression of 113 more genes, including induction of genes involved in Fe-S cluster assembly, oxidative stress and heat shock response, as well as repression of genes implicated in amino acid metabolism, chemotaxis and motility.ConclusionsUsing a global transcriptional approach, we determined the C. crescentus iron stimulon. Many but not all of iron responsive genes were directly or indirectly controlled by Fur. The iron limitation stimulon overlaps with other regulatory systems, such as the RpoH and FixK regulons. Altogether, our results showed that adaptation of C. crescentus to iron limitation not only involves increasing the transcription of iron-acquisition systems and decreasing the production of iron-using proteins, but also includes novel genes and regulatory mechanisms.
Highlights
In the alpha subclass of proteobacteria iron homeostasis is controlled by diverse iron responsive regulators
Iron homeostasis is mediated by ferric uptake repressor (Fur), an iron-sensing repressor protein, that controls the expression of genes involved in iron uptake, storage and usage
To define the C. crescentus iron limitation stimulon, we compared the transcriptome of wild-type cells treated with 100 μM FeSO4 with that of wild-type cells treated with 100 μM 2,2-dipyridyl for 2 hours, an experimental condition previously established to study gene expression of iron-responsive genes in C. crescentus [6]
Summary
In the alpha subclass of proteobacteria iron homeostasis is controlled by diverse iron responsive regulators. The most common Fur-mediated activation mechanism occurs indirectly via small regulatory RNAs (sRNA), such as RyhB in Escherichia coli [10], PrrF1 and PrrF2 in Pseudomonas aeruginosa [11], NrrF in Neisseria meningitidis [12] and FsrA in Bacillus subtilis [13]. In all these cases, the sRNAs inhibit the production of non-essential ironusing proteins under iron limitation, allowing relocation of the intracellular iron for essential proteins [14]
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