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How we learnt about iron acquisition in Pseudomonas aeruginosa: a series of very fortunate events

University of Colorado at Denver and Health Sciences Center, 12800 E. 19th Avenue RC-1 North P18-9127, Microbiology Mail Stop 8333, P.O. Box 6511, Aurora, CO 80045, USA.
BioMetals (Impact Factor: 2.69). 06/2007; 20(3-4):587-601. DOI: 10.1007/s10534-006-9067-2
Source: PubMed

ABSTRACT The ferric uptake repressor (Fur) of Pseudomonas aeruginosa, and a wide assortment of other prokaryotic organisms, has been mostly regarded as a negative regulator (repressor) of genes involved in iron acquisition (e.g., expression and utilization of siderophores) or of iron-regulated genes involved in virulence (e.g., toxins). However, there is an emerging picture of an even broader role for this protein in basic bacterial biology. Evidence has now accumulated indicating that Fur acts in a positive manner as well, and that it has a considerably wider impact on gene expression than originally perceived. We discovered that in P. aeruginosa Fur directly (i.e., negatively) regulates the expression of two, nearly identical tandem small (<200nt) RNA transcripts (sRNA). Our initial experiments showed that these Fur-regulated sRNAs (PrrF) affected expression of certain genes we initially thought might be directly, but positively, regulated by Fur. However, with discovery of the Fur-regulated sRNAs, first in Escherichia coli and then in P. aeruginosa, it became clear that Fur, in at least some cases, exerts its positive regulatory effect on gene expression by repressing the expression a negative regulatory factor (i.e., PrrF), which acts at the posttranscriptional level. While a clear picture was already available regarding the function of genes (see above) that are directly repressed by Fur (negative regulation), the functional classes of genes that are influenced by Fur-repressed sRNAs (positive regulation) had not been identified for P. aeruginosa. Accordingly we established a set of rigorous criteria, based on microarray experimental data, to identify the cohort of genes that are likely to be directly influenced by Fur-regulated PrrFs. More than 60 genes that fulfilled these strict criteria were identified. These include genes encoding proteins required for the sequestration of iron (e.g., bacterioferritins) and genes encoding enzymes (superoxide dismutase) vital to defense against iron catalyzed oxidative stress. More notably however, we identified more than 30 genes encoding proteins involved in carbon catabolism and aerobic or anaerobic respiration that are regulated by PrrFs. A significant number of genes encoding enzymes (e.g., aconitase, citrate synthase) involved in the TCA cycle are controlled by the PrrFs however, in quite a few instances there are genes encoding proteins with redundant functions (i.e., aconitase, citrate synthase) that do not appear to be influenced in any way by PrrFs. Based on our microarray experiments, as well as on phenotypic data, we propose that the Fur regulated sRNAs (i.e., PrrFs) exert a powerful regulatory influence that permits the sparing of vital metabolic compounds (e.g., citrate) during periods of iron limitation. These and other data to be presented indicate that Fur controlled gene expression in bacteria like P. aeruginosa is considerably more imperative and intricate than previously appreciated.

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    • "Indeed, alterations in the activities of several TCA cycle and glyoxylate shunt pathway enzymes funnel acetyl-CoA and oxaloacetate towards the production of an Al-detoxifying agent. The regulation of TCA cycle enzymes by Fe and small RNA transcripts has also been shown [23]. Enzymes in this metabolic network have been demonstrated to confer resistance to vanadium in P. fluorescens [24]. "
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    • "It is not clear whether and to what extent bacterial anaerobic metabolism takes place during tuber maceration. However, switching from aerobic to anaerobic energy pathways is an important aspect of bacterial pathogenesis (Tang et al. 2005; Vasil 2007). Therefore, citrate import by Cit1 might be part of this complex and adaptable machinery, allowing P. atrosepticum to deal with the specific environment within the host tissue and the corresponding metabolite spectrum. "
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    • "Fur is further considered to have essential but unknown function(s) in some bacterial strains because null mutants have not been obtained despite repeated attempts. Pseudomonas aeruginosa is one such bacterium, and analysis of a fur point mutant in this species has revealed that its wild-type Fur protein most probably regulates the expression of various genes, including those involved in the catabolism of carbon sources, and aerobic and anaerobic respiration (Vasil, 2007). "
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