Abstract
BackgroundFerredoxins are small iron-sulfur proteins belonging to all domains of life. A sub-group binds two [4Fe-4S] clusters with unequal and extremely low values of the reduction potentials. These unusual properties are associated with two specific fragments of sequence. The functional importance of the very low potential ferredoxins is unknown.ResultsA bioinformatic screening of the sequence features defining very low potential 2[4Fe-4S] ferredoxins has revealed the almost exclusive presence of the corresponding fdx gene in the Proteobacteria phylum, without occurrence in Archaea and Eukaryota. The transcript was found to be monocistronic in Pseudomonas aeruginosa, and not part of an operon in most bacteria. Only fdx genes of bacteria which anaerobically degrade aromatic compounds belong to operons. As this pathway is not present in all bacteria having very low potential 2[4Fe-4S] ferredoxins, these proteins cannot exclusively be reductants of benzoyl CoA reductases. Expression of the ferredoxin gene did not change in response to varying growth conditions, including upon macrophage infection or aerobic growth with 4-hydroxy benzoate as carbon source. However, it increased along the growth curve in Pseudomonas aeruginosa and in Escherichia coli. The sequence immediately 5' upstream of the coding sequence contributed to the promotor activity. Deleting the fdx gene in Pseudomonas aeruginosa abolished growth, unless a plasmid copy of the gene was provided to the deleted strain.ConclusionsThe gene of the very low potential 2[4Fe-4S] ferredoxin displays characteristics of a housekeeping gene, and it belongs to the minority of genes that are essential in Pseudomonas aeruginosa. These data identify a new potential antimicrobial target in this and other pathogenic Proteobacteria.
Highlights
Ferredoxins are small iron-sulfur proteins belonging to all domains of life
Curing the mutants of the plasmid copy of fdx1 did not allow us to select colonies lacking the chromosomal copy of the gene. These results indicate that the plasmids bearing fdx1 rescued the cells that had lost chromosomal fdx1, but complete lack of the gene was deleterious to P. aeruginosa growth
We could not demonstrate iron regulation for the single fdx gene of P. aeruginosa or E. coli, in line with previous results obtained with H. pylori [32] and P. aeruginosa [33]
Summary
A sub-group binds two [4Fe-4S] clusters with unequal and extremely low values of the reduction potentials These unusual properties are associated with two specific fragments of sequence. Ferredoxin (Fdx) is the name given to a variety of small proteins binding inorganic clusters organized around two to four iron atoms and a complementary number of sulfur atoms [1]. Functional substitution among Fdxs may occur, and other soluble electron shuttles, such as flavodoxins, may act as Fdx-substitutes. This is the case upon iron starvation for a 2[4Fe-4S] Fdx in glycolytic. Clostridia [6] or a [2Fe-2S] Fdx in some photosynthetic organisms [7], for instance Despite this apparent functional redundancy, most sequenced genomes display a wealth of genes encoding various Fdxs.
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