Iron transport and regulation, cell signalling and genomics: lessons from Escherichia coli and Pseudomonas

A variety of bacterial species secrete and take up chelating compounds that enable acquisition of iron (siderophores). It has become clear that a common feature in regulation of different iron acquisition systems is the involvement of alternative sigma factor proteins of the extracytoplasmic function (ECF) family. Two of these proteins, PvdS from Pseudomonas aeruginosa and FecI from Escherichia coli K-12, have been studied extensively. PvdS directs transcription of genes required for the biosynthesis of a siderophore, pyoverdine, and FecI causes expression of genes for uptake of ferric citrate. FecI forms part of a signalling system that responds to the presence of ferric citrate. Here, we review recent advances in understanding of PvdS and of the Fec signalling system. PvdS and FecI are part of a distinct subfamily of ECF sigma factors involved in iron acquisition and hence named the iron-starvation sigmas. Analysis of microbial genome sequences shows that Fec-like signalling systems are present in a wide range of species and many such systems may be present in a single species. The availability of tools for large-scale genome analysis is likely to lead to rapid advances in our understanding of this expanding family of proteins.