Role for the outer membrane ferric siderophore receptor PupB in signal transduction across the bacterial cell envelope

The outer membrane protein PupB of Pseudomonas putida WCS358 facilitates transport of iron complexed to the siderophores pseudobactin BN8 and pseudobactin BN7 into the cell. Its synthesis is induced by the presence of these specific siderophores under iron limitation. The signal transduction pathway regulating siderophore-dependent expression of pupB was shown to consist of two regulatory proteins, PupI and PupR, and the PupB receptor itself. Mutational analysis of the regulatory genes suggested that PupI acts as a positive regulator of pupB transcription, whereas PupR modifies PupI activity dependent on the presence of pseudobactin BN8. PupI and PupR do not share homology with the classical bacterial two-component systems but display significant similarity to the FecI and FecR proteins of Escherichia coli involved in regulation of ferric dicitrate transport. The function of the PupB receptor in pupB regulation was studied by the use of chimeric receptor proteins composed of PupB and the ferric pseudobactin 358 receptor PupA. This experiment revealed that PupB is involved in the initiation of the signal transduction pathway, implying a so far unique role for an outer membrane protein in signal transduction.