M. Koster et al., ROLE FOR THE OUTER-MEMBRANE FERRIC SIDEROPHORE RECEPTOR PUPB IN SIGNAL-TRANSDUCTION ACROSS THE BACTERIAL-CELL ENVELOPE, EMBO journal, 13(12), 1994, pp. 2805-2813
The outer membrane protein PupB of Pseudomonas putida WCS358 facilitat
es transport of iron complexed to the siderophores pseudobactin BN8 an
d pseudobactin BN7 into the cell. Its synthesis is induced by the pres
ence of these specific siderophores under iron limitation. The signal
transduction pathway regulating siderophore-dependent expression of pu
pB 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 transcriptio
n, whereas PupR modifies PupI activity dependent on the presence of ps
eudobactin 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 p
upB regulation was studied by the use of chimeric receptor proteins co
mposed of PupB and the ferric pseudobactin 358 receptor PupA. This exp
eriment revealed that PupB is involved in the initiation of the signal
transduction pathway, implying a so far unique role for an outer memb
rane protein in signal transduction.