IDENTIFICATION AND CHARACTERIZATION OF THE PUPB GENE ENCODING AN INDUCIBLE FERRIC-PSEUDOBACTIN RECEPTOR OF PSEUDOMONAS-PUTIDA WCS358

Pseudomonas putida WCS358 can transport iron complexed to a wide varie ty of pseudobactins produced by other Pseudomonas strains. The pupB ge ne encoding an outer membrane ferric-pseudobactin receptor was isolate d from a genomic library of P. putida WCS358. The PupB receptor facili tated iron transport via two distinct heterologous siderophores, i.e. pseudobactin BN8 and pseudobactin BN7. The amino acid sequence deduced from the nucleotide sequence consisted of 804 amino acids (molecular weight 88369) of which the N-terminal part was very similar to a proka ryotic leader peptide. The mature protein shared significant homology with the receptor for ferric-pseudobactin 358 (PupA) and contained thr ee regions common to TonB-dependent receptor proteins of Escherichia c oli. Interestingly, PupB expression was only observed in cells culture d in iron-deficient medium containing pseudobactin BN8 or pseudobactin BN7. This expression required a transcriptional unit, pupR, identifie d upstream of the structural pupB gene. Transposon Tn5 insertion mutan ts defective in PupB production still exhibited uptake of iron via pse udobactin BN8, although with reduced efficiency. Apparently, an additi onal transport system for this ferric-siderophore complex operates in this strain. In addition to pseudobactin BN8 also other heterologous s iderophores were capable of inducing synthesis of specific high-molecu lar-weight outer membrane proteins in strain WCS358, which suggests th e existence of multiple siderophore-inducible iron transport systems i n this strain.