Study on Iron Regulon Pseudomonas aeruginosa

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Under iron limitation, Pseudomonas aeruginosa secretes pyoverdin, a low-molecular-weight siderophore able to capture ferric iron with a very high affinity for uptake of this ion into the cell. A number of pvd genes involved in pyoverdin production were found to be clustered and organized in many operons at the 47 min region (pvd region) on the chromosomal genetic map. Genetic analysis of the three pvd operons demonstrated that their transcription (1) was repressed under iron-rich conditions and (2) required a positive regulator, PvdS,that was encoded in the pvd region. Based on uncleotide sequencing and subsequent homology search, it was inferred that PvdS is a member of subfamily of RNA polymerase sigma factors regulating "extracytoplasmic functions" . The promoter region of pvdS contained the sequence that matched well with the consensus binding sequence of Fur, a global repressor of the iron regulon. Consistent with the presence of such a sequence (Fur box) , transcription of pvdS was repressed under iron-rich conditions. The data obtained in this study leads to the proposal that (i) an active form of Fur under an iron-rich condition binds to the Fur box at the pvdS promoter to repress the transcription of pvdS and (ii) no production of PvdS thereafter does not permit the transcription of other structural pvd genes. Because of the most probable involvement of Fur in pyoverdin production, allelic exchange mutagenesis was carried out to isolate a chromosomal fur mutation. Our repeated attempt to obtain such a mutation was unsuccessful, suggesting the essential role of Fur on the viability of P.aeruginosa.