Functional analysis of PvdS, an iron starvation sigma factor of Pseudomonas aeruginosa

Analysis of promoters recognized by PvdS, an extracytoplasmic-function sigma factor protein from Pseudomonas aeruginosa

The alternative sigma factor PvdS is required by Pseudomonas aeruginosa for initiation of transcription from pyoverdine (pvd) promoters. Two divergent PvdS-dependent promoters (pvdE and pvdF) were characterized by deletion analysis, and the minimal promoter region for each included a sequence element, the iron starvation (IS) box, that is present in other pvd promoters. Site-directed mutagenesis showed that the IS box elements were essential for promoter activity in vivo. Band shift assays and in vitro transcription experiments showed that a complex of PvdS and core RNA polymerase required the presence of an IS box in order to bind to and initiate transcription from pvd promoters. These results indicate that IS box elements participate in sequence-specific recognition by PvdS to enable initiation of transcription from pvd promoters and are likely to represent a -35 sequence element for this sigma factor.

Pseudobactin biogenesis in the plant growth-promoting rhizobacterium Pseudomonas strain B10: identification and functional analysis of the L-ornithine N(5)-oxygenase (psbA) gene

Pseudobactin(B10), the fluorescent siderophore produced by the rhizobacterium Pseudomonas strain B10, contains the hydroxamate ligand D-N(5)-hydroxyornithine (D-N(5)-OH-Orn). We cloned the L-Orn N(5)-oxygenase (psbA) gene from a genomic library of Pseudomonas strain B10 and demonstrated that PsbA is involved in the conversion of L-Orn to its N(5)-OH derivative. PsbA shows significant similarity to microbial omega-amino acid hydroxylases containing flavin adenine dinucleotide and NADP cofactor-binding sites and the FATGY signature of the putative substrate recognition pocket. The psbA gene is monocistronic, and its transcription is negatively controlled by iron. A site-specific psbA mutant of Pseudomonas strain B10 was biochemically complemented with the precursor L-N(5)-OH-Orn, suggesting that L-Orn is hydroxylated before conversion to the D isomer. The L-Orn N(5)-hydroxylase-defective mutants of Pseudomonas strain B10 and Pseudomonas aeruginosa PAO1 were much less effective than the parental strains in suppressing the growth of the phytopathogen Erwinia carotovora in iron-poor medium. The extent of in vitro inhibition of E. carotovora was strictly iron dependent and directly correlated with the amount of released siderophores. These data strengthen the role of fluorescent siderophores in biocontrol of deleterious rhizomicroorganisms.

Characterization of an ECF sigma factor protein from Pseudomonas aeruginosa

The PvdS protein is essential for synthesis of the siderophore pyoverdine by Pseudomonas aeruginosa. PvdS has some sequence similarity to a family of alternative sigma factor proteins (the ECF [extracytoplasmic factor] family) that direct bacterial RNA polymerases to transcribe genes encoding extracytoplasmic factors. PvdS was purified as a His-tagged protein (hPvdS) and this was used to test the hypothesis that PvdS is a sigma factor protein. The purified protein caused core RNA polymerase from Escherichia coli to bind specifically to the promoters of pyoverdine synthesis genes and enabled transcription from these promoters in vitro. In addition, PvdS was found to co-purify with RNA polymerase from P. aeruginosa, indicating that PvdS associates with RNA polymerase inside the bacteria. These results show that PvdS is a sigma factor protein.