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 rhizobacteri um Pseudomonas strain B10, contains the hydroxamate ligand D-N-5-hydroxyorn ithine (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 in volved in the conversion of L-Orn to its N-5-OH derivative. PsbA shows sign ificant similarity to microbial omega-amino acid hydroxylases containing fl avin adenine dinucleotide and NADP cofactor-binding sites and the FATGY sig nature of the putative substrate recognition pocket. The psbA gene is monoc istronic, and its transcription is negatively controlled by iron. A site-sp ecific psbA mutant of Pseudomonas strain B10 was biochemically complemented with the precursor L-N-5-OH-Orn, suggesting that L-Orn is hydroxylated bef ore conversion to the D isomer. The L-Orn N-5-hydroxylase-defective mutants of Pseudomonas strain B10 and Pseudomonas aeruginosa PAO1 were much less e ffective than the parental strains in suppressing the growth of the phytopa thogen Erwinia carotovora in iron-poor medium. The extent of in vitro inhib ition 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.