The periplasmic transaminase PtaA of Pseudomonas fluorescens converts the glutamic acid residue at the pyoverdine fluorophore to a ketoglutaric acid
The periplasmic conversion of ferribactin to pyoverdine is essential for siderophore biogenesis in fluorescent pseudomonads, such as pathogenic Pseudomonas aeruginosa or plant growth promoting Pseudomonas fluorescens. The non ribosomal peptide ferribactin undergoes cyclizations and oxidations that result in the fluorophore, and a strictly conserved fluorophore bound glutamic acid residue is converted to a range of variants, including succinamide, succinic acid, and a ketoglutaric acid residues. We recently discovered that the pyridoxal phosphate containing enzyme PvdN is responsible for the generation of the succinamide, which can be hydrolyzed to succinic acid. Based on this, a distinct unknown enzyme was postulated to be responsible for the conversion of the glutamic acid to a ketoglutaric acid. Here we report the identification and characterization of this enzyme in P. fluorescens strain A506. In silico analyses indicated a periplasmic transaminase in fluorescent pseudomonads and other proteobacteria that we termed PtaA for "periplasmic transaminase A." An in frame deleted ptaA mutant selectively lacked the a ketoglutaric acid form of pyoverdine, and recombinant PtaA complemented this phenotype. The ptaA/pvdN double mutant produced exclusively the glutamic acid form of pyoverdine. PtaA is homodimeric and contains a pyridoxal phosphate cofactor. Mutation of the active site lysine abolished PtaA activity and affected folding as well as Tat dependent transport of the enzyme. In pseudomonads, the occurrence of ptaA correlates with the occurrence of a ketoglutaric acid forms of pyoverdines. As this enzyme is not restricted to pyoverdine producing bacteria, its catalysis of periplasmic transaminations is most likely a general tool for specific biosynthetic pathways.
The Journal of Biological Chemistry