PvdM of fluorescent pseudomonads is required for the oxidation of ferribactin by PvdP in periplasmic pyoverdine maturation.

Fluorescent pseudomonads such as Pseudomonas aeruginosa or Pseudomonas fluorescens produce pyoverdine siderophores that ensure iron-supply in iron-limited environments. After its synthesis in the cytoplasm, the nonfluorescent pyoverdine precursor ferribactin is exported into the periplasm, where the enzymes PvdQ, PvdP, PvdO, PvdN, and PtaA are responsible for fluorophore maturation and tailoring steps. While the roles of all these enzymes are clear, little is known about the role of PvdM, a human renal dipeptidase-related protein that is predicted to be periplasmic and that is essential for pyoverdine biogenesis. Here, we reveal the subcellular localization and functional role of PvdM. Using the model organism P. fluorescens, we show that PvdM is anchored to the periplasmic side of the cytoplasmic membrane, where it is indispensable for the activity of the tyrosinase PvdP. While PvdM does not share the metallopeptidase function of renal dipeptidase, it still has the corresponding peptide-binding site. The substrate of PvdP, deacylated ferribactin, is secreted by a ΔpvdM mutant strain, indicating that PvdM prevents loss of this periplasmic biosynthesis intermediate into the medium by ensuring the efficient transfer of ferribactin to PvdP in vivo. We propose that PvdM belongs to a new dipeptidase-related protein subfamily with inactivated Zn2+ coordination sites, members of which are usually genetically linked to TonB-dependent uptake systems and often associated with periplasmic FAD-dependent oxidoreductases related to d-amino acid oxidases. We suggest that these proteins are necessary for selective binding, exposure, or transfer of specific d- and l-amino acid-containing peptides and other periplasmic biomolecules in manifold pathways.

A tunable anthranilate-inducible gene expression system for Pseudomonas species.

Pseudomonads are among the most common bacteria in soils, limnic ecosystems, and human, animal, or plant host environments, including intensively studied species such as Pseudomonas aeruginosa, P. putida, or P. fluorescens. Various gene expression systems are established for some species, but there is still a need for a simple system that is suitable for a wide range of pseudomonads and that can be used for physiological applications, i.e., with a tuning capacity at lower expression levels. Here, we report the establishment of the anthranilate-dependent PantA promoter for tunable gene expression in pseudomonads. During studies on P. fluorescens, we constructed an anthranilate-inducible AntR/PantA-based expression system, named pUCP20-ANT, and used GFP as reporter to analyze gene expression. This system was compared with the rhamnose-inducible RhaSR/PrhaB-based expression system in an otherwise identical vector background. While the rhamnose-inducible system did not respond to lower inducer concentrations and always reached high levels over time when induced, expression levels of the pUCP20-ANT system could be adjusted to a range of distinct lower or higher levels by variation of anthranilate concentrations in the medium. Importantly, the anthranilate-inducible expression system worked also in strains of P. aeruginosa and P. putida and therefore will be most likely useful for physiological and biotechnological purposes in a wide range of pseudomonads. KEY POINTS: • We established an anthranilate-inducible gene expression system for pseudomonads. • This system permits tuning of gene expression in a wide range of pseudomonads. • It will be very useful for physiological and biotechnological applications.