Abstract
Pyoverdines are high affinity siderophores produced by a broad range of pseudomonads to enhance growth under iron deficiency. They are especially relevant for pathogenic and mutualistic strains that inhabit iron-limited environments. Pyoverdines are generated from non-ribosomally synthesized highly modified peptides. They all contain an aromatic chromophore that is formed in the periplasm by intramolecular cyclization steps. Although the cytoplasmic peptide synthesis and side-chain modifications are well characterized, the periplasmic maturation steps are far from understood. Out of five periplasmic enzymes, PvdM, PvdN, PvdO, PvdP, and PvdQ, functions have been attributed only to PvdP and PvdQ. The other three enzymes are also regarded as essential for siderophore biosynthesis. The structure of PvdN has been solved recently, but no function could be assigned. Here we present the first in-frame deletion of the PvdN-encoding gene. Unexpectedly, PvdN turned out to be required for a specific modification of pyoverdine, whereas the overall amount of fluorescent pyoverdines was not altered by the mutation. The mutant strain grew normally under iron-limiting conditions. Mass spectrometry identified the PvdN-dependent modification as a transformation of the N-terminal glutamic acid to a succinamide. We postulate a pathway for this transformation catalyzed by the enzyme PvdN, which is most likely functional in the case of all pyoverdines.
Highlights
Pyoverdines are high affinity siderophores produced by a broad range of pseudomonads to enhance growth under iron deficiency
Pseudomonas fluorescens A506 Produces a Typical Pyoverdine—Based on genetic analyses, it was recently proposed that P. fluorescens strain A506 produces a typical pyoverdine, which is expected to bind FeIII via six ligands provided by two hydroxamates of the peptide moiety and two oxo groups of the catechol fluorophore (Fig. 1A) [4]
While the cytoplasmically generated peptide moiety varies in sequence, length, and cyclic/ linear structure among the many known pyoverdines, the enzymes involved in the periplasmic maturation of this siderophore are highly conserved in all pyoverdine-producing species and strains, indicating that their activities are required for all known pyoverdines (Fig. 1B)
Summary
Pyoverdines are high affinity siderophores produced by a broad range of pseudomonads to enhance growth under iron deficiency. Pyoverdines are generated from non-ribosomally synthesized highly modified peptides They all contain an aromatic chromophore that is formed in the periplasm by intramolecular cyclization steps. Of the five periplasmic enzymes, PvdM, PvdN, PvdO, PvdP, and PvdQ, which are found in all known pyoverdine-producing species, functions have been assigned so far only to PvdQ and PvdP, which are involved in a precursor deacylation step and the chromophore cyclization, respectively [5,6,7,8]. Fluorescent pseudomonads usually produce more than one isoform of pyoverdine, which is exploited by a method known as siderotyping [13] This method utilizes the different pI values of siderophores, which include modified pyoverdine isoforms. Lys-261 in the cofactor-binding pocket was essential for activity and translocation into the periplasm, suggesting that the Tat system has to translocate PvdN in an active, cofactor-containing conformation
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