5 Fluorescent Pseudomonads …
109
iron complex is taken up via the cognate-specific receptor and a transport pathway
(Neilands 1981; Neilands et al. 1987). Microorganisms have developed following
methods of iron acquisition, membrane-bound chelator (Royt 1988), and reduction
of iron chelates (Emery 1987; Lesuisse and Labbe 1989; Zimmermann et al. 1989).
5.4 Siderophores
The production of siderophores by microorganisms in slightly acidic, neutral, or
alkaline soils is a general phenomenon (Kloepper et al. 1980) . No system analogous
to the siderophores has been found for any other metal ion and Fe
3+ seems to be
unique in requiring such specific ligands. Siderophores are low molecular weight
iron-chelating agents produced by virtually all bacteria and fungi under iron-limiting
conditions. The fluorescent pseudomonads producing siderophores (water-soluble,
yellow-green fluorescent pigments) viz. P. aeruginosa, P. putida, and P. fluorescens all
belong to the same inter-generic homology group (Palleroni et al. 1973; Palleroni and
Doudoroff 1974). Turfreijer (1942) proposed the term “pyoverdine” for the pigment
of P. fluorescens by its analogy with that of phenazine pigment, pyocyanine produced
by P. aeruginosa. The term pyoverdine has been extended to include all pigments
produced by fluorescent pseudomonads. However, due to differences in structure,
these are now named differently.
Mayer and Abdallah (1978) like to designate the pigments of this class by a suffix
including the species responsible for their production e.g., pyoverdine Pf for the
pyoverdine of P. fluorescens. The main differences observed between those produced
by different strains are the number, composition, and sequence of their L- and Damino acids which are thought to give the molecule their receptor specification
(Hohnadel and Mayer 1988).
During the past years, the chemical structure of a large number of microbial
siderophores has been elucidated by NMR spectroscopy, Mass spectroscopy, Chemical degradation, and X-ray diffraction. Two different names have been used for
siderophores of Pseudomonas sp. pyoverdine, and pseudobactin. The complete structure for pseudobactin, the siderophore of Pseudomonas BlO, has been determined. It
consists of a linear hex peptide: L-Lys-D-Threo-BOH-Asp-L-Ala-D-allo-Thr-L-AlaD-N-OH-Orn, in which the ornithine residue is cyclized into an N-hydroxypiperidone
ring and the lysine residue, is linked to a fluorescent quinoline derivative. The iron
chelation to the hydroxamate, the a-hydroxy acid, and the -o-dihydroxy group1,
establishes pseudobactin as one of the unique groups of siderophores. The occurrence of both L- and D-amino acids is also unusual and their alternate sequence in
pseudobactin explains why the compound is not affected by proteolytic enzymes. The
pyoverdine for P. fluorescens has been partially characterized. It contains seven amino
acid residues with two hydroxamate groups and an o-dihydroxy aromatic group. The
complete structure of pyoverdine Pa from P. aeruginosa has been determined (Dileep
et al. 1998).
109
iron complex is taken up via the cognate-specific receptor and a transport pathway
(Neilands 1981; Neilands et al. 1987). Microorganisms have developed following
methods of iron acquisition, membrane-bound chelator (Royt 1988), and reduction
of iron chelates (Emery 1987; Lesuisse and Labbe 1989; Zimmermann et al. 1989).
5.4 Siderophores
The production of siderophores by microorganisms in slightly acidic, neutral, or
alkaline soils is a general phenomenon (Kloepper et al. 1980) . No system analogous
to the siderophores has been found for any other metal ion and Fe
3+ seems to be
unique in requiring such specific ligands. Siderophores are low molecular weight
iron-chelating agents produced by virtually all bacteria and fungi under iron-limiting
conditions. The fluorescent pseudomonads producing siderophores (water-soluble,
yellow-green fluorescent pigments) viz. P. aeruginosa, P. putida, and P. fluorescens all
belong to the same inter-generic homology group (Palleroni et al. 1973; Palleroni and
Doudoroff 1974). Turfreijer (1942) proposed the term “pyoverdine” for the pigment
of P. fluorescens by its analogy with that of phenazine pigment, pyocyanine produced
by P. aeruginosa. The term pyoverdine has been extended to include all pigments
produced by fluorescent pseudomonads. However, due to differences in structure,
these are now named differently.
Mayer and Abdallah (1978) like to designate the pigments of this class by a suffix
including the species responsible for their production e.g., pyoverdine Pf for the
pyoverdine of P. fluorescens. The main differences observed between those produced
by different strains are the number, composition, and sequence of their L- and Damino acids which are thought to give the molecule their receptor specification
(Hohnadel and Mayer 1988).
During the past years, the chemical structure of a large number of microbial
siderophores has been elucidated by NMR spectroscopy, Mass spectroscopy, Chemical degradation, and X-ray diffraction. Two different names have been used for
siderophores of Pseudomonas sp. pyoverdine, and pseudobactin. The complete structure for pseudobactin, the siderophore of Pseudomonas BlO, has been determined. It
consists of a linear hex peptide: L-Lys-D-Threo-BOH-Asp-L-Ala-D-allo-Thr-L-AlaD-N-OH-Orn, in which the ornithine residue is cyclized into an N-hydroxypiperidone
ring and the lysine residue, is linked to a fluorescent quinoline derivative. The iron
chelation to the hydroxamate, the a-hydroxy acid, and the -o-dihydroxy group1,
establishes pseudobactin as one of the unique groups of siderophores. The occurrence of both L- and D-amino acids is also unusual and their alternate sequence in
pseudobactin explains why the compound is not affected by proteolytic enzymes. The
pyoverdine for P. fluorescens has been partially characterized. It contains seven amino
acid residues with two hydroxamate groups and an o-dihydroxy aromatic group. The
complete structure of pyoverdine Pa from P. aeruginosa has been determined (Dileep
et al. 1998).