High-molecular-mass, iron-repressed cytoplasmic proteins in fluorescent Pseudomonas: potential peptide-synthetases for pyoverdine biosynthesis.

High molecular-mass cytoplasmic proteins were detected in iron-starved, pyoverdine-producing Pseudomonas aeruginosa, P. chlororaphis, P. fluorescens, P. putida, P. aptata and P. tolaasii. They appeared to be specifically located in the cytoplasm and thus were termed 'IRCPs', for iron-repressed cytoplasmic proteins. A strain-dependent gel electrophoresis pattern with multiple bands of M(r) values ranging from 180 to 600 kDa was usually observed for these proteins. Strains synthesizing pyoverdines differing in their peptide part presented different IRCP gel electrophoresis profiles, whereas strains synthesizing identical pyoverdines had identical IRCP gel electrophoresis profiles. Some mutants affected in pyoverdine biosynthesis presented a perturbed IRCP pattern, and no IRCPs were detected in non-fluorescent Pseudomonas strains either unable to synthesize siderophores or synthesizing non-peptidic siderophores. The data strongly suggest that the IRCPs could be related to peptide synthetases involved in the biosynthesis of the peptidic part of pyoverdine-type siderophores.