Cloning and Characterization of pvdS, a Gene Required for Pyoverdine Synthesis in Pseudomonas aeruginosa: PvdS Is Probably an Alternative Sigma Factor

Cells of Pseudomonas aeruginosa secrete a fluorescent yellow-green siderophore, pyoverdine, when grown under iron-deficient conditions. We describe here the cloning and characterization of a gene, pvdS, which is required for this process. The pvdS gene is required for expression from promoters of at least two pyoverdine synthesis genes and can cause expression from these promoters in Escherichia coli, where they are otherwise inactive. Sequencing of pvdS revealed that it is a member of a subfamily of RNA polymerase sigma factors which direct the synthesis of extracellular products by bacteria. The pvdS gene is expressed only in iron-starved bacteria, and in E. coli cells at least, expression is regulated by the Fur repressor protein. We propose that in iron-rich cells of P. aeruginosa, Fur binds to the pvdS promoter and prevents expression of the gene; under conditions of iron starvation, repression is relieved and PvdS is made, reprogramming the cells for pyoverdine synthesis. Fluorescent pseudomonads are characterized by the produc-tion of yellow-green fluorescent pigments. These act as sid-erophores and are variously termed pyoverdines or pseudobac-tins. The fluorescence and color are due to the presence of a dihydroxyquinoline group; this is attached to a peptide of 6 t