Expression of pyoverdine genes in Pseudomonas aeruginosa PAO1: intracellular levels of PvdS

In response to iron-limitation, Pseudomonas aeruginosa PAO1 produces the fluorescent siderophore pyoverdine. Transcription of pyoverdine biosynthetic (pvd) genes depends on the extracytoplasmic function (ECF) sigma factor PvdS, which is negatively regulated by the Fur repressor. An additional level of regulation of pvd genes relies upon the surface signalling mechanism that involves the ferripyoverdine outer membrane receptor FpvA and the pyoverdine molecule itself. In the absence of ferripyoverdine, the activity of PvdS is antagonized by the antisigma factor FpvR. Binding of ferripyoverdine to FpvA initiates a signal transduction cascade that involves FpvR and leads to the transcription of several virulence genes (e.g. pvd, aprA, prpL and toxA) by PvdS-dependent RNA polymerase. To evaluate the competition between PvdS and the major sigma subunit RpoD (s70) for RNA polymerase binding, we have determined the intracellular levels of PvdS and RpoD in iron-starved P. aeruginosa PAO1 cells during the exponential and stationary phase. By means of quantitative immunoblot analysis, we found that PvdS attains 60% of RpoD levels in exponential phase, while it drastically decreases at the stationary phase. PvdS also exhibits reduced stability in the presence of an excess of iron in vivo. Experiments are in progress to investigate the effect surface signalling components on PvdS activity and stability.