Functional analysis of PvdS, an iron starvation sigma factor of Pseudomonas aeruginosa

In Pseudomonas aeruginosa, iron modulates gene expression through a cascade of negative and positive regulatory proteins. The master regulator Fur is involved in iron-dependent repression of several genes. One of these genes, pvdS, was predicted to encode a putative sigma factor responsible for the transcription of a subset of genes of the Fur regulon. PvdS appears to belo ng to a structurally and functionally distinct subgroup of the extracytopla smic function family of alternative sigma factors. Members of this subgroup , also including PbrA from Pseudomonas fluorescens, PfrI and PupI from Pseu domonas putida, and FecI from Escherichia coli, are controlled by the Fur r epressor, and they activate transcription of genes for the biosynthesis or the uptake of siderophores. Evidence is provided that the PvdS protein of P . aeruginosa is endowed with biochemical properties of eubacterial sigma fa ctors, as it spontaneously forms 1:1 complexes with the core fraction of RN A polymerase (RNAP, alpha(2)beta beta' subunits), thereby promoting in vitr o binding of the PVdS-RNAP holoenzyme to the promoter region of the pvdA ge ne. These functional features of PvdS are consistent with the presence of s tructural domains predicted to be involved in core RNAP binding, promoter r ecognition, and open complex formation. The activity of pyoverdin biosynthe tic (pvd) promoters was significantly lower in E. coli overexpressing the m ulticopy pvdS gene than in wild-type P. aeruginosa PAO1 carrying the single gene copy, and pvd::lacZ transcriptional fusions were silent in both pfrI (the pvdS homologue) and pfrA (a positive regulator of pseudobactin biosynt hetic genes) mutants of P. putida WCS358, while they are expressed at PAO1 levels in wild-type WCS358. Moreover, the PvdS-RNAP holoenzyme purified fro m E. coli lacked the ability to generate in vitro transcripts from the pvdA promoter. These observations suggest that at least one additional positive regulator could be required for full activity of the PvdS-dependent transc ription complex both in vivo and in vitro. This is consistent with the pres ence of a putative activator binding site (the iron starvation box) at vari able distance from the transcription initiation sites of promoters controll ed by the iron starvation sigma factors PvdS, PfrI, and PbrA of fluorescent pseudomonads.