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Journal of Bacteriology, December 1998, p. 6784-6788, Vol. 180, No. 24
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

The Fur-Regulated Gene Encoding the Alternative Sigma Factor PvdS Is Required for Iron-Dependent Expression of the LysR-Type Regulator PtxR in Pseudomonas aeruginosa

Michael L. Vasil,1,* Urs A. Ochsner,1 Zaiga Johnson,1 Jane A. Colmer,2 and Abdul N. Hamood2

Department of Microbiology, University of Colorado Health Sciences Center, Denver, Colorado 80262,1 and Department of Microbiology and Immunology, Texas Tech University Health Sciences Center, Lubbock, Texas 740302

Received 18 March 1998/Accepted 1 October 1998

We previously identified a novel regulator of the exotoxin A gene (toxA) in Pseudomonas aeruginosa, PtxR, that belongs to the LysR family of prokaryotic regulatory proteins. Preliminary data also suggest that PtxR affects the expression of siderophores in P. aeruginosa. Because toxA expression and siderophore production in this organism are coordinately regulated by the ferric uptake regulator (Fur) and the Fur-regulated alternative sigma factor PvdS, regulation of ptxR itself in the context of these regulators was examined. RNase protection analyses of ptxR transcription revealed that there are two independent transcription initiation sites (T1 and T2). While transcription from the promoter of T1 is constitutive throughout the growth cycle of PAO1, transcription from the second promoter (P2) is negatively affected by iron. Transcription from the P2 promoter is constitutive in a fur mutant under microaerobic conditions but still iron regulated during aerobic growth. High concentrations (>100 nM) of the ferric uptake regulatory protein (Fur) failed to bind to either of the promoter regions of ptxR in either gel mobility shift assays or DNase I footprint experiments. These results indicate that Fur indirectly regulates the iron-dependent expression of ptxR. Iron-regulated transcription of ptxR from the P2 promoter, but not constitutive expression from the P1 promoter, was dependent on the Fur-regulated alternative sigma factor gene pvdS, even under aerobic conditions. Consequently, there are two levels of iron-regulated expression of ptxR. The iron-regulated expression of ptxR under microaerobic conditions from the P2 promoter of ptxR is mediated indirectly by Fur through the iron-regulated expression of pvdS. In contrast, pvdS-mediated iron regulation of ptxR under aerobic conditions is Fur independent.

* Corresponding author. Mailing address: Department of Microbiology, Box B175, University of Colorado Health Sciences Center, 4200 E. Ninth Ave., Denver, CO 80262. Phone: (303) 315-8627. Fax: (303) 315-6785. E-mail: Mike.Vasil{at}uchsc.edu.

Journal of Bacteriology, December 1998, p. 6784-6788, Vol. 180, No. 24
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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