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Applied and Environmental Microbiology, March 2006, p. 1910-1924, Vol. 72, No. 3
0099-2240/06/$08.00+0     doi:10.1128/AEM.72.3.1910-1924.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Conservation of the Pho regulon in Pseudomonas fluorescens Pf0-1

Russell D. Monds, Peter D. Newell, Julia A. Schwartzman, and George A. O'Toole^*

Department of Microbiology and Immunology, Dartmouth Medical School, Hanover, NH 03755

Received 30 September 2005/ Accepted 27 December 2005

The Pho regulon integrates the sensing of environmental inorganic phosphate (P_i) availability with coregulation of gene expression, mediating an adaptive response to P_i limitation. Many aspects of the Pho regulon have been addressed in studies of Escherichia coli; however, it is unclear how transferable this knowledge is to other bacterial systems. Here, we report work to discern the conservation of the Pho regulon in Pseudomonas fluorescens Pf0-1. We demonstrate by mutational studies that PhoB/PhoR and the Pst system have conserved functions in the regulation of P_i-induced phosphatase activities, as well as expression of other P_i-regulated genes. A genetic screen was carried out to isolate factors that affect Pho-regulated phosphatase activity. We identified the Pho-regulated phosphatases PhoX and PhoD and present evidence that these enzymes are exported via the Tat system. The phoX and phoD genes were shown to be members of the Pho regulon by reverse transcription-PCR, as well as by functional assessment of putative PhoB binding sites (Pho boxes). Our data also suggested that at least one other non-Tat-secreted Pho-regulated phosphatase exists. From the genetic screen, numerous siderophore mutants that displayed severe defects in Pho-activated phosphatase activity were isolated. Subsequently, iron was shown to be important for modulating the activity of Pho-regulated phosphatases, but it does not regulate this activity at the level of transcription. We also identify and demonstrate a novel role in siderophore production and Pho-regulated phosphatase activity for ApaH, the hydrolase for the nucleotide-signaling molecule AppppA. Finally, numerous mutations in multiple cellular pathways were recovered that may be required for maximal induction of the Pho regulon under P_i-limiting conditions.

Present address: Middlebury College, Middlebury, VT 05753.

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