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Journal of Bacteriology, September 2004, p. 5876-5882, Vol. 186, No. 17
0021-9193/04/$08.00+0     DOI: 10.1128/JB.186.17.5876-5882.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

The htx and ptx Operons of Pseudomonas stutzeri WM88 Are New Members of the Pho Regulon

Department of Microbiology, University of Illinois, Urbana, Illinois

Received 12 April 2004/ Accepted 7 June 2004

The htx and ptx operons of Pseudomonas stutzeri WM88 allow for the use of the inorganic reduced phosphorus (P) compounds hypophosphite (P valence, +1) and phosphite (P valence, +3) as sole P sources. To support the proposed in vivo role for the htx and ptx operons, namely the use of phosphite and hypophosphite as alternative P sources, we used reporter gene fusions to examine their expression levels with respect to various P conditions. Expression of the htx and ptx operons was induced up to 17- and 22-fold, respectively, in cultures grown under phosphate starvation conditions relative to expression in medium with excess phosphate (P_i). However, the presence of the reduced P substrate hypophosphite, phosphite, or methylphosphonate, in addition to excess P_i, did not result in an increase in the expression of either operon. To provide further support for a role of the htx and ptx operons in P_i acquisition, we identified P. stutzeri phoBR homologs and constructed deletion mutants. Induction of the htx and ptx reporter gene fusions in response to growth on limiting P_i was abolished in phoB, phoR, and phoBR mutants, demonstrating that htx and ptx expression is phoBR dependent. The putative LysR-type regulator encoded by ptxE has no apparent role in the expression of the htx and ptx operons, as no effect was observed on the level of induction of either operon in a ptxE mutant.

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