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Journal of Bacteriology, March 2007, p. 1765-1773, Vol. 189, No. 5
0021-9193/07/$08.00+0     doi:10.1128/JB.00776-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

The pio Operon Is Essential for Phototrophic Fe(II) Oxidation in Rhodopseudomonas palustris TIE-1{triangledown}

Yongqin Jiao1 and Dianne K. Newman1,2,3*

Division of Geological and Planetary Sciences,1 Division of Biology,2 Howard Hughes Medical Institute, California Institute of Technology, Pasadena, California3

Received 30 May 2006/ Accepted 12 December 2006

Phototrophic Fe(II)-oxidizing bacteria couple the oxidation of ferrous iron [Fe(II)] to reductive CO2 fixation by using light energy, but until recently, little has been understood about the molecular basis for this process. Here we report the discovery, with Rhodopseudomonas palustris TIE-1 as a model organism, of a three-gene operon, designated the pio operon (for phototrophic iron oxidation), that is necessary for phototrophic Fe(II) oxidation. The first gene in the operon, pioA, encodes a c-type cytochrome that is upregulated under Fe(II)-grown conditions. PioA contains a signal sequence and shares homology with MtrA, a decaheme c-type cytochrome from Shewanella oneidensis MR-1. The second gene, pioB, encodes a putative outer membrane beta-barrel protein. PioB is a homologue of MtrB from S. oneidensis MR-1. The third gene, pioC, encodes a putative high potential iron sulfur protein (HiPIP) with a twin-arginine translocation (Tat) signal sequence and is similar to the putative Fe(II) oxidoreductase (Iro) from Acidithiobacillus ferrooxidans. Like PioA, PioB and PioC appear to be secreted proteins. Deletion of the pio operon results in loss of Fe(II) oxidation activity and growth on Fe(II). Complementation studies confirm that the phenotype of this mutant is due to loss of the pio genes. Deletion of pioA alone results in loss of almost all Fe(II) oxidation activity; however, deletion of either pioB or pioC alone results in only partial loss of Fe(II) oxidation activity. Together, these results suggest that proteins encoded by the pio operon are essential and specific for phototrophic Fe(II) oxidation in R. palustris TIE-1.

* Corresponding author. Mailing address: California Institute of Technology, Division of Geological and Planetary Sciences, Mail Stop 100-23, Pasadena, CA 91125. Phone: (626) 395-6790. Fax: (626) 683-0621. E-mail: dkn{at}gps.caltech.edu.

{triangledown} Published ahead of print on 22 December 2006.

Journal of Bacteriology, March 2007, p. 1765-1773, Vol. 189, No. 5
0021-9193/07/$08.00+0     doi:10.1128/JB.00776-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.



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