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Journal of Bacteriology, March 2009, p. 1361-1368, Vol. 191, No. 5
0021-9193/09/$08.00+0     doi:10.1128/JB.01571-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Positive Control of Ferric Siderophore Receptor Gene Expression by the Irr Protein in Bradyrhizobium japonicum{triangledown}

Sandra K. Small, Sumant Puri, Indu Sangwan, and Mark R. O'Brian*

Department of Biochemistry, State University of New York at Buffalo, Buffalo, New York

Received 4 November 2008/ Accepted 16 December 2008

Ferric siderophore receptors are components of high-affinity iron-chelate transport systems in gram-negative bacteria. The genes encoding these receptors are generally regulated by repression. Here, we show that the ferrichrome receptor gene bll4920 and four additional putative ferric siderophore receptor genes in Bradyrhizobium japonicum are positively controlled by the regulatory protein Irr, as observed by the low level of mRNA transcripts in an irr mutant in iron-limited cells. Potential Irr binding sites with iron control element (ICE)-like motifs were found upstream and distal to the transcription start sites of the five receptor genes. However, purified recombinant Irr bound only some of those elements. Nevertheless, dissection of the bll4920 promoter region showed that a component in extracts of wild-type cells grown in iron-limited media bound only in the ICE motif region of the promoter. This binding was not observed with extracts of cells from the parent strain grown under high-iron conditions or from an irr mutant strain. Furthermore, gel mobility supershift experiments identified Irr as the binding protein in cell extracts. Chromatin immunoprecipitation experiments demonstrated that Irr occupies the promoters of the five ferric iron transport genes in vivo. We conclude that Irr is a direct positive regulator of ferric iron transport in B. japonicum.

* Corresponding author. Mailing address: Department of Biochemistry, 140 Farber Hall, State University of New York at Buffalo, Buffalo, NY 14214. Phone: (716) 829-3200. Fax: (716) 829-2725. E-mail: mrobrian{at}buffalo.edu

{triangledown} Published ahead of print on 29 December 2008.

Journal of Bacteriology, March 2009, p. 1361-1368, Vol. 191, No. 5
0021-9193/09/$08.00+0     doi:10.1128/JB.01571-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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