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Journal of Bacteriology, March 2008, p. 1631-1637, Vol. 190, No. 5
0021-9193/08/$08.00+0     doi:10.1128/JB.01570-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

The Intracellular Pathogen Rhodococcus equi Produces a Catecholate Siderophore Required for Saprophytic Growth

School of Biomolecular and Biomedical Science, Conway Institute, University College Dublin, Dublin 4, Ireland,¹ Department of Pathobiology, University of Guelph, Guelph, Ontario, Canada,² Veterinary Molecular Microbiology Section, Faculty of Medical and Veterinary Sciences, Veterinary School, University of Bristol, Langford, United Kingdom³

Received 28 September 2007/ Accepted 14 December 2007

Little is known about the iron acquisition systems of the soilborne facultative intracellular pathogen Rhodococcus equi. We previously reported that expression of iupABC, encoding a putative siderophore ABC transporter system, is iron regulated and required for growth at low iron concentrations. Here we show that disruption of iupA leads to the concomitant accumulation of catecholates and a chromophore with absorption maxima at 341 and 528 nm during growth under iron-replete conditions. In contrast, the wild-type strain produces these compounds only in iron-depleted medium. Disruption of iupU and iupS, encoding nonribosomal peptide synthetases, prevented growth of the corresponding R. equi SID1 and SID3 mutants at low iron concentrations. However, only R. equi SID3 did not produce the chromophore produced by the wild-type strain during growth at low iron concentrations. The phenotype of R. equi SID3, but not that of R. equi SID1, could be rescued by coculture with the wild type, allowing growth at low iron concentrations. This strongly suggests that the product of the iupS gene is responsible for the synthesis of a diffusible compound required for growth at low iron concentrations. Transcription of iupU was constitutive, but that of iupS was iron regulated, with an induction of 3 orders of magnitude during growth in iron-depleted compared to iron-replete medium. Neither mutant was attenuated in vivo in a mouse infection model, indicating that the iupU- and iupS-encoded iron acquisition systems are primarily involved in iron uptake during saprophytic life.

Published ahead of print on 21 December 2007.