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J. Biol. Chem., Vol. 282, Issue 43, 31812-31820, October 26, 2007

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RsmA Is an Anti-sigma Factor That Modulates Its Activity through a [2Fe-2S] Cluster Cofactor^*

Alisa A. Gaskell, Jason C. Crack, Gabriella H. Kelemen, Matthew I. Hutchings^¶, and Nick E. Le Brun¹

From the Centre for Metalloprotein Spectroscopy and Biology, School of Chemical Sciences and Pharmacy, School of Biological Sciences, and ^¶School of Medicine, Health Policy and Practice, University of East Anglia, Norwich NR4 7TJ, United Kingdom

The rsmA gene of Streptomyces coelicolor lies directly upstream of the gene encoding the group 3 sigma factor ^M. The RsmA protein is a putative member of the HATPase_c family of anti-sigma factors but is unusual in that it contains seven cysteine residues. Bacterial two-hybrid studies demonstrate that it interacts specifically with ^M, and in vitro studies of the purified proteins by native PAGE and transcription assays confirmed that they form a complex. Characterization of RsmA revealed that it binds ATP and that, as isolated, it contains significant quantities of iron and inorganic sulfide, in equal proportion, with spectroscopic properties characteristic of a [2Fe-2S] cluster-containing protein. Importantly, the interaction between RsmA and ^M is dependent on the presence of the iron-sulfur cluster. We propose a model in which RsmA regulates the activity of ^M. Loss of the cluster, in response to an as yet unidentified signal, activates ^M by abolishing its interaction with the anti-sigma factor. This represents a major extension of the functional diversity of iron-sulfur cluster proteins.

Received for publication, June 22, 2007 , and in revised form, August 30, 2007.

^* This work was supported by the Biotechnology and Biological Sciences and Research Council, United Kingdom, through the award of a studentship (to A. A. G.), and the Wellcome Trust through an award from the Joint Infra-structure Fund for equipment. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1 and S2 and Table S1.

¹ To whom correspondence should be addressed. Tel.: 44-1-603-592-699; Fax: 44-1-603-592-003; E-mail: n.le-brun{at}uea.ac.uk.

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