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J. Biol. Chem., Vol. 283, Issue 20, 14092-14099, May 16, 2008

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A Proposed Role for the Azotobacter vinelandii NfuA Protein as an Intermediate Iron-Sulfur Cluster Carrier^*

Sibali Bandyopadhyay, Sunil G. Naik, Ina P. O'Carroll^¶, Boi-Hanh Huynh, Dennis R. Dean^¶, Michael K. Johnson, and Patricia C. Dos Santos^¶1

From the Department of Chemistry, University of Georgia, Athens, Georgia 30602, the Department of Physics, Emory University, Atlanta, Georgia 30322, and the ^¶Department of Biochemistry, Virginia Tech, Blacksburg, Virginia 24061

Iron-sulfur clusters ([Fe-S] clusters) are assembled on molecular scaffolds and subsequently used for maturation of proteins that require [Fe-S] clusters for their functions. Previous studies have shown that Azotobacter vinelandii produces at least two [Fe-S] cluster assembly scaffolds: NifU, required for the maturation of nitrogenase, and IscU, required for the general maturation of other [Fe-S] proteins. A. vinelandii also encodes a protein designated NfuA, which shares amino acid sequence similarity with the C-terminal region of NifU. The activity of aconitase, a [4Fe-4S] cluster-containing enzyme, is markedly diminished in a strain containing an inactivated nfuA gene. This inactivation also results in a null-growth phenotype when the strain is cultivated under elevated oxygen concentrations. NifU has a limited ability to serve the function of NfuA, as its expression at high levels corrects the defect of the nfuA-disrupted strain. Spectroscopic and analytical studies indicate that one [4Fe-4S] cluster can be assembled in vitro within a dimeric form of NfuA. The resultant [4Fe-4S] cluster-loaded form of NfuA is competent for rapid in vitro activation of apo-aconitase. Based on these results a model is proposed where NfuA could represent a class of intermediate [Fe-S] cluster carriers involved in [Fe-S] protein maturation.

Received for publication, November 7, 2007 , and in revised form, March 11, 2008.

^* This work was supported, in whole or in part, by National Institutes of Health Grants GM47295 (to B. H. H.) and GM62524 (to M. K. J.). This work was also supported by Grant MCB0717710 (to D. R. D.) from the National Science Foundation. 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.

¹ To whom correspondence should be addressed: 108 Fralin Biotech Center, West Campus Dr., VA Tech, Blacksburg, VA 24061. Tel.: 540-231-7977; Fax: 540-231-7126; E-mail: pdossant{at}vt.edu.