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A more recent version of this article appeared on December 1, 2008
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Submitted on June 19, 2008
Revised on September 5, 2008
Accepted on September 25, 2008

*Department of Biochemistry,
Graduate Program in Biomolecular Chemistry, University of Wisconsin, Madison WI 53706
Monitoring Editor: Thomas D. Fox
Isu, the scaffold protein on which Fe-S clusters are built in the mitochondrial matrix, plays a central role in the biogenesis of Fe-S cluster proteins. We report that the reduction in the activity of several components of the cluster biogenesis system, including the specialized Hsp70 Ssq1, causes a 15–20-fold upregulation of Isu. This upregulation results from changes at both the transcriptional and posttranslational level - an increase in ISU mRNA levels and in stability of ISU protein. Its biological importance is demonstrated by the fact that cells lacking Ssq1 grow poorly when Isu levels are prevented from rising above those found in wild type cells. Of the biogenesis factors tested, Nfs1, the sulfur donor, was unique. Little increase in Isu levels occurred when Nfs1 was depleted. However, its presence was required for the upregulation caused by reduction in activity of other components. Our results are consistent with the existence of a mechanism to increase the stability of Isu, and thus its level, that is dependent on the presence of the cysteine desulfurase Nfs1.
Present address: Viral Biochemistry Section, National Institutes of Allergy and Infectious Disease, NIH, Bethesda MD 20892.
Address correspondence to:
Elizabeth A. Craig (ecraig{at}wisc.edu)
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