Two transcriptional activators, Aft1 and Aft2, regulate iron homeostasis in 
Saccharomyces cerevisiae. These factors induce the expression of iron regulon 
genes in iron-deficient yeast but are inactivated in iron-replete cells. Iron 
inhibition of Aft1/Aft2 is abrogated in cells defective for Fe-S cluster 
biogenesis within the mitochondrial matrix (Chen, O. S., Crisp, R. J., 
Valachovic, M., Bard, M., Winge, D. R., and Kaplan, J. (2004) J. Biol. Chem. 
279, 29513-29518). To determine whether iron sensing by Aft1/Aft2 requires the 
function of the mitochondrial Fe-S export and cytosolic Fe-S protein assembly 
systems, we evaluated the expression of the iron regulon in cells depleted of 
glutathione and in cells depleted of Atm1, Nar1, Cfd1, and Nbp35. The iron 
regulon is induced in cells depleted of Atm1 with Aft1 largely responsible for 
the induced gene expression. Aft2 is activated at a later time in Atm1-depleted 
cells. Likewise, the iron regulon is induced in cells depleted of glutathione. 
In contrast, repression of NAR1, CFD1, or NBP35 fails to induce the iron regulon 
despite strong inhibition of cytosolic/nuclear Fe-S protein assembly. Thus, iron 
sensing by Aft1/Aft2 is not linked to the maturation of cytosolic/nuclear Fe-S 
proteins, but the mitochondrial inner membrane transporter Atm1 is important to 
transport the inhibitory signal. Although Aft1 and Aft2 sense a signal emanating 
from the Fe-S cluster biogenesis pathway, there is no indication that the 
proteins are inhibited by direct binding of an Fe-S cluster.