Transcription of the Yeast Iron Regulon Does Not Respond Directly to Iron but Rather to Iron-Sulfur Cluster Biosynthesis

Abstract

Saccharomyces cerevisiae responds to iron deprivation by increased transcription of the iron regulon, including the high affinity cell-surface transport system encoded by FET3 and FTR1. Here we demonstrate that transcription of these genes does not respond directly to cytosolic iron but rather to the mitochondrial utilization of iron for the synthesis of iron-sulfur (Fe-S) clusters. We took advantage of a mutant form of an iron-dependent enzyme in the sterol pathway (Erg25-2p) to assess cytosolic iron levels. We showed that disruption of mitochondrial Fe-S biosynthesis, which results in excessive mitochondrial iron accumulation, leads to transcription of the iron transport system independent of the cytosolic iron level. There is an inverse correlation between the activity of the mitochondrial Fe-S-containing enzyme aconitase and the induction of FET3. Regulation of transcription by Fe-S biosynthesis represents a mechanism by which cellular iron acquisition is integrated with mitochondrial iron metabolism. Saccharomyces cerevisiae responds to iron deprivation by increased transcription of the iron regulon, including the high affinity cell-surface transport system encoded by FET3 and FTR1. Here we demonstrate that transcription of these genes does not respond directly to cytosolic iron but rather to the mitochondrial utilization of iron for the synthesis of iron-sulfur (Fe-S) clusters. We took advantage of a mutant form of an iron-dependent enzyme in the sterol pathway (Erg25-2p) to assess cytosolic iron levels. We showed that disruption of mitochondrial Fe-S biosynthesis, which results in excessive mitochondrial iron accumulation, leads to transcription of the iron transport system independent of the cytosolic iron level. There is an inverse correlation between the activity of the mitochondrial Fe-S-containing enzyme aconitase and the induction of FET3. Regulation of transcription by Fe-S biosynthesis represents a mechanism by which cellular iron acquisition is integrated with mitochondrial iron metabolism. High affinity iron uptake in Saccharomyces cerevisiae is mediated by a transport system comprising the multicopper oxidase Fet3p and the transmembrane permease Ftr1p (1Van Ho A. Ward D.M. Kaplan J. Annu. Rev. Microbiol. 2002; 56: 237-261Crossref PubMed Scopus (181) Google Scholar). Genes that encode these proteins are part of an iron regulon in which transcription is activated by Aft1p. Aft1p is cytosolic when iron is replete and translocates to the nucleus when iron is limited (2Yamaguchi-Iwai Y. Ueta R. Fukunaka A. Sasaki R. J. Biol. Chem. 2002; 277: 18914-18919Abstract Full Text Full Text PDF PubMed Scopus (143) Google Scholar). It has been speculated that Aft1p responds to cytosolic levels of elemental iron, as conditions that lower cytosolic iron result in increased transcription of the iron regulon. Binding of iron to Aft1p, however, has not been demonstrated. Mutations that affect mitochondrial Fe-S biosynthesis or export result in increased cellular and mitochondrial iron as a result of decreased mitochondrial iron efflux (3Chen O.S. Hemenway S. Kaplan J. Proc. Natl. Acad. Sci. (U. S. A.). 2002; 99: 16922-16927Crossref PubMed Scopus (37) Google Scholar). It has been thought that the increase in mitochondrial iron occurs at the expense of cytosolic iron, and the decrease in cytosolic iron is responsible for the increase in Aft1p-mediated transcription of the iron regulon (3Chen O.S. Hemenway S. Kaplan J. Proc. Natl. Acad. Sci. (U. S. A.). 2002; 99: 16922-16927Crossref PubMed Scopus (37) Google Scholar, 4Lill R. Diekert K. Kaut A. Lange H. Pelzer W. Prohl C. Kispal G. Biol. Chem. 1999; 380: 1157-1166Crossref PubMed Scopus (125) Google Scholar). Herein, we demonstrate that activation of Aft1p does not directly respond to cytosolic iron. Using a novel measure of cytosolic iron, we showed that activation of the iron regulon can occur when cytosolic iron levels are high. We further demonstrated that activation of the iron regulon is controlled by the synthesis of Fe-S clusters, which in yeast is localized within mitochondria. Our studies revealed that cellular iron acquisition is coordinated with the mitochondrial use of iron. Yeast Strains—All yeast strains used in this study were derived from a W303 background strain. The following yeast strains have been described previously: DY150 (Mata ade2-1 his3-11 leu2-3,112 trp1-1 ura3-52 can1-100(oc)), METYFH1 (Matα ade2-1 his3-11 Δyfh1::HIS3 leu2-3,112 trp1-1 ura3-52 can1-100(oc), pMET3YFH1 [URA3]) (5Chen O.S. Kaplan J. J. Biol. Chem. 2000; 275: 7626-7632Abstract Full Text Full Text PDF PubMed Scopus (68) Google Scholar), METNFS1 (Matα ade2-1 his3-11 Δnfs1::HIS3 leu2-3,112 trp1-1 ura3-52 can1-100(oc), pMET3NFS1 [URA3]) (3Chen O.S. Hemenway S. Kaplan J. Proc. Natl. Acad. Sci. (U. S. A.). 2002; 99: 16922-16927Crossref PubMed Scopus (37) Google Scholar), and erg25-2 (Matα erg25-2 ade2-1 his3-11 leu2-3,112 trp1-1 ura3-52 can1-100(oc)) (6Li L. Kaplan J. J. Biol. Chem. 1996; 271: 16927-16933Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar). DNA transformations of Escherichia coli and S. cerevisiae were performed by standard procedures (7Sambrook J. Fritisch E.F. Maniatis T. Molecular Cloning: A Laboratory Manual. 2nd Ed. Cold Spring Harbor Laboratory, Cold Spring Harbor, NY1989Google Scholar). An erg25-2 METCCC1 strain was generated by transforming pMET3CCC1 (5Chen O.S. Kaplan J. J. Biol. Chem. 2000; 275: 7626-7632Abstract Full Text Full Text PDF PubMed Scopus (68) Google Scholar) into the erg25-2 strain. An erg25-2 METYFH1 strain was generated by crossing the METYFH1 strain with an erg25-2 strain. The erg25-2 METNSF1 strain was generated by crossing the METNSF1 strain with an erg25-2 strain. Sporulation and dissection of diploids generated the haploid strains erg25-2 METYFH1 and erg25-2 METNFS1. To generate methionine-regulated genes, the promoter region of MET3 was fused in-frame to the indicated open reading frame (3Chen O.S. Hemenway S. Kaplan J. Proc. Natl. Acad. Sci. (U. S. A.). 2002; 99: 16922-16927Crossref PubMed Scopus (37) Google Scholar). Gene expression was achieved by incubating the strain that contained the construct in methionine-free medium. To turn the methionine-regulated gene off, 10-fold normal medium methionine was supplemented to the medium (final concentration 0.56 mg/ml). To generate strains carrying a FET3-LacZ reporter construct, a plasmid containing the FET3-LacZ reporter was integrated at the HO locus (8Crisp R.J. Pollington A. Galea C. Jaron S. Yamiguchi-Iwai Y. Kaplan J. J. Biol. Chem. 2003; 278: 45499-45506Abstract Full Text Full Text PDF PubMed Scopus (74) Google Scholar). Strains carrying a PGK1-regulated FET3/FTR1 were generated by integrating a plasmid (pFET3/FTR1) at the ade2-1 locus (9Stearman R. Dancis A. Klausner R.D. Gene. 1998; 212: 197-202Crossref PubMed Scopus (17) Google Scholar). Media—Yeast strains were grown in rich medium (YPD; 1% yeast extract, 2% peptone, 2% dextrose) or in synthetic complete medium (CM; 0.7% yeast nitrogen base without amino acids, 2% dextrose, and 0.12% amino acid drop-out mix). The media were made iron-limited by the addition of 80 μm bathophenanthroline disulfonate (BPS) 1The abbreviations used are: BPS, bathophenanthroline disulfonate; 4,4-DMZ, 4,4-dimethylzymosterol. to YPD and 40 μm BPS to the CM. To vary the concentration of media iron, FeSO4 (50 mm stock in 0.1 m HCl) was added back to the media at the indicated concentrations. Experimental Protocols—Cells were iron-loaded by the addition of 50 μm FeSO4 in the presence of 1 mm ascorbate for 30 min. Ascorbate-reduced iron is the substrate for Fet3p/Ftr1p and bypasses the rate-limiting cell-surface reductases. β-Galactosidase was assayed as described by Li et al. (10Li L. Chen O.S. McVey Ward D. Kaplan J. J. Biol. Chem. 2001; 276: 29515-29519Abstract Full Text Full Text PDF PubMed Scopus (277) Google Scholar), sterols were extracted and analyzed by gas chromatography as described by Kennedy and Bard (11Kennedy M.A. Bard M. Biochim. Biophys. Acta. 2001; 1517: 177-189Crossref PubMed Scopus (38) Google Scholar), and aconitase was assayed as described by Chen et al. (3Chen O.S. Hemenway S. Kaplan J. Proc. Natl. Acad. Sci. (U. S. A.). 2002; 99: 16922-16927Crossref PubMed Scopus (37) Google Scholar). Decreased Cytosolic Iron Levels Induce the Iron Regulon—We developed a system in which we could determine the effect of cytosolic iron on induction of the iron transport system. To increase cellular iron, we utilized an integrating plasmid that contains FET3/FTR1 under the control of a constitutive iron-independent promoter (PGK1) (9Stearman R. Dancis A. Klausner R.D. Gene. 1998; 212: 197-202Crossref PubMed Scopus (17) Google Scholar). For these and other experiments, ascorbate-reduced iron was employed to bypass the need for a ferrireductase to provide reduced iron, which is the substrate for the Fet3p/Ftr1p transport system. Increased expression of FET3/FTR1 led to a >3-fold increase in cellular iron uptake above that seen in cells where the endogenous FET3/FTR1 is maximally induced (8Crisp R.J. Pollington A. Galea C. Jaron S. Yamiguchi-Iwai Y. Kaplan J. J. Biol. Chem. 2003; 278: 45499-45506Abstract Full Text Full Text PDF PubMed Scopus (74) Google Scholar). To determine whether the increase in iron due to the integrated FET3/FTR1 plasmid was detrimental to cell survival (i.e. increased oxidative damage), we examined cell growth in response to iron loading. Cells containing the integrated FET3/FTR1 plasmid that had been incubated with iron grew as well as cells without the plasmid or cells with the plasmid not exposed to iron (data not shown). To determine the effect of iron loading on induction of the iron regulon, cells (containing a FET3-LacZ reporter construct) both with and without the integrated FET3/FTR1 plasmid were exposed to ascorbate-reduced iron for 10 min and then incubated in media without bioavailable iron. As might be expected from the increased cytosolic iron, the cells that were exposed to iron and expressing higher levels of FET3/FTR1 showed a lag in the induction of the FET3-LacZ reporter construct (Fig. 1A). Overexpression of the vacuolar iron transporter Ccc1p leads to the increased expression of FET3 mRNA, even in cells grown in iron-replete media (5Chen O.S. Kaplan J. J. Biol. Chem. 2000; 275: 7626-7632Abstract Full Text Full Text PDF PubMed Scopus (68) Google Scholar, 10Li L. Chen O.S. McVey Ward D. Kaplan J. J. Biol. Chem. 2001; 276: 29515-29519Abstract Full Text Full Text PDF PubMed Scopus (277) Google Scholar). We examined whether increased cytosolic iron could attenuate induction of a FET3 reporter construct in cells with increased vacuolar iron storage. Increased expression of the vacuolar iron storage transporter Ccc1p was accomplished through the use of a methionine-regulated plasmid. Strains were engineered that contained a methionine-regulated CCC1 plasmid and a FET3-LacZ reporter construct but differed in that one of the two contained the integrated FET3/FTR1 plasmid. Cells with and without the integrated FET3/FTR1 plasmid were exposed to ascorbate-reduced iron 30 min before induction of CCC1. Overexpression of CCC1 in the strain lacking the integrated FET3/FTR1 construct resulted in immediate activation of FET3-LacZ (Fig. 1B). However, there was a delay in the induction of FET3-LacZ when CCC1 was overexpressed in cells containing the integrated FET3/FTR1 construct. Western analysis showed that Ccc1p was expressed with similar kinetics in both strains (data not shown). Control experiments have shown that the addition or subtraction of methionine does not affect the induction of the FET3-LacZ reporter construct in response to low iron. These results indicate that increased cytosolic iron can affect the rate of induction of the FET3-LacZ reporter construct when cytosolic iron is depleted by storage in the vacuole. Expression of the iron transport system has often been used as a measure of cytosolic iron. We developed an alternative method of measuring cytosolic iron. To examine the relationship between iron and the expression of iron transport genes, we utilized strains that contained a mutant allele of ERG25, erg25-2, which is responsive to cytosolic iron (6Li L. Kaplan J. J. Biol. Chem. 1996; 271: 16927-16933Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar). ERG25 encodes C-4 methyl sterol oxidase, an enzyme essential for ergosterol biosynthesis. Erg25p has been shown to localize to the endoplasmic reticulum and contains histidine motifs thought to coordinate oxo-diiron as a catalytic group (6Li L. Kaplan J. J. Biol. Chem. 1996; 271: 16927-16933Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar). ERG25-2 has a point mutation that appears to result in an enzyme with a lower affinity for iron. When cytosolic iron levels are low, iron is released from the mutant enzyme abolishing its catalytic activity. Thus, enzyme activity is a reflection of cytosolic iron concentration. Under low iron conditions, the erg25-2 strain does not synthesize ergosterol and accumulates the ergosterol intermediate 4,4-dimethylzymosterol (4,4-DMZ). Initially, we constructed a standard curve where we assayed both FET3-LacZ induction and the sterol profile in erg25-2 cells with respect to varying concentrations of media iron. Decreased media iron resulted in an induction in β-galactosidase activity, a decrease in ergosterol biosynthesis, and an increase in 4,4-DMZ (Fig. 2). This result confirms previous work (6Li L. Kaplan J. J. Biol. Chem. 1996; 271: 16927-16933Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar) showing that ergosterol synthesis in cells with the erg25-2 allele responds to cytosolic iron. Consequently, the levels of the sterol intermediate 4,4-DMZ can be used as a measure of cytosolic iron, as the concentration of 4,4-DMZ varies inversely with cytosolic iron. We utilized the erg25-2 strain to confirm that constitutive expression of FET3/FTR1 results in increased cytosolic iron and that induction of Ccc1p can deplete cytosolic iron. We constructed two strains that contained the erg25-2 allele and a regulated CCC1 plasmid. One of the two strains also contained the integrated FET3/FTR1 construct. The two strains were incubated with ascorbate iron for 30 min before the induction of CCC1. At the indicated times, the cells were harvested, and the sterol profile was determined. As expected, the induction of CCC1 in cells containing the erg25-2 allele resulted in an increase in 4,4-DMZ (Fig. 3). No increase in 4,4-DMZ was seen in cells with a wild-type ERG25 allele (data not shown). In cells containing the integrated FET3/FTR1 plasmid, there was approximately a 2-h delay before the level of 4,4-DMZ increased. These results support the data utilizing the FET3-LacZ reporter construct and demonstrate that reduction in cytosolic iron, either through decreased external iron or increased vacuolar iron transport, induces the iron regulon. Decreased Fe-S Cluster Synthesis Increases the Iron Regulon in Iron-replete Cells—To determine whether increased cytosolic iron would affect expression of the FET3-LacZ reporter construct when Fe-S biosynthesis is disrupted, a yeast strain was constructed that permitted the regulated expression of YFH1, the yeast orthologue of the mammalian gene frataxin (responsible for the human disease Friedreich ataxia (12Radisky D.C. Babcock M.C. Kaplan J. J. Biol. Chem. 1999; 274: 4497-4499Abstract Full Text Full Text PDF PubMed Scopus (251) Google Scholar)). Loss of YFH1 expression results in decreased Fe-S biosynthesis and increased cellular and mitochondrial iron accumulation (3Chen O.S. Hemenway S. Kaplan J. Proc. Natl. Acad. Sci. (U. S. A.). 2002; 99: 16922-16927Crossref PubMed Scopus (37) Google Scholar). We examined whether increased cytosolic iron would affect the kinetics of the expression of a FET3 reporter construct in cells that did not express YFH1. Cells containing a chromosomal deletion of YFH1 (a plasmid containing a methionine-regulated YFH1 gene) and an integrated FET3-LacZ reporter construct were incubated with iron ascorbate for 30 min to allow the accumulation of intracellular iron. The cultures were then supplemented with methionine to repress the expression of YFH1. As expected, the inhibition of YFH1 expression resulted in activation of the FET3-LacZ reporter construct (Fig. 4). Despite the increase in cytosolic iron, similar kinetics were observed for activation of FET3-LacZ in the strain carrying the integrated FET3/FTR1 construct. This is in contrast to the delay observed when iron levels are decreased by either low media iron or by CCC1 overexpression. Induction of FET3-LacZ independent of cellular iron content was also seen when expression of the FET3-LacZ reporter construct was assayed after cells had been incubated for 6 h with a range of media iron. In the absence of Yfh1p, there was a significant activation of β-galactosidase, even in the presence of 250 μm iron (data not shown). These results suggest that activation of the iron regulon in the absence of Yfh1p is independent of cytosolic iron levels and appears to be dependent on a mitochondrial product. This result was confirmed by measuring the sterol levels in cells carrying the erg25-2 allele and a regulated YFH1 construct, erg25-2 METYFH1. The absence of Yfh1p in iron-replete media results in the induction of the FET3-LacZ reporter construct comparable with that seen in severely iron-starved cells. Compared with wild-type cells, however, there was no change in sterol profile (ergosterol was high and 4,4-DMZ was low), suggesting that cytosolic iron levels are sufficient to iron load Erg25-2p (Fig. 5). Although studies suggest that the primary function of Yfh1p/frataxin is involvement in Fe-S biosynthesis (3Chen O.S. Hemenway S. Kaplan J. Proc. Natl. Acad. Sci. (U. S. A.). 2002; 99: 16922-16927Crossref PubMed Scopus (37) Google Scholar, 13Muhlenhoff U. Richhardt N. Ristow M. Kispal G. Lill R. Hum. Mol. Genet. 2002; PubMed Scopus Google Scholar, G. A. J. T. Hum. Mol. Genet. 2002; PubMed Scopus Google Scholar), other for Yfh1p have been including on mitochondrial iron storage J. S. G. J. Hum. Genet. 2000; Full Text Full Text PDF PubMed Scopus Google Scholar) and synthesis R. Dancis A. Hum. Mol. Genet. 2003; PubMed Scopus Google Scholar). The deletion of other genes that affect Fe-S biosynthesis also leads to increased mitochondrial iron accumulation and increased expression of the iron regulon. encodes a which for Fe-S biosynthesis J. M. D. Dancis A. J. Biol. Chem. 1999; 274: Full Text Full Text PDF PubMed Scopus Google Scholar, G. Prohl C. Lill R. J. 1999; PubMed Scopus Google Scholar). We examined the effect of reduced expression of on both FET3-LacZ induction and the sterol profile in an erg25-2 METNFS1 strain. to the of Yfh1p, the inhibition of expression resulted in high levels of β-galactosidase activity with a wild-type sterol profile (Fig. 5). This result confirms the that in Fe-S synthesis can the iron regulon even when cytosolic iron is high. of erg25-2 cells can also be used to cytosolic iron levels in cells depleted of In the absence of Yfh1p, iron accumulates in at the expense of cytosolic iron, then of cytosolic iron the low iron growth of the erg25-2 In iron-replete erg25-2 METYFH1 grew as well as wild-type or erg25-2 cells independent of YFH1 expression (Fig. iron-limited erg25-2 cells showed the expected growth as did erg25-2 METYFH1 cells that expressed YFH1. In the absence of YFH1 erg25-2 METYFH1 cells grew erg25-2, suggesting that cytosolic iron was sufficient to Erg25-2p and ergosterol biosynthesis. Although of CCC1 also leads to induction of the erg25-2 METCCC1 strain to on iron-limited These results indicate that deletion of YFH1 mitochondrial iron, does not deplete the of cytosolic iron. Our data suggest that cellular iron acquisition is regulated by the of iron for the of Fe-S biosynthesis. We that in cytosolic iron, from media iron deprivation or of the vacuolar iron to a decrease in the activity of the Fe-S containing the enzyme aconitase and also to a induction of FET3 We aconitase activity and FET3-LacZ induction in cells incubated under varying media iron concentrations (Fig. We performed a analysis on the FET3-LacZ and aconitase activity data from (Fig. There was a inverse correlation between FET3 induction and aconitase activity We examined FET3-LacZ induction and aconitase activity in and METNFS1 strains under conditions in which the gene was either on or (Fig. open there was a inverse correlation between aconitase activity and FET3 these in the analysis increased the to these data suggest that expression of FET3 is regulated by mitochondrial Fe-S synthesis and not directly through the cytosolic iron The control of cellular iron acquisition by mitochondrial Fe-S synthesis is a mechanism that would both cytosolic iron levels and mitochondrial iron metabolism. Our data suggest that Aft1p not directly elemental iron but rather Fe-S or a from an Fe-S-containing This is on the that induction of the iron regulon by the inhibition of Fe-S synthesis occurs when cytosolic iron levels are high. of cytosolic iron has been for as as yeast and The is that cell of cytosolic iron. The cytosolic has been used to cytosolic iron in cells W. S. J. PubMed Google Scholar). has not been of use in We took advantage of a mutant enzyme in the sterol pathway to provide a measure of cytosolic iron. Erg25p is an enzyme localized to the endoplasmic on Erg25p is an and iron is essential for activity. The erg25-2 allele was the activity of the and cell was dependent on cytosolic iron. in cytosolic iron led to decreased growth as a of decreased ergosterol biosynthesis (6Li L. Kaplan J. J. Biol. Chem. 1996; 271: 16927-16933Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar). In this we have shown that in sterol profile are responsive to media iron. Thus, we not the of the of the enzyme the or the the activity of the mutant enzyme is responsive to cytosolic iron. We utilized the erg25-2 allele to that reduction in Fe-S which induces transcription of the iron regulon, does not to a reduction in cytosolic the sterol profile of the erg25-2 cell iron-limited we have shown that induction of the iron regulon by decreased Fe-S synthesis can cells to iron, which can allow erg25-2 cells to Although cytosolic iron is the substrate for mitochondrial Fe-S biosynthesis, conditions that to cytosolic iron deprivation reduced media iron iron transport, or increased vacuolar iron result in decreased Fe-S biosynthesis and increased expression of the iron regulon (3Chen O.S. Hemenway S. Kaplan J. Proc. Natl. Acad. Sci. (U. S. A.). 2002; 99: 16922-16927Crossref PubMed Scopus (37) Google Scholar). of the iron regulon by mitochondrial Fe-S biosynthesis can the in which of Fe-S leads to mitochondrial iron accumulation but when the cytosolic iron level is high (12Radisky D.C. Babcock M.C. Kaplan J. J. Biol. Chem. 1999; 274: 4497-4499Abstract Full Text Full Text PDF PubMed Scopus (251) Google Scholar, 1999; PubMed Scopus (143) Google Scholar). that result in cytosolic iron mitochondrial iron accumulation in strains in Fe-S biosynthesis. Our results suggest that in the absence of Fe-S biosynthesis, the cytosolic iron level is mitochondrial iron The from data is that Aft1p, or a that with Aft1p, be an Fe-S-containing and that the of the Fe-S transcription of genes for iron Aft1p contains a that could in mutation in one of the which might Aft1p from an results in a allele Y. Dancis A. Klausner R.D. J. PubMed Scopus Google Scholar). Expression of Aft1p in under conditions in which the Fe-S was overexpressed did not provide that Aft1p contains Fe-S clusters. R. It is that Aft1p between and the the transcription for the regulon and regulon are (1Van Ho A. Ward D.M. Kaplan J. Annu. Rev. Microbiol. 2002; 56: 237-261Crossref PubMed Scopus (181) Google Scholar). We that the in the of transcription is of the of elemental by and in contrast to Aft1p, which a mitochondrial rather elemental iron. A for Fe-S of gene expression is demonstrated by mammalian in which the of an Fe-S the and expression of in iron transport and storage T. Klausner R. PubMed Scopus Google Scholar). In the Fe-S of and the transcription of genes for and the H. Microbiol. 2003; PubMed Scopus Google Scholar, T. C. H. Proc. Natl. Acad. Sci. U. S. A. 2001; PubMed Scopus Google Scholar). It well be that Aft1p to other proteins that Fe-S or that Aft1p the of an Fe-S Cells respond to Fe-S synthesis by the of iron in the and in the iron in the the concentration of one of the essential for Fe-S synthesis is also increased. we demonstrated that transcription of the cell-surface iron transport system (8Crisp R.J. Pollington A. Galea C. Jaron S. Yamiguchi-Iwai Y. Kaplan J. J. Biol. Chem. 2003; 278: 45499-45506Abstract Full Text Full Text PDF PubMed Scopus (74) Google Scholar). In the absence of transcription is even Aft1p the of genes as FET3. This that of Aft1p is regulated by other gene In however, synthesis is as yeast can without Fe-S however, is transcription of the iron transport system to the presence of or the synthesis of Fe-S for integrating mitochondrial iron with cellular iron