The AFT1 Transcriptional Factor is Differentially Required for Expression of High‐Affinity Iron Uptake Genes in Saccharomyces cerevisiae

Abstract

High-affinity iron uptake in Saccharomyces cerevisiae involves the extracytoplasmic reduction of ferric ions by FRE1 and FRE2 reductases. Ferrous ions are then transported across the plasma membrane through the FET3 oxidase-FTR1 permease complex. Expression of the high-affinity iron uptake genes is induced upon iron deprivation. We demonstrate that AFT1 is differentially involved in such regulation. Aft1 protein is required for maintaining detectable non-induced levels of FET3 expression and for induction of FRE2 in iron starvation conditions. On the contrary, FRE1 mRNA induction is normal in the absence of Aft1, although the existence of AFT1 point mutations causing constitutive expression of FRE1 (Yamaguchi-Iwai et al., EMBO J. 14: [1231][1232][1233][1234][1235][1236][1237][1238][1239] 1995) indicates that Aft1 may also participate in FRE1 expression in a dispensable way. The alterations in the basal levels of expression of the high-affinity iron uptake genes may explain why the AFT1 mutant is unable to grow on respirable carbon sources. Overexpression of AFT1 leads to growth arrest at the G 1 stage of the cell cycle. Aft1 is a transcriptional activator that would be part of the different transcriptional complexes interacting with the promoter of the high-affinity iron uptake genes. Aft1 displays phosphorylation modifications depending on the growth stage of the cells, and it might link induction of genes for iron uptake to other metabolically dominant requirements for cell growth.

The yeast A kinases differentially regulate iron uptake and respiratory function

“…FRE2, FET3, FTR1, and CCC2 are transcriptionally activated by Aft1 under conditions of iron deprivation, and FRE2 mRNA is undetectable in an aft1 mutant (34). FRE2, FET3, FTR1, and CCC2, which are under Tpk2 control, contain Aft1 consensus binding sites in their promoter regions, and Aft1 has been shown to bind these sites (35).…”

“…FRE2, FET3, FTR1, and CCC2, which are under Tpk2 control, contain Aft1 consensus binding sites in their promoter regions, and Aft1 has been shown to bind these sites (35). AFT1 null mutants grow on fermentable carbon sources but not on nonfermentable carbon sources (34). This inability to grow on nonfermentable carbon sources is suppressed by the addition of ferrous iron to the growth medium, suggesting that the aft1 growth defect is due to poor iron uptake (34).…”

“…AFT1 null mutants grow on fermentable carbon sources but not on nonfermentable carbon sources (34). This inability to grow on nonfermentable carbon sources is suppressed by the addition of ferrous iron to the growth medium, suggesting that the aft1 growth defect is due to poor iron uptake (34). The simplest explanation for this circuitry would be that Tpk2 negatively regulates Aft1 activity, which is required for expression of these high-affinity iron-uptake genes after the switch to respiratory growth.…”

Aft1p and Aft2p Mediate Iron-responsive Gene Expression in Yeast through Related Promoter Elements

“…It was noted that both these FeRE sequences deviate from the consensus FeRE at the most 3Ј-nucleotide. These data are consistent with the observation that a strain containing an AFT1 allele (aft1-1⌬) encoding only the N-terminal 440 residues of Aft1p, and thereby lacking the C-terminal transactivation domain, is more sensitive to low iron conditions than the aft1⌬ strain (17). Such a phenotype might arise if the truncated Aft1p was able to compete successfully with Aft2p for binding to particular FeREs, and thereby prevent Aft2p from activating the iron regulon.…”

“…A strain that lacks a functional Aft1p (aft1⌬) grows poorly in iron-limiting conditions and exhibits reduced iron uptake and cell surface iron-reductase activity (14,17). The vector-borne AFT2-1 up allele is able to partially restore iron uptake in the aft1⌬ strain (15).…”