A second iron-regulatory system in yeast independent of Aft1p

Abstract

Iron homeostasis in the yeast Saccharomyces cerevisiae is regulated at the transcriptional level by Aft1p, which activates the expression of its target genes in response to low-iron conditions. The yeast genome contains a paralog of AFT1, which has been designated AFT2. To establish whether AFT1 and AFT2 have overlapping functions, a mutant containing a double aft1⌬aft2⌬ deletion was generated. Growth assays established that the single aft2⌬ strain exhibited no iron-dependent phenotype. However, the double-mutant aft1⌬aft2⌬ strain was more sensitive to lowiron growth conditions than the single-mutant aft1⌬ strain. A mutant allele of AFT2 (AFT2-1 up ), or overexpression of the wildtype AFT2 gene, led to partial complementation of the respiratorydeficient phenotype of the aft1⌬ strain. The AFT2-1 up allele also increased the uptake of 59 Fe in an aft1⌬ strain. DNA microarrays were used to identify genes regulated by AFT2. Some of the AFT2-regulated genes are known to be regulated by Aft1p; however, AFT2-1 up -dependent activation was independent of Aft1p. The kinetics of induction of two genes activated by the AFT2-1 up allele are consistent with Aft2p acting as a direct transcriptional factor. Truncated forms of Aft1p and Aft2p bound to a DNA duplex containing the Aft1p binding site in vitro. The wild-type allele of AFT2 activated transcription in response to growth under low-iron conditions. Together, these data suggest that yeast has a second regulatory pathway for the iron regulon, with AFT1 and AFT2 playing partially redundant roles. metalloregulation ͉ iron homeostasis

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

“…Genes within the iron regulon are induced under low iron conditions and are regulated by the transcription factors, Aft1p and Aft2p (14,15,18). Aft1p and Aft2p are 39% identical within their N-terminal regions, which contain the DNA binding domain of each protein (15).…”

“…Aft1p and Aft2p are 39% identical within their N-terminal regions, which contain the DNA binding domain of each protein (15). The activation domain of Aft1p is within its C-terminal 413-572 amino acid residues which, when fused to the Gal4p DNA binding domain, activates transcription in an iron-independent manner (16).…”

“…Aft1p is localized to the cytosol under iron-replete conditions, but is nuclear and active in iron-deficient conditions. Mutant alleles of AFT1 and AFT2 (AFT1-1 up and AFT2-1 up respectively) have been identified that result in the iron-independent activation of the iron regulon (14,15). These gain of function mutations result from a single cysteine to phenylalanine substitution in identical regions of Aft1p and Aft2p.…”

“…The vector-borne AFT2-1 up allele is able to partially restore iron uptake in the aft1⌬ strain (15). The aft2⌬ strain has no growth phenotype on iron-limiting medium (15).…”

“…The vector-borne AFT2-1 up allele is able to partially restore iron uptake in the aft1⌬ strain (15). The aft2⌬ strain has no growth phenotype on iron-limiting medium (15). However, the aft1⌬ aft2⌬ strain is more sensitive to iron-limiting conditions than the single aft1⌬ strain (15,18).…”