Metalloregulation of FRE1and FRE2Homologs in Saccharomyces cerevisiae*

The high affinity uptake systems for iron and copper ions in Saccharomyces cerevisiaeinvolve metal-specific permeases and two known cell surface Cu(II) and Fe(III) metalloreductases, Fre1 and Fre2. Five novel genes found in theS. cerevisiaegenome exhibit marked sequence similarity to Fre1 and Fre2, suggesting that the homologs are part of a family of proteins related to Fre1 and Fre2. The homologs are expressed genes in S. cerevisiae,and their expression is metalloregulated as is true with FRE1and FRE2. Four of the homologs (FRE3-FRE6) are specifically iron-regulated through the Aft1 transcription factor. These genes are expressed either in cells limited for iron ion uptake by treatment with a chelator or in cells lacking the high affinity iron uptake system. Expression of FRE3-FRE6is elevated in AFT1–1cells and attenuated in aft1null cells, showing that iron modulation occurs through the Aft1 transcriptional activator. The fifth homolog FRE7is specifically copper-metalloregulated. FRE7is expressed in cells limited in copper ion uptake by a Cu(I)-specific chelator or in cells lacking the high affinity Cu(I) permeases. The constitutive expression of FRE7in MAC1cells and the lack of expression in mac1–1cells are consistent with Mac1 being the critical transcriptional activator of FRE7expression. The 5′ promoter sequence of FRE7contains three copper-responsive promoter elements. Two elements are critical for Mac1-dependent FRE7expression. Combinations of either the distal and central elements or the central and proximal elements result in copper-regulated FRE7expression. Spacing between Mac1-responsive sites is important as shown by the attenuated expression of FRE7and CTR1when two elements are separated by over 100 base pairs. From the three Mac1-responsive elements in FRE7, a new consensus sequence for Mac1 binding can be established as TTTGC(T/G)C(A/G).