Homeostatic regulation of copper uptake in yeast via direct binding of MAC1 protein to upstream regulatory sequences of FRE1 and CTR1

Copper deprivation of Saccharomyces cerevisiae induces transcription of the FRE1 and CTR1 genes. FRE1 encodes a surface reductase capable of reducing and mobilizing copper chelates outside the cell, and CTR1 encodes a protein mediating copper uptake at the plasma membrane. In this paper, the protein encoded by MAC1 is identified as the factor mediating this homeostatic control. A novel dominant allele of MAC1, MAC1(up2), is mutated in a Cys- rich domain that may function in copper sensing (a G to A change of nucleotide 812 resulting in a Cys-271 to Tyr substitution). This mutant is functionally similar to the MAC1(up1) allele in which His-279 in the same domain has been replaced by Gln. Both mutations confer constitutive copper- independent expression of FRE1 and CTR1. A sequence including the palindrome TTTGCTCA... TGAGCAAA, appearing within the 5'-flanking region of the CTR1 promoter, is necessary and sufficient for the copper- and MAC1-dependent CTR1 transcriptional regulation. An identical sequence appears as a direct repeat in the FRE1 promoter. The data indicate that the signal resulting from copper deprivation is transduced via the Cys-rich motif of MAC1 encompassing residues 264-279. MAC1 then binds directly and specifically to the CTR1 and FRE1 promoter elements, inducing transcription of those target genes. This model defines the homeostatic mechanism by which yeast regulates the cell acquisition of copper in response to copper scarcity or excess.