Sensors that mediate copper-specific activation and repression of gene expression

Copper ion homeostasis in yeast is maintained, in part, through regulated expression of genes involved in copper ion uptake, Cu(I) sequestration and defense against reactive oxygen intermediates. Positive and negative copper ion regulation is observed, and both effects occur at the level of transcription. The mechanism of Cu(I) regulation is distinct for transcriptional activation versus transcriptional inhibition. Cu(I) activation of gene expression occurs through Cu-induced DNA binding by the transcription factors Ace1 in Saccharomyces cerevisiae and Amt1 in Candida glabrata. Cu(I) ion binding within a regulatory domain of each molecule stabilizes a specific tertiary fold capable of high affinity interaction with specific DNA promoter sequences. Cu(I)-activated transcription factors are modular proteins in which the DNA binding domain is distinct from the domain that mediates transcriptional activation through assembly of the preinitiation complex. Cu(I) triggering involves formation of a tetracopper thiolate cluster within a regulatory domain. Formation of the tetracopper cluster occurs in an all-or-nothing fashion. Thus, the concentration of Cu-activated factor is proportional to the Cu(I) concentration, thereby directly coupling the intracellular Cu(I) concentration to transcriptional activation of a subset of genes. Cu-mediated inhibition of gene expression in S. cerevisiae occurs through copper regulation of the Mad transcription factor. Genes inhibited in their expression in Cu-treated cells encode proteins involved in Cu ion uptake across the plasma membrane. The activation domain of Mad is repressed in Cu-treated cells. The presence of duplicated cysteine-rich sequences within the activation domain is consistent with Cu(I) binding within this domain.