SENSORS THAT MEDIATE COPPER-SPECIFIC ACTIVATION AND REPRESSION OF GENE-EXPRESSION

Copper ion homeostasis in yeast is maintained, in part, through regula ted expression of genes involved in copper ion uptake, Cu(I) sequestra tion and defense against reactive oxygen intermediates. Positive and n egative copper ion regulation is observed, and both effects occur at t he level of transcription. The mechanism of Cu(I) regulation is distin ct for transcriptional activation versus transcriptional inhibition. C u(I) activation of gene expression occurs through Cu-induced DNA bindi ng by the transcription factors Ace1 in Saccharomyces cerevisiae and A mt1 in Candida glabrata. Cu(I) ion binding within a regulatory domain of each molecule stabilizes a specific tertiary fold capable of high a ffinity interaction with specific DNA promoter sequences. Cu(I)-activa ted transcription factors are modular proteins in which the DNA bindin g domain is distinct from the domain that mediates transcriptional act ivation through assembly of the preinitiation complex. Cu(I) triggerin g involves formation of a tetracopper thiolate cluster within a regula tory domain. Formation of the tetracopper cluster occurs in an all-or- nothing fashion. Thus, the concentration of Cu-activated factor is pro portional to the Cu(I) concentration, thereby directly coupling the in tracellular Cu(I) concentration to transcriptional activation of a sub set of genes. Cu-mediated inhibition of gene expression in S. cerevisi ae occurs through copper regulation of the Mad transcription factor. G enes 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 duplic ated cysteine-rich sequences within the activation domain is consisten t with Cu(I) binding within this domain.