Copper-mediated repression of the activation domain in the yeast Mac1p transcription factor

The expression of a number of genes encoding products involved in copper ion uptake in yeast is specifically inhibited by copper ions. We show here that copper metalloregulation occurs through Cu-dependent repression of the transactivation activity of Mac1p. A segment of the yeast transcription factor Mac1p was identified that activated transcription in vivo in a heterologous system using fusion polypeptides with the yeast Gal4 DNA-binding domain. The Gal4/Mac1p hybrid exhibits transactivation activity that is repressed in cells cultured in the presence of copper salts and derepressed in cells with reduced copper uptake. The repressive effect is specific for copper ions. The concentration dependency of the Cu-inactivation of Gal4/Mac1p is similar to that of Cu-inhibition of CTR1 expression, a known Cu-regulated gene in vivo. Copper inhibition of gene expression is not observed with a Gal4/Mac1p chimera containing the MAC1(up1) substitution within the transactivation domain. Cells harboring the MAC1(up1) allele fail to attenuate FRE1 and CTR1 expression in a Cu dependent manner. Additional MAC1(up) alleles exist within the first of two cysteine-rich sequence motifs adjacent to the His --> Gin MAC1(up1) encoded substitution. Thus, Cu-regulation of Mac1p function arises from a novel Cu-specific repression of the transactivation domain function. Models for the mechanism of Cu-repression of Mac1p function will be discussed.