EXPRESSION OF A YEAST METALLOTHIONEIN GENE FAMILY IS ACTIVATED BY A SINGLE METALLOREGULATORY TRANSCRIPTION FACTOR

The opportunistic pathogenic yeast Candida glabrata elicits at least two major responses in the presence of high environmental metal levels: transcriptional induction of the metallothionein gene family by copper and the appearance of small (gamma-Glu-Cys)(n)Gly peptides in the presence of cadmium. On the basis of a trans-activation selection scheme in the baker's yeast Saccharomyces cerevisiae, we previously isolated a C. glabrata gene which encodes a copper-activated DNA-binding protein designated AMT1. AMT1 forms multiple specific DNA-protein complexes with both C. glabrata MT-I and MT-II(a) promoter DNA fragments. In this report, we localize and define the AMT1-binding sites in the MT-I and MT-II(a) promoters and characterize the mode of AMT1 binding. Furthermore, we demonstrate that the AMT1 protein trans activates both the MT-I and MT-II(a) genes in vivo in response to copper and that this activation is essential for high-level copper resistance in C. glabrata. Although AMT1-mediated trans activation of the C. glabrata metallothionein genes is essential for copper resistance, AMT1 is completely dispensable for cadmium tolerance. The distinct function that metallothionein genes have in copper but not cadmium detoxification in C. glabrata is in contrast to the role that metallothionein genes play in tolerance to multiple metals in higher organisms.