Roles of two activation domains in Zap1 in the response to zinc deficiency in Saccharomyces cerevisiae.

Previous studies suggested that the zinc-responsive Zap1 transcriptional activator directly regulates the expression of over 80 genes in Saccharomyces cerevisiae. Many of these genes play key roles to enhance the ability of yeast cells to grow under zinc-limiting conditions. Zap1 is unusual among transcriptional activators in that it contains two activation domains, designated AD1 and AD2, which are regulated independently by zinc. These two domains are evolutionarily conserved among Zap1 orthologs suggesting that they are both important for Zap1 function. In this study, we have examined the roles of AD1 and AD2 in low zinc growth and the regulation of Zap1 target gene expression. Using alleles that are specifically disrupted for either AD1 or AD2 function, we found that these domains are not redundant, and both are important for normal growth in low zinc. AD1 plays the primary role in zinc-responsive gene regulation, whereas AD2 is required for maximal expression of only a few target promoters. AD1 alone is capable of driving full expression of most Zap1 target genes and dictates the kinetics of Zap1 gene induction in response to zinc withdrawal. Surprisingly, we found that AD1 is less active in zinc-limited cells under heat stress and AD2 plays a more important role under those conditions. These results suggest that AD2 may contribute more to Zap1 function when zinc deficiency is combined with other environmental stresses. In the course of these studies, we also found that the heat shock response is induced under conditions of severe zinc deficiency.