A dual role for zinc fingers in both DNA binding and zinc sensing by the Zap1 transcriptional activator

The Zap1 transcriptional activator of Saccharomyces cerevisiae controls zin c homeostasis. Zap1 induces target gene expression in zinc-limited cells an d is repressed by high zinc. One such target gene is ZAP1 itself. In this r eport, we examine how zinc regulates Zap1 function. First, we show that tra nscriptional autoregulation of Zap1 is a minor component of zinc responsive ness; most regulation of Zap1 activity occurs post-translationally. Secondl y, nuclear localization of Zap1 does not change in response to zinc, sugges ting that zinc regulates DNA binding and/or activation domain function. To understand how Zap1 responds to zinc, we performed a functional dissection of the protein. Zap1 contains two activation domains. DNA-binding activity is conferred by five C-terminal C2H2 zinc fingers and each finger is requir ed for high-affinity DNA binding. The zinc-responsive domain of Zap1 also m aps to the C-terminal zinc fingers. Furthermore, mutations that disrupt som e of these fingers cause constitutive activity of a bifunctional Gal4 DNA-b inding domain-Zap1 fusion protein. These results demonstrate a novel functi on of Zap1 zinc fingers in zinc sensing as well as DNA binding.