The Yeast Fre1p/Fre2p Cupric Reductases Facilitate Copper Uptake and Are Regulated by the Copper-modulated Mac1p Activator

Fre1p and Fre2p are ferric reductases which account for the total plasma membrane associated activity, a prerequisite for iron uptake, in Saccharomyces cerevisiae. The two genes are transcriptionally induced by iron depletion. In this communication, we provide evidence that Fre2p has also cupric reductase activity, as has been previously shown for Fre1p (Hassett, R., and Kosman, D.J. (1995) J. Biol. Chem. 270, 128-134). Both Fre1p and Fre2p enzymes are functionally significant for copper uptake, as monitored by the accumulation of the copper-regulated CUP1 and CTR1 mRNAs in fre1Delta, fre2Delta, and fre1Deltafre2Delta mutant strains. However, only Fre1p activity is induced by copper depletion, even in the presence of iron. This differential copper-dependent regulation of Fre1p and Fre2p is exerted at the transcriptional level of the two genes. We have shown that Mac1p, known to affect the basal levels of FRE1 gene expression (Jungmann, J., Reins, H.-A., Lee, J., Romeo, A., Hassett, R., Kosman, D., and Jentsch, S. (1993) EMBO J. 12, 5051-5056), accounts for both the copper-dependent induction of FRE1 and down-regulation of FRE2 gene. Finally, Mac1p transcriptional activation function is itself modulated by the availability of copper.