In this study, we examine the fate of the nuclear pool of the 
Schizosaccharomyces pombe transcription factor Cuf1 in response to variations in 
copper levels. A nuclear pool of Cuf1-green fluorescent protein (GFP) was 
generated by expressing a functional cuf1(+)-GFP allele in the presence of a 
copper chelator. We then extinguished cuf1(+)-GFP expression and tracked the 
changes in the localization of the nuclear pool of Cuf1-GFP in the presence of 
low or high copper concentrations. Treating cells with copper as well as silver 
ions resulted in the nuclear export of Cuf1. We identified a leucine-rich 
nuclear export signal (NES), (349)LAALNHISAL(358), within the C-terminal region 
of Cuf1. Mutations in this sequence abrogated Cuf1 export from the nucleus. 
Furthermore, amino acid substitutions that impair Cuf1 NES function resulted in 
increased target gene expression and a concomitant cellular hypersensitivity to 
copper. Export of the wild-type Cuf1 protein was inhibited by leptomycin B 
(LMB), a specific inhibitor of the nuclear export protein Crm1. We further show 
that cells expressing a temperature-sensitive mutation in crm1(+) exhibit 
increased nuclear accumulation of Cuf1 at the nonpermissive temperature. 
Although wild-type Cuf1 is localized in the nucleus in both conditions, we 
observed that the protein can still be inactivated by copper, resulting in the 
repression of ctr4(+) gene expression in the presence of exogenous copper. These 
results demonstrate that nuclear accumulation of Cuf1 per se is not sufficient 
to cause the unregulated expression of the copper transport genes like ctr4(+). 
In addition to nuclear localization, a functional Cys-rich domain or NES element 
in Cuf1 is required to appropriately regulate copper transport gene expression 
in response to changes in intracellular copper concentration.