In trace amounts, copper is essential for the function of key enzymes in prokaryotes and eukaryotes. Organisms have developed sophisticated mechanisms to control the cytosolic level of the metal, manage its toxicity and survive in copper-rich environments. Here we show that the Sulfolobus CopR represents a novel class of copper-responsive regulators, unique to the archaeal domain. Furthermore, by disruption of the ORF Sso2652 (copR) of the Sulfolobus solfataricus genome, we demonstrate that the gene encodes a transcriptional activator of the copper-transporting ATPase CopA gene and co-transcribed copT, encoding a putative copper-binding protein. Disruption resulted in a loss of copper tolerance in two copR-knockout mutants, while metals such as zinc, cadmium and chromium did not affect their growth. Copper sensitivity in the mutant was linked to insufficient levels of expression of CopA and CopT. The findings were further supported by time-course inductively coupled plasma optical emission spectrometry measurements, whereby continued accumulation of copper in the S. solfataricus mutant was observed. In contrast, copper accumulation in the wild-type stabilized after reaching approximately 6 pg (µg total protein)(-1). Complementation of the disrupted mutant with a wild-type copy of the copR gene restored the wild-type phenotype with respect to the physiological and transcriptional response to copper. These observations, taken together, lead us to propose that CopR is an activator of copT and copA transcription, and the member of a novel class of copper-responsive regulators.