The Cu(I) sensor Mycobacterium tuberculosis CsoR is a founding member of a new metalloregulatory protein family. Here we show that two "atom" substitutions of the N epsilon 2 face of a Cu(1) coordinating histisine-61 allosterically uncouple Cu(1) and DNA binding, with no effect on Cu(1) binding affinity and coordination structure. A model analogous to the allosteric switch mechanism in Staphylococcus aureus CzrA, a zinc sensor protein with a completely different fold, is proposed.