X-ray absorption spectroscopy on the minimal copper-regulatory domains of the two copper-regulated transcription factors (Ace1 and Mac1) in Saccharomyces cerevisiae revealed the presence of a remarkably similar polycopper cluster in both proteins. The Cu-regulatory switch motif of Mac1 consisting of the C-terminal first Cys-rich motif, designated the C1 domain, binds four Cu(I) ions as does the Cu-regulatory domain of Ace1. The four Cu(I) ions are bound to each molecule in trigonal geometry. An extended X-ray absorption fine structure (EXAFS) arising from outer-shell Cu.Cu interactions at 2.7 and 2.9 A was apparent in each Cu(I) complex indicative of a polycopper cluster. The intensity of the 2.9 A Cu.Cu backscatter peak, apparently diminished by partial cancellation, dominates the EXAFS. The results suggest that CuAce1 and CuMac1(C1) contain somewhat distorted forms of a known [Cu(4)-S(6)] cage in which a core of Cu atoms forming an approximate tetrahedron is bound by bridging thiolates above each of the six edges. The tetracopper clusters bound by Ace1 and Mac1 differ in that the Ace1 cluster is coordinated entirely by cysteinyl thiolate, whereas the cysteine-deficient Mac1 cluster appears to consist of a Cu(4)(S-Cys)(5)(N-His) cluster with a bridging histidyl-derived nitrogen. ; Kenneth R. Brown, Greg L. Keller, Ingrid J. Pickering, Hugh H. Harris, Graham N. George, and Dennis R. Winge ; Copyright © 2002 American Chemical Society