The urease accessory protein encoded by ureE from Klebsiella aerogenes is proposed to function in Ni(II) delivery to the urease apoprotein. Wild-type UreE contains a histidine-rich region at its carboxyl terminus and binds 5-6 Ni per dimer, whereas the functionally active but truncated H144*UreE lacks the histidine-rich motif and binds only two Ni per dimer [Brayman TG, Hausinger RP (1996) J Bacteriol 178:5410-5416]. For both proteins, Gu(II), Co(II), and Zn(II) ions compete for the Ni-binding sites. In order to characterize the coordination environments of bound metals, especially features that are unique to Ni, the Ni-, Cu-, and Go-bound forms of H144*UreE were studied by a combination of EPR, ESEEM, hyperfine-shifted H-1-NMR, XAS, and RR spectroscopic methods. For each metal ion, the two binding sites per homodimer were spectroscopically distinguishable. For example, the two Ni-binding sites each have pseudo-octahedral geometry in an N/O coordination environment, but differ in their number of histidine donors. The two Cu-binding sites have tetragonal geometry with two histidine donors each; however, the second Cu ion is bound by at least one cysteine donor in addition to the N/O-type donors found for the first Cu ion. Two Co ions are bound to H144*UreE in pseudo-octahedral geometry with N/O coordination, but the sites differ in the number of histidine donors that can be observed by NMR, The differences in coordination for each type of metal ion are relevant to the proposed function of UreE to selectively facilitate Ni insertion into urease in vivo.