The control of mom (COM) protein from bacteriophage Mu, a translational activator of the mom gene, is 62 amino acids in length. We have shown previously that COM binds a single Zn(II) ion using four cysteine residues as ligands. COM regulates the translation initiation of the mom mRNA in bacteriophage Mu. In this study the metal specificity of COM for Zn(II), Cd(II), and Co(II) was determined using nuclear magnetic resonance and UV/vis spectroscopic methods. The conditional stability constants were obtained and compared to those of Ether zinc fingers. The results show that the relative metal specificity is quite similar to that of other classical zinc fingers. (Zn(II) much greater than CdO much greater than Co(II), Fe(II)). However, COM shows an unusually high relative affinity for Zn; it binds Zn(II) 100 000-fold more strongly than it binds Co(II). Thus, strong binding is retained at pH 4, where Zn(II) binding is abolished for the other zinc finger binding domains. We speculate that this affinity is important for the physiological function of COM, where the protein may have to compete for a limited pool of "free" zinc in a critical stage in the phage growth cycle.