Retroviral gag gene-encoded core nucleic acid binding proteins contain either one or two sequences of the form Cys-Xaa2-Cys-Xaa4-His-Xaa4-Cys. Previously, one of us has proposed that these sequences form metal-binding domains in analogy with the "zinc ringer" domains first observed in transcription factor MA. We report that an 18-amino acid peptide derived from the core nucleic acid binding protein from Rauscher murine leukemia virus binds metal ions such as Co2' and Zn2+. The absorption spectrum of the peptide-Co2 complex is highly suggestive of tetrahedral coordination involving three cysteinates and one histidine. Titration experiments indicate that the dissociation constant for the peptideCo2+ complex is 1.0 ,uM and that Zn2+ binds more tightly than Co2+. A detailed three-dimensional structure for this domain based on conserved substructures in other crystallographically characterized metalloproteins and on a detailed analysis of the Cys-Xaa2-Cys-Xaa4-His-Xaa4-Cys sequences from retroviruses and other related sources is proposed.In 1985, two groups observed the occurrence of nine tandem sequences of the form Cys-Xaa4-Cys-Xaa12-His-Xaa3-His in the deduced amino acid sequence of Xenopus transcription factor IIIA (TFIIIA) (1,2). Based on the presence of zinc in a purified TFIIIA-5S RNA complex (1, 3), it was proposed that each of these sequences forms a metal-binding domainthat is, a relatively discrete structural unit stabilized by the tetrahedral coordination of a zinc ion to the invariant cysteine and histidine residues. These domains were termed "zinc fingers" (1). Subsequently, numerous other deduced protein sequences have been found that contain quite similar sequences that match the template described above (4-6).Where it is known, the function of these proteins is to act as specific nucleic acid binding proteins. Studies with several proteins have revealed that zinc is required for this activity (3,(7)(8)(9). The hypothesis that these sequences do indeed form metal-binding domains has been amply supported by a wide variety ofmethods including limited proteolysis studies ofthe TFIIIA-SS RNA complex (1), extended x-ray absorption fine structure spectroscopic studies of the zinc sites in the TF-IIIA-5S RNA complex (10), studies of the structure of the TFIIIA gene (11), hydroxyl radical footprinting studies of a series of shortened versions of TFIIIA on a 5S RNA gene (12), and studies of single domain peptides (13,14 (20) that appear to share the property that they undergo a reverse transcription step at some point in their life cycles (21). The importance ofthe conserved cysteine and histidine residues for viral replication has been directly demonstrated by site-directed mutagenesis in two systems (22,23). Results obtained by using a radioactive zinc blotting technique indicated that these proteins have an affinity for zinc under certain conditions (24). We report herein that an 18-amino acid sequence Asp-Gln-Cys-Ala-Tyr-Cys-Lys-Glu-LysGly-His-Trp-Ala-Lys-Asp-Cys-Pro-Lys derived from the sequence o...