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Abstract. Xenopus nuclear factor 7 (xnf7) is a maternally expressed protein that belongs to the B-box zinc finger gene family consisting of transcription factors, protooncogenes, and ribonucleoproteins.

A 42 amino acid synthetic peptide corresponding to a newly defined cysteitmkistidine-rich protein motif called B-box, from the Xenopur protein XNF7 has been character&d. The metal-binding stokhiometry and dissociation constant for zinc were determined by competition with the chromopho~c chelator Br,BAPTA, demons~ating that one zinc atom binds per molecule of peptide despite the presence of seven putative metal ligands, and represents the t&t application of this method to measuring zinc stoichiometry of proteins and/or peptides. Cobalt binding studies indicate that the motif binds zinc more tightly than cobalt, that cysteines are used as ligands and that the cation is co-ordinated tetrahedrally. Circular dichroism and NMR studies both indicate that the B-box peptide is structured only in the presence of zinc, copper and to a lesser extent cobalt.