Zinc finger domains [ 1 , 2 ] are nucleic acid-binding protein structures first identified in the Xenopus laevis transcription factor TFIIIA. These domains have since been found in numerous nucleic acid-binding proteins. A zinc finger domain is composed of 25 to 30 amino-acid residues including 2 conserved Cys and 2 conserved His residues in a C-2-C-12-H-3-H type motif. The 12 residues separating the second Cys and the first His are mainly polar and basic, implicating this region in particular in nucleic acid binding. The zinc finger motif is an unusually small, self-folding domain in which Zn is a crucial component of its tertiary structure. All bind 1 atom of Zn in a tetrahedral array to yield a finger-like projection, which interacts with nucleotides in the major groove of the nucleic acid. The Zn binds to the conserved Cys and His residues. Fingers have been found to bind to about 5 base pairs of nucleic acid containing short runs of guanine residues. They have the ability to bind to both RNA and DNA, and it has been suggested that the zinc finger may thus represent the original nucleic acid binding protein. It has also been suggested that a Zn-centred domain could be used in a protein interaction, e.g. in protein kinase C. Many classes of zinc fingers are characterized according to the number and positions of the histidine and cysteine residues involved in the zinc atom coordination. In the first class to be characterized, called C2H2, the first pair of zinc coordinating residues are cysteines, while the second pair are histidines.

U1-type zinc fingers are a family of C2H2-type zinc fingers present in matrin, U1 small nuclear ribonucleoprotein C and other RNA-binding proteins.