The three zinc fingers of the Zif protein are arranged as separate domains in a C-shaped structure that fits readily into the major groove of DNA and wraps partially around the double helix (Figure 11). Each finger contacts with 3-bp segments of the guanin-rich strand DNA, and most of the zinc finger contacts are along a single strand of the DNA (Pabo,1991). Same as other DNA-binding proteins, zinc fingers contact the DNA bases with the N-terminal portion of its a helix.
Figure 11. The three Zif268 zinc fingers are arranged in a C-shaped structure that fits readily into the major groove of DNA and wraps partially around the double helix. The cylinders and arrows indicate the a helicial and b-sheet regions of each finger. The zinc ions are shown in yellow.
Two hydrogen bonds are immediately formed between the DNA guanine residue and the arginine residue in each of the three zinc-finger a helices. Interestedly, each of the arginine-guanine interactions is stabilized by an aspartic acid which occurs as the second reside in each a helices (Figure 12). This side chain-side chain interaction enables the zinc fingers to position and stabilize the long side chain of the arginine. In addition, it enhances the specificity of the arginine-guanine contacts during the transcriptional activation. Unlike the helix-turn-helix proteins, the zinc finger complex is based on its individual fingers which are repeated in a way that allows the finger structure to contact 3 bp of the DNA. As reported by Rosenberg (1976) , the backbone of the double helical structure can establish a frame of reference for proteins to recognize the base pairs. The structure of the zinc fingers, in some sense, just provides another mechanism for getting its a helix interact with the DNA major groove.
