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The zinc finger is a sequence motif involved in binding of DNA:

The C2H2 class

 	Cys-(Xaa)2-Cys-(Xaa)12-His-(Xaa)3-His.

Xaa - nonspecific amino acid.

This motif was first discoved in TFIIIA ( an RNA polymerase III associated transcription factor) isolated from Xenopus laevis (African clawed toad). In TFIIIA, this sequence is repeated nine times in the protein. Each repeat can coordinate a zinc ion with the two cysteines and two histidines:
        ======
       =      =
      =        =
       =      =
      CYS    HIS
       = \  / =
       =  Zn   =
      CYS/  \ =
       =     HIS
       =      =
      =        =
 =====          =====>

The twelve residues between the cysteine and histidine loop out to form a DNA binding interface. When TFIIIA associates with DNA, it binds along the major groove with each finger binding about five nucleotides of DNA. In all cases known, the finger is repeated at least two times. Successive fingers are linked by seven or eight amino acids which form a helical spacer. These modular units may have evolved to make DNA binding surfaces more adaptable to changes in DNA sequence.

The Cx class

The Cx class of zinc fingers have a variable number of cysteines that can chelate a Zn ion. These are also involved in DNA binding such as the GAL4 protein (yeast transcripiton factor involved in galactose metabolism.) They're also found in a number of steriod binding proteins which act as transcription factors. The cysteines are closely spaced and can vary from 4 to 6 in number.
        ======
       =      =
      =        =
       =      =
      CYS    CYS
       = \  / =
       =  Zn   =
      CYS/  \ =
       =     CYS
       =      =
      =        =
 =====          =====>

Zinc fingers function in helping some proteins that bind to DNA recognize that DNA. A zinc finger is a specialized protein group that is characterized by a single zinc atom linked to four cysteine residues or two histidine residues and two cysteine residues holding that part of the polypeptide in a finger-shaped loop.

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.

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