P22 c2 Repressor, DNA-Binding Domain (1-76)
This is amino acid residues 1-76 of the amino-terminus of the P22 c2 repressor protein. The P22 c2 repressor is a DNA-binding protein that regulates phage gene expression in the lysogenic state by allowing the phage to reside in the chromosome of the host bacterial organism. This state is maintained from the binding of c2, the regulatory protein, to the or and ol operators. This binding prevents any proteins necessary for lytic development from being transcribed.
The P22 c2 repressor binds as a dimer to specific operator sites in the DNA. The amino-terminal controls the binding of the DNA while the carboxy-terminal controls the dimerization of the protein.
This 76 amino acid residue polypeptide is made up of five alpha helices. Residues 21 to 39 form a helix-turn-helix motif common of proteins involved in gene regulation. In the picture shown above, all alpha helices are shown in magenta, turns are shown in blue, and random coils are shown in green.
This view of P22 c2 repressor protein shows all five alpha helices colored separately: Helix 1=Lavender, helix 2=Violet, helix 3=Aqua, helix 4=Pink, and helix 5=Dark purple. The helix-turn-helix motif can be easily seen here. This motif consists of residues 21-39, or helices 2 (res.21-28) and 3 (res.32-39) connected by a turn (Gly29) shown here in gold.