Opening Image: λ repressor-operator complex. The operator site is 17 base pairs of double-stranded DNA. The repressor is a dimeric protein. Each chain contains an N-terminal domain (residues 1 through 92) that binds to the DNA, and a C-terminal domain. The C-terminal domains interact with each other to hold the dimer together. The isolated N-terminal domains retain their ability to bind specifically to the operator subsites.
cartoon model of one subunit. Notice how one of the α-helices fits into the major groove of the double helix.