Lambda repressor, or cI repressor, binds operator sites as a dimer. This image shows cI repressor binding to the OL1 site via a helix-turn-helix motif. The OL1 site is 17 base pairs in length with symmetry about the central base pair at position 9. The consensus half of the OL1 site (base-pairs 1-8) match the TATCACCG consensus sequence determined from the 12 operator sites in lambda phage. cI repressor binds these the two half sites of the OL1 operator slightly different.

Display of individual dimer subunits.

Helix-Turn-Helix Motif

Helices 2 and 3 form the highly conserved HTH structural motif. This motif is comonly found in bacterial and bacterialphage regulatory proteins. This domain, in conjunction with the N-terminal arm, is responsible for the binding of cI repressor to DNA. Helix 2 contacts the DNA backbone while helix 3 fits directally into the major groove of the DNA. Helix 2 includes residues Gln33-Lys 39. Helix 2 makes no significant DNA binding contributions in cI repressor binding. The binding of helix 3 in the major groove is covered in the major groove binding section of this web page.

Significance of the Turn in HTH

Significant protein-protien interactions occur in the turn that contribute to the overall stability of the HTH motif. Residues Met40-Gly43 compose the turn. The backbone carbonyl of Ala37 forms H-bonds with both Gly40 and Met 42; More H-bonding occurs between the backbone carboyl of Val36 and the amide of Met40.