The tetramerization domain of the Mnt repressor consists of two right-handed coiled coils

The tetrameric Mnt repressor is involved in the genetic switch between the lysogenic and lytic growth of Salmonella bacteriophage P22, The solution st ructure of its C-terminal tetramerization domain, which holds together the two dimeric DNA-binding domains, has been determined by NMR spectroscopy. T his structure reveals an assembly of four alpha-helical subunits, consistin g of a dimer of two antiparallel coiled coils with a unique right-handed tw ist. The superhelical winding is considerably stronger and the interhelical separation closer than those found in the well-known left-handed coiled co ils in fibrous proteins and leucine zippers. An unusual asymmetry arises be tween the two monomers that comprise one right-handed coiled coil. A differ ence in the packing to the adjacent monomer of the other coiled coil occurs with an offset of two helical turns. The two asymmetric monomers within ea ch coiled coil interconvert on a time scale of seconds. Both with respect t o symmetry and handedness of helical packing, the C-2 symmetric four-helix bundle of Mnt differs from other oligomerization domains that assemble DNA- binding modules, such as that in the tumor suppressor p53 and the E. coli l ac repressor.