Structure and Dynamics of the Tetrameric Mnt Repressor and a Model for its DNA Complex

Abstract

The tetrameric Mnt repressor of bacteriophage P22 consists of two dimeric DNA-binding domains and a tetramerization domain. The NOE and chemical shift data demonstrate that the structures of the domains in the wild-type repressor protein are similar to those of the separate domains, the three-dimensional structures of which have been determined previously. ¹⁵N relaxation measurements show that the linker that connects the anti-parallel four-helix bundle with the two β-sheet DNA-binding dimers is highly flexible. No evidence was found for interactions between the distinct modules. The ¹⁵N relaxation properties of the two domains differ substantially, confirming their structural independence. A model in which one two-stranded coiled coil of the four-helix bundle is attached to one N-terminal dimer is most consistent with the biochemical data and ¹⁵N relaxation data. For the Mnt-DNA complex this geometry fits with a model in which the two β-sheet DNA-binding domains are bound at two successive major grooves of the Mnt operator and the tetramerization domain is packed between these two DNA-bound dimers. In such a model the two-fold symmetry axis of the four-helix bundle coincides with that of the operator sequence and the two bound dimers. Bending of the Mnt operator of approximately 30° upon binding of the tetramer, as measured by gel-shift assays, is in agreement with this model of the Mnt-DNA complex.