Bacteriophage λ is a paradigm for understanding the role of cooperativity in gene regulation. Comparison of the regulatory regions of λ and the unrelated temperate bacteriophage 186 provides insight into alternate ways to assemble functional genetic switches. The structure of the C-terminal domain of the 186 repressor, determined at 2.7 Å resolution, reveals an unusual heptamer of dimers, consistent with presented genetic studies. In addition, the structure of a cooperativity mutant of the full-length 186 repressor, identified by genetic screens, was solved to 1.95 Å resolution. These structures provide a molecular basis for understanding lysogenic regulation in 186. Whereas the overall fold of the 186 and λ repressor monomers is remarkably similar, the way the two repressors cooperatively assemble is quite different and explains in part the differences in their regulatory activity.
