The bacteriophage lambda CI protein finds an asymmetric solution.

The CI protein of bacteriophage lambda (lambdaCI) is both a repressor and activator of transcription that has served as a model for understanding how gene regulatory proteins work. A dimeric DNA-binding protein, lambdaCI also forms higher-order oligomers that allow it to bind cooperatively to both adjacent and nonadjacent operator sites within the phage genome. The ability of phage lambda to transition efficiently from one program of gene expression to another depends upon the formation of these higher-order protein-DNA complexes. A recently determined crystal structure of a DNA-bound lambdaCI dimer reveals that the two subunits of the dimer adopt different conformations. This unexpected asymmetry helps explain how these higher-order complexes are assembled.