The bacteriophage l CI protein finds an asymmetric solution

The CI protein of bacteriophage l (lCI) 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, lCI 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 l 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 lCI dimer reveals that the two subunits of the dimer adopt different conformations. This unexpected asymmetry helps explain how these higher-order complexes are assembled.