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The bacteriophage λ CI protein finds an asymmetric solution

The CI protein of bacteriophage λ ( λ CI) 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, λ CI 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 λ 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 λ CI dimer reveals that the two subunits of the dimer adopt different conformations. This unexpected asymmetry helps explain how these higher-order complexes are assembled. authors show that genetically disrupting λ CI cooperativity in the context of an otherwise wild-type phage prevents the formation of stable lysogens. They then demonstrate that compensatory mutations can be introduced into this phage that restore qualitatively wild-type behavior (both stable lysogeny and inducibility) without restoring λ CI cooperativity a rigorous test of the idea that Cro-mediated repression of P RM transcription during prophage induction is required for an efficient transition from lysogeny to lytic