Crystal structure of bacteriophage lambda cII and its DNA complex.

The tetrameric cII protein from bacteriophage λ activates transcription from the phage promoters PRE, PI, and PAQ by binding to two direct repeats that flank the promoter −35 element. Here, we present the X-ray crystal structure of cII alone (2.8 Å resolution) and in complex with its DNA operator from PRE (1.7 Å resolution). The structures provide a basis for modeling of the activation complex with the RNA polymerase holoenzyme, and point to the key role for the RNA polymerase α subunit C-terminal domain (αCTD) in cII-dependent activation, which forms a bridge of protein/protein interactions between cII and the RNA polymerase σ subunit. The model makes specific predictions for protein/protein interactions between cII and αCTD, and between αCTD and σ, which are supported by previous genetic studies.