Structural basis of λCII-dependent transcription activation.

The CII protein of bacteriophage λ activates transcription from the phage promoters P, P, and P by binding to two direct repeats that straddle the promoter -35 element. Although genetic, biochemical, and structural studies have elucidated many aspects of λCII-mediated transcription activation, no precise structure of the transcription machinery in the process is available. Here, we report a 3.1-Å cryo-electron microscopy (cryo-EM) structure of an intact λCII-dependent transcription activation complex (TAC-λCII), which comprises λCII, E. coli RNAP-σ holoenzyme, and the phage promoter P. The structure reveals the interactions between λCII and the direct repeats responsible for promoter specificity and the interactions between λCII and RNAP α subunit C-terminal domain responsible for transcription activation. We also determined a 3.4-Å cryo-EM structure of an RNAP-promoter open complex (RPo-P) from the same dataset. Structural comparison between TAC-λCII and RPo-P provides new insights into λCII-dependent transcription activation.