Structural basis of XCII-dependent transcription activation

The CII protein of bacteriophage X activates transcription from the phage promoters PRE, PI, and PAQ by bind-ing to two direct repeats that straddle the promoter -35 element. Although genetic, biochemical, and struc-tural studies have elucidated many aspects of XCII-mediated transcription activation, no precise structure of the transcription machinery in the process is available. Here, we report a 3.1-A cryo-electron microscopy (cryo-EM) structure of an intact XCII-dependent transcription activation complex (TAC-XCII), which com-prises XCII, E. coli RNAP-& sigma;70 holoenzyme, and the phage promoter PRE. The structure reveals the interactions between XCII and the direct repeats responsible for promoter specificity and the interactions between XCII and RNAP a subunit C-terminal domain responsible for transcription activation. We also determined a 3.4-A cryo-EM structure of an RNAP-promoter open complex (RPo-PRE) from the same dataset. Structural com-parison between TAC-XCII and RPo-PRE provides new insights into XCII-dependent transcription activation.