The CII protein of bacteriophage λ activates transcription from the phage promoters P_RE, P_I, and P_AQ 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-σ⁷⁰ holoenzyme, and the phage promoter P_RE. 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_RE) from the same dataset. Structural comparison between TAC-λCII and RPo-P_RE provides new insights into λCII-dependent transcription activation.

噬菌体 λ 的 CII 蛋白通过与跨启动子 -35 元件的两个同向重复序列结合，激活噬菌体启动子PRE、P _I和P _{AQ的转录。}尽管遗传、生化和结构研究已经阐明了 λCII 介导的转录激活的许多方面，但尚无该过程中转录机制的精确结构。在这里，我们报告了完整的 λCII 依赖性转录激活复合物 (TAC-λCII) 的 3.1-Å 冷冻电子显微镜 (cryo-EM) 结构，该复合物包含 λCII、大肠杆菌 RNAP-σ 70 全酶和噬菌体^启动子预。_{_} 该结构揭示了 λCII 和负责启动子特异性的同向重复序列之间的相互作用，以及 λCII 和负责转录激活的 RNAP α 亚基 C 端结构域之间的相互作用。我们还从同一数据集中确定了 RNAP 启动子开放复合物 (RPo- P _{RE ) 的 3.4-Å 冷冻电镜结构。}TAC-λCII 和 RPo- P _RE之间的结构比较为 λCII 依赖性转录激活提供了新的见解。