Transcription factors respond to multilevel stimuli and co-occupy promoter regions of target genes to activate RNA polymerase (RNAP) in a cooperative manner. To decipher the molecular mechanism, here we report two cryo-electron microscopy structures of Anabaena transcription activation complexes (TACs): NtcA-TAC composed of RNAP holoenzyme, promoter and a global activator NtcA, and NtcA–NtcB-TAC comprising an extra context-specific regulator, NtcB. Structural analysis showed that NtcA binding makes the promoter DNA bend by ∼50°, which facilitates RNAP to contact NtcB at the distal upstream NtcB box. The sequential binding of NtcA and NtcB induces looping back of promoter DNA towards RNAP, enabling the assembly of a fully activated TAC bound with two activators. Together with biochemical assays, we propose a ‘DNA looping’ mechanism of cooperative transcription activation in bacteria.

转录因子响应多级刺激并共同占据靶基因的启动子区域，以协作方式激活RNA聚合酶（RNAP）。为了破译其分子机制，我们在这里报道了鱼腥藻转录激活复合物（TAC）的两种冷冻电子显微镜结构：由RNAP全酶、启动子和全局激活子NtcA组成的NtcA-TAC，以及包含额外上下文的NtcA–NtcB-TAC。特定调节器，NtcB。结构分析表明，NtcA 结合使启动子 DNA 弯曲约50°，这有利于 RNAP 在远端上游 NtcB 盒处接触 NtcB。 NtcA 和 NtcB 的顺序结合诱导启动子 DNA 向 RNAP 环回，从而能够组装与两个激活剂结合的完全激活的 TAC。结合生化分析，我们提出了细菌中协同转录激活的“DNA 循环”机制。