Transcription factors control gene expression; among these, transcriptional repressors must liberate the promoter for derepression to occur. Toxin–antitoxin (TA) modules are bacterial elements that autoregulate their transcription by binding the promoter in a T:A ratio-dependent manner, known as conditional cooperativity. The molecular basis of how excess toxin triggers derepression has remained elusive, largely because monitoring the rearrangement of promoter–repressor complexes, which underpin derepression, is challenging. Here, we dissect the autoregulation of the Salmonella enterica tacAT3 module. Using a combination of assays targeting DNA binding and promoter activity, as well as structural characterization, we determine the essential TA and DNA elements required to control transcription, and we reconstitute a repression-to-derepression path. We demonstrate that excess toxin triggers molecular stripping of the repressor complex off the DNA through multiple allosteric changes causing DNA distortion and ultimately leading to derepression. Thus, our work provides important insight into the mechanisms underlying conditional cooperativity.

转录因子控制基因表达；其中，转录抑制因子必须释放启动子才能发生去抑制。毒素-抗毒素 (TA) 模块是细菌元件，通过以 T:A 比例依赖的方式（称为条件协同性）结合启动子来自动调节其转录。过量毒素如何触发去抑制的分子基础仍然难以捉摸，主要是因为监测支持去抑制的启动子-阻遏物复合物的重排具有挑战性。在这里，我们剖析了肠沙门氏菌 tacAT3模块的自动调节。通过结合针对 DNA 结合和启动子活性的检测以及结构表征，我们确定了控制转录所需的基本 TA 和 DNA 元件，并重建了抑制到去抑制的路径。我们证明，过量的毒素会通过多种变构变化触发阻遏复合物从 DNA 上分子剥离，从而导致 DNA 扭曲并最终导致去阻遏。因此，我们的工作为了解条件合作性的机制提供了重要的见解。