Department of Biochemistry, Duke University School of Medicine, Durham, NC 27710, USA.
Division of Infectious Diseases, Boston Children's Hospital, Harvard Medical School, Boston, MA 02115, USA; Department of Cell and Molecular Biology and Department of Biomedical and Pharmaceutical Sciences, University of Rhode Island, Kingston, RI 02881, USA.
Mol Cell. 2021 Jan 7;81(1):139-152.e10. doi: 10.1016/j.molcel.2020.10.035. Epub 2020 Nov 19.
The bacterium Francisella tularensis (Ft) is one of the most infectious agents known. Ft virulence is controlled by a unique combination of transcription regulators: the MglA-SspA heterodimer, PigR, and the stress signal, ppGpp. MglA-SspA assembles with the σ-associated RNAP holoenzyme (RNAPσ), forming a virulence-specialized polymerase. These factors activate Francisella pathogenicity island (FPI) gene expression, which is required for virulence, but the mechanism is unknown. Here we report FtRNAPσ-promoter-DNA, FtRNAPσ-(MglA-SspA)-promoter DNA, and FtRNAPσ-(MglA-SspA)-ppGpp-PigR-promoter DNA cryo-EM structures. Structural and genetic analyses show MglA-SspA facilitates σ binding to DNA to regulate virulence and virulence-enhancing genes. Our Escherichia coli RNAPσhomodimeric EcSspA structure suggests this is a general SspA-transcription regulation mechanism. Strikingly, our FtRNAPσ-(MglA-SspA)-ppGpp-PigR-DNA structure reveals ppGpp binding to MglA-SspA tethers PigR to promoters. PigR in turn recruits FtRNAP αCTDs to DNA UP elements. Thus, these studies unveil a unique mechanism for Ft pathogenesis involving a virulence-specialized RNAP that employs two (MglA-SspA)-based strategies to activate virulence genes.
弗氏柠檬酸杆菌（Ft）是已知最具传染性的病原体之一。Ft 的毒力由独特的转录调控因子组合控制：MglA-SspA 异二聚体、PigR 和应激信号 ppGpp。MglA-SspA 与 σ 相关的全酶 RNA 聚合酶（RNAPσ）组装，形成一种毒力特异性聚合酶。这些因子激活弗朗西斯氏菌致病性岛（FPI）基因表达，这是毒力所必需的，但机制尚不清楚。在这里，我们报告了 FtRNAPσ-启动子-DNA、FtRNAPσ-（MglA-SspA）-启动子 DNA 和 FtRNAPσ-（MglA-SspA）-ppGpp-PigR-启动子 DNA 的冷冻电镜结构。结构和遗传分析表明，MglA-SspA 有助于 σ 与 DNA 结合，从而调节毒力和毒力增强基因。我们的大肠杆菌 RNAPσ 同源二聚体 EcSspA 结构表明，这是一种普遍的 SspA 转录调控机制。引人注目的是，我们的 FtRNAPσ-（MglA-SspA）-ppGpp-PigR-DNA 结构揭示了 ppGpp 与 MglA-SspA 的结合将 PigR 固定在启动子上。PigR 反过来招募 FtRNAPαCTDs 到 DNA UP 元件。因此，这些研究揭示了一种涉及专门用于毒力的 RNAP 的独特 Ft 发病机制机制，该机制采用两种基于 MglA-SspA 的策略来激活毒力基因。
