Department of Crop Sciences, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA.
Mol Plant Pathol. 2014 Jan;15(1):58-66. doi: 10.1111/mpp.12065. Epub 2013 Aug 13.
In Erwinia amylovora, ECF (extracytoplasmic functions) alternative sigma factor HrpL regulates the transcription of hrp (hypersensitive response and pathogenicity)-type III secretion system (T3SS) genes by binding to a consensus sequence known as the hrp box in hrp gene promoters. In turn, the expression of hrpL has been proposed to be positively controlled by alternative sigma factor 54 (σ(54)) (RpoN) and HrpS, a member of the σ(54) enhancer-binding proteins (EBPs). However, the function of RpoN has not been characterized genetically in E. amylovora. In this study, we investigated the role of RpoN, a nitrogen limitation sigma factor, and its modulation protein YhbH, a novel ribosome-associated protein, in E. amylovora virulence. Our results showed that mutations in hrpS, hrpL, rpoN and yhbH, but not yfiA and rmf3, resulted in a nonpathogenic phenotype on immature pear fruits and apple shoots. Consistently, the expression of T3SS genes, including hrpL, dspE, hrpN and hrpA, was barely detected in hrpS, hrpL, rpoN and yhbH mutants. These mutants were also not capable of eliciting a hypersensitive response (HR) on tobacco; however, the overexpression of hrpL using an inducible promoter rescued the HR-eliciting abilities of these mutants. These results suggest that a sigma factor cascade exists in the regulatory networks of E. amylovora and regulates important virulence factors. On the basis of this study and previously reported data, a model is proposed for the regulation of T3SS in E. amylovora.
在果胶杆菌中，ECF（细胞外功能）替代σ因子 HrpL 通过与 hrp 基因启动子中称为 hrp 框的共有序列结合来调节 hrp（过敏反应和致病性）-III 型分泌系统（T3SS）基因的转录。反过来，hrpL 的表达被提议通过替代σ因子 54（RpoN）和 HrpS（σ（54）增强子结合蛋白（EBPs）的成员）进行正调控。然而，在果胶杆菌中，RpoN 的功能尚未通过遗传方法进行表征。在这项研究中，我们研究了氮限制σ因子 RpoN 及其调节蛋白 YhbH（一种新型核糖体相关蛋白）在果胶杆菌毒力中的作用。我们的结果表明，hrpS、hrpL、rpoN 和 yhbH 的突变导致在不成熟的梨果实和苹果嫩枝上表现出非致病性表型，但 yfiA 和 rmf3 的突变则没有。一致地，T3SS 基因的表达，包括 hrpL、dspE、hrpN 和 hrpA，在 hrpS、hrpL、rpoN 和 yhbH 突变体中几乎检测不到。这些突变体也不能在烟草上引发过敏反应（HR）;然而，使用诱导型启动子过表达 hrpL 挽救了这些突变体引发 HR 的能力。这些结果表明，在果胶杆菌的调控网络中存在一个σ因子级联，并调节重要的毒力因子。基于这项研究和以前报道的数据，提出了果胶杆菌 T3SS 调控的模型。
