Department of Plant Microbiology and Pathology, University of Missouri at Columbia, 108 Waters Hall, Columbia, MO 65211, USA.
Mol Plant Pathol. 2002 Sep 1;3(5):359-70. doi: 10.1046/j.1364-3703.2002.00128.x.
Summary Erwinia carotovora ssp. carotovora (Ecc) possesses hrpN(Ecc)[hrp = gene for hypersensitive reaction (HR) and pathogenicity], the structural gene for Harpin(Ecc), the inducer of the HR-like response and genes for the type III secretion system. In Ecc, RsmA, an RNA-binding protein responsible for the accelerated decay of RNA species, tightly controls the expression of the Hrp regulon. We document here that Ecc strain 71 possesses several presumed Hrp regulators: HrpX(Ecc), HrpY(Ecc), HrpS(Ecc), and HrpL(Ecc). Nucleotide sequence data indicate that the regulatory genes occur as a cluster. The data also suggest that HrpX(Ecc) is a putative sensor kinase, HrpY(Ecc) is the cognate response regulator, and HrpS(Ecc) is an NtrC-like regulator. hrpL(Ecc) consists of a 543-bp open reading frame (ORF), which encodes a c. 21-kDa protein product. This protein shares significant homology with members of the extracytoplasmic function (ECF) subfamily of alternative sigma factors, including SigE. To examine the regulatory role of hrpL(Ecc), we constructed by marker exchange an HrpL(Ecc) (-) derivative of an RsmA(-) strain. The levels of transcripts of hrcC(Ecc), a gene for the type III secretion system, and hrpN(Ecc) were lower in the RsmA(-)HrpL(Ecc) (-) mutant compared to its RsmA(-)HrpL(Ecc) (+) parent. The RsmA(-)HrpL(Ecc) (-) strain, like the RsmA(-)HrpL(Ecc) (+) parent, caused maceration of celery petioles and produced extracellular pectinases, cellulase, and protease, indicating that the alternative sigma factor is not required for the type I and type II secretion systems, for tissue maceration, or for the production of proteins secreted by these pathways. However, the RsmA(-)HrpL(Ecc) (-) strain, unlike the RsmA(-)HrpL(Ecc) (+) strain, did not elicit the HR in tobacco leaves. In addition, the RsmA(-)HrpL(-) strain failed to cause symptoms in Arabidopsis thaliana. Genetic and biochemical data revealed that Harpin(Ecc) is required for symptom production in Arabidopsis, but is not sufficient by itself to cause necrosis. These observations raise the possibility that another 'effector' protein of Ecc, secreted by the type III system, acts in conjunction with Harpin(Ecc) to trigger responses resulting in cell death in Arabidopsis.
胡萝卜软腐果胶杆菌亚种胡萝卜软腐果胶杆菌（Ecc）拥有 hrpN（Ecc）[hrp = 过敏反应（HR）和致病性基因]，是 Harpin（Ecc）的结构基因，是 HR 样反应和 III 型分泌系统基因的诱导剂。在 Ecc 中，RsmA 是一种负责加速 RNA 物种降解的 RNA 结合蛋白，它严格控制 Hrp 调节子的表达。我们在这里证明 Ecc 菌株 71 拥有几种假定的 Hrp 调节剂：HrpX（Ecc）、HrpY（Ecc）、HrpS（Ecc）和 HrpL（Ecc）。核苷酸序列数据表明，调节基因作为一个簇存在。该数据还表明，HrpX（Ecc）是一种假定的传感器激酶，HrpY（Ecc）是同源的反应调节剂，HrpS（Ecc）是一种 NtrC 样调节剂。hrpL（Ecc）由一个 543 个碱基对的开放阅读框（ORF）组成，编码一个约 21kDa 的蛋白质产物。该蛋白与细胞外功能（ECF）替代 sigma 因子亚家族的成员，包括 SigE，具有显著的同源性。为了研究 HrpL（Ecc）的调节作用，我们通过标记交换构建了 RsmA（-）菌株的 HrpL（Ecc）（-）衍生物。与 RsmA（-）HrpL（Ecc）（+）亲本相比，III 型分泌系统基因 hrcC（Ecc）和 hrpN（Ecc）的转录水平在 RsmA（-）HrpL（Ecc）（-）突变体中较低。与 RsmA（-）HrpL（Ecc）（+）亲本一样，RsmA（-）HrpL（Ecc）（-）菌株引起芹菜叶柄的软化，并产生细胞外果胶酶、纤维素酶和蛋白酶，表明替代 sigma 因子不是 I 型和 II 型分泌系统、组织软化或这些途径分泌的蛋白质所必需的。然而，与 RsmA（-）HrpL（Ecc）（+）菌株不同，RsmA（-）HrpL（Ecc）（-）菌株在烟草叶片中没有引起过敏反应。此外，RsmA（-）HrpL（-）菌株在拟南芥中不能引起症状。遗传和生化数据表明，Harpin（Ecc）是拟南芥症状产生所必需的，但本身不足以引起坏死。这些观察结果提出了一种可能性，即 Ecc 的另一种“效应子”蛋白，由 III 型系统分泌，与 Harpin（Ecc）一起作用，引发导致拟南芥细胞死亡的反应。
