The pathogenicity of the plant-pathogenic bacterium Xanthomonas campestris pv. vesicatoria depends on a type III secretion system which is encoded by the 23-kb hrp (hypersensitive response and pathogenicity) gene cluster. Expression of the hrp operons is strongly induced in planta and in a special minimal medium and depends on two regulatory proteins, HrpG and HrpX. In this study, DNA affinity enrichment was used to demonstrate that the AraC-type transcriptional activator HrpX binds to a conserved cis-regulatory element, the plant-inducible promoter (PIP) box (TTCGC-N(15)-TTCGC), present in the promoter regions of four hrp operons. No binding of HrpX was observed when DNA fragments lacking a PIP box were used. HrpX also bound to a DNA fragment containing an imperfect PIP box (TTCGC-N(8)-TTCGT). Dinucleotide replacements in each half-site of the PIP box strongly decreased binding of HrpX, while simultaneous dinucleotide replacements in both half-sites completely abolished binding. Based on the complete genome sequence of Xanthomonas campestris pv. vesicatoria, putative plant-inducible promoters consisting of a PIP box and a -10 promoter motif were identified in the promoter regions of almost all HrpX-activated genes. Bioinformatic analyses and reverse transcription-PCR experiments revealed novel HrpX-dependent genes, among them a NUDIX hydrolase gene and several genes with a predicted role in the degradation of the plant cell wall. We conclude that HrpX is the most downstream component of the hrp regulatory cascade, which is proposed to directly activate most genes of the hrpX regulon via binding to corresponding PIP boxes.

译文

:植物致病性细菌Xanthomonas campestris pv的致病性。 vesicatoria依赖于由23-kb hrp(过敏反应和致病性)基因簇编码的III型分泌系统。 hrp操纵子的表达在植物和特殊的基本培养基中强烈诱导,并依赖于两种调节蛋白HrpG和HrpX。在这项研究中,DNA亲和力富集用于证明AraC型转录激活因子HrpX与保守的顺式调控元件,即植物诱导型启动子(PIP)框(TTCGC-N(15)-TTCGC)结合,四个hrp操纵子的启动子区域。当使用缺少PIP盒的DNA片段时,未观察到HrpX的结合。 HrpX还绑定到包含不完整的PIP盒(TTCGC-N(8)-TTCGT)的DNA片段。 PIP盒每个半位中的二核苷酸替换强烈降低了HrpX的结合,而两个半位中同时进行的二核苷酸替换则完全消除了结合。基于Xanthomonas campestris pv的完整基因组序列。在几乎所有的HrpX激活基因的启动子区域中,鉴定出了由vesptoria组成的假定的植物诱导型启动子,该启动子由PIP框和-10个启动子组成。生物信息学分析和逆转录-PCR实验揭示了新的HrpX依赖性基因,其中包括NUDIX水解酶基因和几个在植物细胞壁降解中具有预测作用的基因。我们得出的结论是,HrpX是hrp调节级联的最下游组件,建议通过与相应的PIP盒结合直接激活hrpX regulon的大多数基因。

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