Guangdong Key Lab of Biotechnology for Plant Development, School of Life Sciences, South China Normal University, Guangzhou, China.
State Key Laboratory of Plant Genomics, Institute of Microbiology, Chinese Academy of Sciences, Beijing, China.
PLoS Pathog. 2019 Apr 18;15(4):e1007673. doi: 10.1371/journal.ppat.1007673. eCollection 2019 Apr.
Many plant bacterial pathogens including Pseudomonas species, utilize the type III secretion system (T3SS) to deliver effector proteins into plant cells. Genes encoding the T3SS and its effectors are repressed in nutrient-rich media but are rapidly induced after the bacteria enter a plant or are transferred into nutrient-deficient media. To understand how the T3SS genes are regulated, we screened for P. savastanoi pv. phaseolicola (Psph) mutants displaying diminished induction of avrPto-luc, a reporter for the T3SS genes, in Arabidopsis. A mutant carrying transposon insertion into a gene coding for a small functional unknown protein, designated as rhpC, was identified that poorly induced avrPto-luc in plants and in minimal medium (MM). Interestingly, rhpC is located immediately downstream of a putative metalloprotease gene named rhpP, and the two genes are organized in an operon rhpPC; but rhpP and rhpC displayed different RNA expression patterns in nutrient-rich King's B medium (KB) and MM. Deletion of the whole rhpPC locus did not significantly affect the avrPto-luc induction, implying coordinated actions of rhpP and rhpC in regulating the T3SS genes. Further analysis showed that RhpC was a cytoplasmic protein that interacted with RhpP and targeted RhpP to the periplasm. In the absence of RhpC, RhpP was localized in the cytoplasm and caused a reduction of HrpL, a key regulator of the T3SS genes, and also reduced the fitness of Psph. Expression of RhpP alone in E. coli inhibited the bacterial growth. The detrimental effect of RhpP on the fitness of Psph and E. coli required metalloprotease active sites, and was repressed when RhpC was co-expressed with RhpP. The coordination between rhpP and rhpC in tuning the T3SS gene expression and cell fitness reveals a novel regulatory mechanism for bacterial pathogenesis. The function of RhpP in the periplasm remains to be studied.
许多植物病原菌，包括假单胞菌属物种，利用 III 型分泌系统（T3SS）将效应蛋白输送到植物细胞中。编码 T3SS 及其效应子的基因在富含营养的培养基中受到抑制，但在细菌进入植物或转移到营养缺乏的培养基后会迅速诱导。为了了解 T3SS 基因是如何调控的，我们筛选了 P. savastanoi pv. phaseolicola（Psph）突变体，这些突变体在拟南芥中显示出 T3SS 基因的 avrPto-luc 报告基因诱导减弱。一个携带转座子插入到编码一个小的功能未知蛋白的基因中的突变体被鉴定出来，该蛋白被命名为 rhpC，它在植物和最低培养基（MM）中诱导 avrPto-luc 的能力很差。有趣的是，rhpC 位于一个假定的金属蛋白酶基因 rhpP 的下游，这两个基因在一个操纵子 rhpPC 中组织；但 rhpP 和 rhpC 在富含 King's B 培养基（KB）和 MM 的营养丰富的培养基中表现出不同的 RNA 表达模式。rhpPC 整个基因座的缺失并没有显著影响 avrPto-luc 的诱导，这表明 rhpP 和 rhpC 在调节 T3SS 基因方面具有协调作用。进一步的分析表明，RhpC 是一种细胞质蛋白，与 RhpP 相互作用，并将 RhpP 靶向周质。在没有 RhpC 的情况下，RhpP 定位于细胞质中，导致 T3SS 基因的关键调控因子 HrpL 减少，同时也降低了 Psph 的适应性。RhpP 单独在大肠杆菌中的表达抑制了细菌的生长。RhpP 对 Psph 和大肠杆菌适应性的有害影响需要金属蛋白酶活性位点，并且当 RhpC 与 RhpP 共同表达时受到抑制。rhpP 和 rhpC 之间在调节 T3SS 基因表达和细胞适应性方面的协调作用揭示了细菌发病机制的一种新的调控机制。RhpP 在周质中的功能仍有待研究。
