Department of Biochemistry, University of Cambridge, Building O, Downing Site, Cambridge, CB2 1QW, UK.
Mol Microbiol. 2013 Nov;90(3):457-71. doi: 10.1111/mmi.12369. Epub 2013 Sep 30.
Pectobacterium atrosepticum (Pca) is a Gram-negative phytopathogen which causes disease by secreting plant cell wall degrading exoenzymes (PCWDEs). Previous studies have shown that PCWDE production is regulated by (i) the intercellular quorum sensing (QS) signal molecule, 3-oxo-hexanoyl-l-homoserine lactone (OHHL), and (ii) the intracellular 'alarmone', (p)ppGpp, which reports on nutrient limitation. Here we show that these two signals form an integrated coincidence circuit which ensures that metabolically costly PCWDE synthesis does not occur unless the population is simultaneously quorate and nutrient limited. A (p)ppGpp null ΔrelAΔspoT mutant was defective in both OHHL and PCWDE production, and nutritional supplementation of wild type cultures (which suppresses (p)ppGpp production) also suppressed OHHL and PCWDE production. There was a substantial overlap in the transcriptome of a (p)ppGpp deficient relA mutant and of a QS defective expI (OHHL synthase) mutant, especially with regards to virulence-associated genes. Random transposon mutagenesis revealed that disruption of rsmA was sufficient to restore PCWDE production in the (p)ppGpp null strain. We found that the ratio of RsmA protein to its RNA antagonist, rsmB, was modulated independently by (p)ppGpp and QS. While QS predominantly controlled virulence by modulating RsmA levels, (p)ppGpp exerted regulation through the modulation of the RsmA antagonist, rsmB.
软腐果胶杆菌（Pectobacterium atrosepticum，Pca）是一种革兰氏阴性植物病原体，通过分泌植物细胞壁降解外切酶（PCWDEs）引起疾病。先前的研究表明，PCWDE 的产生受（i）细胞间群体感应（QS）信号分子 3-氧代己酰基-L-高丝氨酸内酯（OHHL）和（ii）报告营养限制的细胞内“警报素”（p）ppGpp 调节。在这里，我们表明这两个信号形成一个集成的巧合电路，确保只有当种群同时达到群体感应和营养限制时，才会发生代谢成本高昂的 PCWDE 合成。一个（p）ppGpp 缺失ΔrelAΔspoT 突变体在 OHHL 和 PCWDE 产生方面都有缺陷，并且野生型培养物的营养补充（抑制（p）ppGpp 产生）也抑制了 OHHL 和 PCWDE 的产生。（p）ppGpp 缺乏的 relA 突变体和 QS 缺陷的 expI（OHHL 合酶）突变体的转录组有很大的重叠，尤其是与毒力相关的基因。随机转座子诱变显示，rsmA 的破坏足以恢复（p）ppGpp 缺失菌株中的 PCWDE 产生。我们发现，RsmA 蛋白与其 RNA 拮抗剂 rsmB 的比率分别受（p）ppGpp 和 QS 调节。虽然 QS 主要通过调节 RsmA 水平来控制毒力，但（p）ppGpp 通过调节 RsmA 拮抗剂 rsmB 发挥调节作用。
