Pereira Catarina S, McAuley J Randall, Taga Michiko E, Xavier Karina B, Miller Stephen T
Bacterial Signaling Laboratory, Instituto Gulbenkian de Ciência, Oeiras, Portugal.
Mol Microbiol. 2008 Dec;70(5):1223-35. doi: 10.1111/j.1365-2958.2008.06477.x.
Many bacterial species respond to the quorum-sensing signal autoinducer-2 (AI-2) by regulating different niche-specific genes. Here, we show that Sinorhizobium meliloti, a plant symbiont lacking the gene for the AI-2 synthase, while not capable of producing AI-2 can nonetheless respond to AI-2 produced by other species. We demonstrate that S. meliloti has a periplasmic binding protein that binds AI-2. The crystal structure of this protein (here named SmlsrB) with its ligand reveals that it binds (2R,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran (R-THMF), the identical AI-2 isomer recognized by LsrB of Salmonella typhimurium. The gene encoding SmlsrB is in an operon with orthologues of the lsr genes required for AI-2 internalization in enteric bacteria. Accordingly, S. meliloti internalizes exogenous AI-2, and mutants in this operon are defective in AI-2 internalization. S. meliloti does not gain a metabolic benefit from internalizing AI-2, suggesting that AI-2 functions as a signal in S. meliloti. Furthermore, S. meliloti can completely eliminate the AI-2 secreted by Erwinia carotovora, a plant pathogen shown to use AI-2 to regulate virulence. Our findings suggest that S. meliloti is capable of 'eavesdropping' on the AI-2 signalling of other species and interfering with AI-2-regulated behaviours such as virulence.
许多细菌物种通过调控不同的生态位特异性基因来响应群体感应信号自诱导物-2（AI-2）。在此，我们表明，苜蓿中华根瘤菌作为一种缺乏AI-2合酶基因的植物共生菌，虽然不能产生AI-2，但仍能对其他物种产生的AI-2作出反应。我们证明苜蓿中华根瘤菌有一种结合AI-2的周质结合蛋白。该蛋白（此处命名为SmlsrB）与其配体的晶体结构表明，它结合（2R，4S）-2-甲基-2,3,3,4-四羟基四氢呋喃（R-THMF），这与鼠伤寒沙门氏菌的LsrB识别的相同AI-2异构体。编码SmlsrB的基因位于一个操纵子中，该操纵子与肠道细菌中AI-2内化所需的lsr基因的直系同源物在一起。因此，苜蓿中华根瘤菌能内化外源性AI-2，并且该操纵子中的突变体在AI-2内化方面存在缺陷。苜蓿中华根瘤菌从内化AI-2中未获得代谢益处，这表明AI-2在苜蓿中华根瘤菌中起信号作用。此外，苜蓿中华根瘤菌能够完全消除胡萝卜软腐欧文氏菌分泌的AI-2，胡萝卜软腐欧文氏菌是一种已证明利用AI-2来调节毒力的植物病原体。我们的研究结果表明，苜蓿中华根瘤菌能够“窃听”其他物种的AI-2信号，并干扰诸如毒力等受AI-2调控的行为。
