McIntosh Matthew, Krol Elizaveta, Becker Anke
Institute of Biology 3, University of Freiburg, Schänzlestr 1, 79104 Freiburg, Germany.
J Bacteriol. 2008 Aug;190(15):5308-17. doi: 10.1128/JB.00063-08. Epub 2008 May 30.
The symbiotic nitrogen-fixing bacterium Sinorhizobium meliloti possesses the Sin quorum-sensing system based on N-acyl homoserine lactones (AHLs) as signal molecules. The Sin system consists of SinI, the AHL synthase, and SinR, the LuxR-type regulator. This system regulates the expression of a multitude of S. meliloti genes through ExpR, another LuxR-type regulator. Analysis of the activity of the sinI promoter showed that the expression of sinI is dependent on sinR and enhanced by a combination of expR and Sin AHLs. The characterization of the ExpR binding site upstream of sinI and the identification of binding sites upstream of the galactoglucan biosynthesis genes wgaA (expA1) and wgeA (expE1) allowed the definition of a consensus sequence for these binding sites. Based on this consensus, two additional ExpR binding sites in the promoter regions of exoI and exsH, two genes related to the production of succinoglycan, were found. The specific binding of ExpR to the wgaA and wgeA promoters was enhanced in the presence of oxo-C(14)-HL. Positive regulation of the galactoglucan biosynthesis genes by ExpR was shown to be dependent on WggR (ExpG) and influenced by MucR, both of which are previously characterized regulators of these genes. Based on these results, a reworked model of the Sin-ExpR quorum-sensing regulation scheme of galactoglucan production in S. meliloti is suggested.
共生固氮细菌苜蓿中华根瘤菌拥有基于N-酰基高丝氨酸内酯（AHLs）作为信号分子的Sin群体感应系统。Sin系统由AHL合成酶SinI和LuxR型调节因子SinR组成。该系统通过另一个LuxR型调节因子ExpR来调节苜蓿中华根瘤菌众多基因的表达。对sinI启动子活性的分析表明，sinI的表达依赖于sinR，并由expR和Sin AHLs的组合增强。对sinI上游ExpR结合位点的表征以及对半乳葡聚糖生物合成基因wgaA（expA1）和wgeA（expE1）上游结合位点的鉴定，使得能够定义这些结合位点的共有序列。基于该共有序列，在与琥珀酰聚糖产生相关的两个基因exoI和exsH的启动子区域中发现了另外两个ExpR结合位点。在氧代-C(14)-HL存在的情况下，ExpR与wgaA和wgeA启动子的特异性结合增强。ExpR对半乳葡聚糖生物合成基因的正调控被证明依赖于WggR（ExpG）并受MucR影响，这两者都是这些基因先前已被表征的调节因子。基于这些结果，提出了苜蓿中华根瘤菌中半乳葡聚糖产生的Sin-ExpR群体感应调节机制的改进模型。
