Binns Andrew N, Zhao Jinlei
Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania, USA
Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania, USA.
J Bacteriol. 2020 Mar 26;202(8). doi: 10.1128/JB.00609-19.
Expression of the tumor-inducing (Ti) plasmid virulence genes of is required for the transfer of DNA from the bacterium into plant cells, ultimately resulting in the initiation of plant tumors. The genes are induced as a result of exposure to certain phenol derivatives, monosaccharides, and low pH in the extracellular milieu. The soil, as well as wound sites on a plant-the usual site of the virulence activity of this bacterium-can contain these signals, but gene expression in the soil would be a wasteful utilization of energy. This suggests that mechanisms may exist to ensure that gene expression occurs only at the higher concentrations of inducers typically found at a plant wound site. In a search for transposon-mediated mutations that affect sensitivity for the virulence gene-inducing activity of the phenol, 3,5-dimethoxy-4-hydroxyacetophenone (acetosyringone [AS]), an RND-type efflux pump homologous to the MexE/MexF/OprN pump of was identified. Phenotypes of mutants carrying an insertion or deletion of pump components included hypersensitivity to the -inducing effects of AS, hypervirulence in the tobacco leaf explant virulence assay, and hypersensitivity to the toxic effects of chloramphenicol. Furthermore, the methoxy substituents on the phenol ring of AS appear to be critical for recognition as a pump substrate. These results support the hypothesis that the regulation of virulence gene expression is integrated with cellular activities that elevate the level of plant-derived inducers required for induction so that this occurs preferentially, if not exclusively, in a plant environment. Expression of genes controlling the virulence activities of a bacterial pathogen is expected to occur preferentially at host sites vulnerable to that pathogen. Host-derived molecules that induce such activities in the plant pathogen are found in the soil, as well as in the plant. Here, we tested the hypothesis that mechanisms exist to suppress the sensitivity of species to a virulence gene-inducing molecule by selecting for mutant bacteria that are hypersensitive to its inducing activity. The mutant genes identified encode an efflux pump whose proposed activity increases the concentration of the inducer necessary for gene expression; this pump is also involved in antibiotic resistance, demonstrating a relationship between cellular defense activities and the control of virulence in .
根癌土壤杆菌的致瘤（Ti）质粒毒力基因的表达是将DNA从细菌转移到植物细胞所必需的，最终导致植物肿瘤的形成。这些基因是由于细胞外环境中暴露于某些酚类衍生物、单糖和低pH值而被诱导表达的。土壤以及植物上的伤口部位——这种细菌毒力活性的常见部位——可能含有这些信号，但在土壤中表达这些基因将是对能量的一种浪费性利用。这表明可能存在一些机制来确保这些基因仅在植物伤口部位通常发现的较高浓度诱导物存在时才表达。在寻找影响对酚类物质（3,5 - 二甲氧基 - 4 - 羟基苯乙酮[乙酰丁香酮（AS）]）毒力基因诱导活性敏感性的转座子介导突变时，鉴定出了一种与铜绿假单胞菌的MexE/MexF/OprN泵同源的RND型外排泵。携带泵组件插入或缺失的突变体的表型包括对AS的诱导作用高度敏感、在烟草叶片外植体毒力测定中具有超强毒力以及对氯霉素的毒性作用高度敏感。此外，AS酚环上的甲氧基取代基似乎对于作为泵底物的识别至关重要。这些结果支持了这样一种假设，即毒力基因表达的调控与提高诱导所需的植物来源诱导物水平的细胞活动相结合，以便这种情况优先（如果不是唯一地）在植物环境中发生。预计控制细菌病原体毒力活性的基因表达会优先在易受该病原体侵害的宿主部位发生。在土壤以及植物中都发现了能在植物病原体根癌土壤杆菌中诱导此类活性的宿主来源分子。在此，我们通过选择对其诱导活性高度敏感的突变细菌来测试是否存在抑制根癌土壤杆菌对毒力基因诱导分子敏感性的机制这一假设。鉴定出的突变基因编码一种外排泵，其推测的活性会增加根癌土壤杆菌基因表达所需诱导物的浓度；这种泵还参与抗生素抗性，表明细胞防御活动与根癌土壤杆菌中毒力控制之间存在关联。
