地衣芽孢杆菌通过糖基化修饰粘球菌素抵御黄色粘球菌捕食的分子机制与生物学意义

捕食行为是不同细菌类群之间广泛存在的相互作用模式，相关研究对于揭示自然界中细菌的多样性种群构成以及生物进化过程中的一些根本科学问题提供了重要的线索和实验证据。黄色粘球菌是第一个被表征为细菌捕食者类群的模式菌，细胞采用群体攻击模式，产生以粘球菌素（抗生素TA）为代表的活性次级代谢产物并且利用水解酶杀伤和降解猎物细胞进行合作捕食。与此同时，被捕食细菌又可以采取不同策略抵御黄色粘球菌的捕食行为。基于前期实验结果，我们提出地衣芽孢杆菌能够通过糖基转移酶YjiC介导的糖基化反应修饰粘球菌素进而获得针对黄色粘球菌的捕食抗性。在本项目中，我们将首先探究糖基化衍生TA的脱毒机理以及地衣芽孢杆菌调控YjiC编码基因过量表达的分子机制，从而阐明其捕食抗性策略。在多个层面探索猎物细胞的捕食抗性对于捕食者细胞行为选择，自然环境中抗生素抗性的广泛分布和进化驱动力，以及被捕食细胞的生态获益所具有的生物学意义。

Predation is a widespread mode of antagonism among living bacteria, and the interaction between predator and prey is the most important factor in bacterial selection and mortality to control and model the bacterial populations in some ecosystems. Myxococcus xanthus is a predatory bacterium that can lyse other bacteria and grow on the nutrients released, which is the model species of the first bacterial group described as micro-predators. As a Gram-negative soil bacterium, M. xanthus predatory behavior is described as a wolf-pack multicellular group attack, in which a quorum of predators is needed to produce secondary metabolites and hydrolytic enzymes to kill and decompose the prey cells. M. xanthus secretes several secondary metabolites with antibiotic properties, among which myxovirescin (a.k.a. antibiotic TA) is considered to be essential for lysing the prey. .The other player in the bacterial predation process is the prey, and the introduction of predators in an ecosystem may cause a rapid evolution of various defensive characteristics in the prey. Although some anti-predation strategies against M. xanthus have been proposed, whether the prey cells can resist M. xanthus predation in natural environments by developing antibiotics resistance remains largely unknown. Based on our preliminary data, we show that some field-isolated strains of Bacillus licheniformis are able to escape from M. xanthus predation by deactivating the antibiotic TA through glucosylation catalyzed by a glycosyltransferase (YjiC). To the best of our knowledge, these results provide the first example for the use of this type of defensive mechanism against M. xanthus predation. In this proposal, we sought to elucidate and prove the detailed molecular mechanisms underlining these observations..A TA glucoside (TA-Glu) was identified in the broth of predation-resistant B. licheniformis J32 cocultured with M. xanthus, and yjiC gene was up-regulated in J32 after the addition of TA. In this proposal, we will elucidate the regulatory mechanism for yjiC gene in the presence of TA and predator M. xanthus. Furthermore, hetero-expressed YjiC modified TA to TA-Glu by conjugating a glucose moiety to the C-21 hydroxyl group. TA-Glu exhibited diminished bactericidal activity due to its weaker binding with bacterial type II signal peptidase (LspA), as suggested by in silico docking data. In this proposal, we would like to obtain the direct evidence for the detoxication of TA by glucosylation. LspA is regarded as an attractive and novel target for antibiotics discovery, and our results will demonstrate that bacteria are able to develop relevant resistance by glycosylation of this LspA inhibitors. .Moreover, we show that the TA resistance in B. licheniformis is induced in a predator-mediated manner and can be rapidly developed under the predatory pressure, supporting the idea that antibiotics resistance in nature is an inducible defense trait for predator avoidance. In the current project, we proposed to investigate the biological significance of our findings, since the bacterial predation is now recognized as a selective driving force in the production and distribution of antibiotic resistance. The expected results from our study will help to understand the specific biological advantages of antibiotic resistance in natural environments. The predation by M. xanthus may induce and select the resistance to TA, and drive the horizontal distribution of resistant genes in the prey populations, which result in that the antibiotic-resistant cells evade predation from antibiotic producers. In addition to serve as a defense mechanism against predation, resistance to antibiotics may also be a strategy for obtaining nutrients without energetic cost associated with production of antibiotics for some predation-resistant bacteria in a nutrient-limited scenario.