Douglas Sarah M, Chubiz Lon M, Harcombe William R, Marx Christopher J
Department of Molecular and Cellular Biology, Harvard University, Cambridge, Massachusetts, United States of America.
Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts, United States of America.
PLoS One. 2017 May 11;12(5):e0174345. doi: 10.1371/journal.pone.0174345. eCollection 2017.
Microbes often engage in cooperation through releasing biosynthetic compounds required by other species to grow. Given that production of costly biosynthetic metabolites is generally subjected to multiple layers of negative feedback, single mutations may frequently be insufficient to generate cooperative phenotypes. Synergistic epistatic interactions between multiple coordinated changes may thus often underlie the evolution of cooperation through overproduction of metabolites. To test the importance of synergistic mutations in cooperation we used an engineered bacterial consortium of an Escherichia coli methionine auxotroph and Salmonella enterica. S. enterica relies on carbon by-products from E. coli if lactose is the only carbon source. Directly selecting wild-type S. enterica in an environment that favored cooperation through secretion of methionine only once led to a methionine producer, and this producer both took a long time to emerge and was not very effective at cooperating. On the other hand, when an initial selection for resistance of S. enterica to a toxic methionine analog, ethionine, was used, subsequent selection for cooperation with E. coli was rapid, and the resulting double mutants were much more effective at cooperation. We found that potentiating mutations in metJ increase expression of metA, which encodes the first step of methionine biosynthesis. This increase in expression is required for the previously identified actualizing mutations in metA to generate cooperation. This work highlights that where biosynthesis of metabolites involves multiple layers of regulation, significant secretion of those metabolites may require multiple mutations, thereby constraining the evolution of cooperation.
微生物常常通过释放其他物种生长所需的生物合成化合物来进行合作。鉴于昂贵的生物合成代谢物的产生通常受到多层负反馈的影响，单个突变往往不足以产生合作表型。因此，多个协同变化之间的协同上位相互作用可能常常是通过代谢物过量产生实现合作进化的基础。为了测试协同突变在合作中的重要性，我们使用了一个由大肠杆菌甲硫氨酸营养缺陷型和肠炎沙门氏菌组成的工程细菌联合体。如果乳糖是唯一的碳源，肠炎沙门氏菌依赖大肠杆菌产生的碳副产物。在一个仅通过甲硫氨酸分泌来促进合作的环境中，直接选择野生型肠炎沙门氏菌，只会产生一个甲硫氨酸生产者，而且这个生产者出现的时间很长，合作效果也不太好。另一方面，当最初选择肠炎沙门氏菌对有毒的甲硫氨酸类似物乙硫氨酸的抗性时，随后选择与大肠杆菌合作的速度很快，产生的双突变体在合作方面效率更高。我们发现，metJ中的增强突变会增加metA的表达，metA编码甲硫氨酸生物合成的第一步。这种表达的增加是之前在metA中鉴定出的实现合作的实际突变所必需的。这项工作强调，在代谢物的生物合成涉及多层调控的情况下，这些代谢物的大量分泌可能需要多个突变，从而限制了合作的进化。
