Quandt Erik M, Gollihar Jimmy, Blount Zachary D, Ellington Andrew D, Georgiou George, Barrick Jeffrey E
Institute for Cellular and Molecular Biology, The University of Texas at Austin, Austin, United States.
BEACON Center for the Study of Evolution in Action, Michigan State University, East Lansing, United States.
Elife. 2015 Oct 14;4:e09696. doi: 10.7554/eLife.09696.
Evolutionary innovations that enable organisms to colonize new ecological niches are rare compared to gradual evolutionary changes in existing traits. We discovered that key mutations in the gltA gene, which encodes citrate synthase (CS), occurred both before and after Escherichia coli gained the ability to grow aerobically on citrate (Cit(+) phenotype) during the Lenski long-term evolution experiment. The first gltA mutation, which increases CS activity by disrupting NADH-inhibition of this enzyme, is beneficial for growth on the acetate and contributed to preserving the rudimentary Cit(+) trait from extinction when it first evolved. However, after Cit(+) was refined by further mutations, this potentiating gltA mutation became deleterious to fitness. A second wave of beneficial gltA mutations then evolved that reduced CS activity to below the ancestral level. Thus, dynamic reorganization of central metabolism made colonizing this new nutrient niche contingent on both co-opting and overcoming a history of prior adaptation.
与现有性状的渐进式进化变化相比，使生物体能够开拓新生态位的进化创新非常罕见。我们发现在莱斯基长期进化实验中，编码柠檬酸合酶（CS）的gltA基因中的关键突变，在大肠杆菌获得在柠檬酸盐上有氧生长的能力（Cit(+)表型）之前和之后均有发生。第一个gltA突变通过破坏该酶的NADH抑制作用来增加CS活性，这有利于在乙酸盐上生长，并有助于在最初进化时保留基本的Cit(+)性状以免灭绝。然而，在Cit(+)通过进一步突变得到优化后，这种增强gltA突变对适应性变得有害。随后又进化出了第二波有益的gltA突变，使CS活性降低到祖先水平以下。因此，中心代谢的动态重组使得开拓这个新的营养生态位既依赖于利用又依赖于克服先前适应的历史。
