Bistability in a metabolic network underpins the de novo evolution of colony switching in Pseudomonas fluorescens.
Bistability in a metabolic network underpins the de novo evolution of colony switching in Pseudomonas fluorescens.
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DOI：
10.1371/journal.pbio.1002109
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发表时间：
2015-03
期刊：
影响因子：
9.8
通讯作者：
Rainey PB
中科院分区：
文献类型：
--
作者：
Gallie J;Libby E;Bertels F;Remigi P;Jendresen CB;Ferguson GC;Desprat N;Buffing MF;Sauer U;Beaumont HJ;Martinussen J;Kilstrup M;Rainey PB
表型转换是自然界中常见的现象。这种普遍性使得许多潜在的分子机制得以阐明。然而，很少有人知道表型开关如何出现和功能在其早期进化阶段。第一次提供经验性见解的机会是通过一项实验提供的，在该实验中，细菌荧光假单胞菌SBW 25的种群从头进化出在两种菌落表型之间切换的能力。在这里，我们揭开了菌落转换背后的分子机制，揭示了一个单一的核苷酸变化，在一个基因陷入中央代谢（carB）产生这样一个惊人的表型。我们表明，菌落切换是由开/关表达的胶囊组成的colanic酸样聚合物的基础。我们使用分子遗传学，生化分析和实验进化，以建立胶囊开关的嘧啶生物合成途径的扰动结果。最重要的是一个分叉点，在该点尿嘧啶三磷酸被分配到核苷酸代谢或聚合物生产。这种分叉标志着细胞命运决定点，其中嘧啶水平相对较高的细胞有利于核苷酸代谢（胶囊关闭），而嘧啶水平较低的细胞将资源转移到聚合物生物合成（胶囊打开）。该决定点在野生型菌株中存在并起作用。最后，我们提出了一个简单的数学模型，证明决策点的分子组成部分能够产生开关。尽管突变的原因很简单，但基因型和表型之间的联系是复杂和多维的，这为我们提供了一个难得的机会，让我们了解调控网络中的噪音如何为进化提供机会。实验进化的集落水平表型开关的分子基础被揭示为影响代谢分叉点，其中核苷酸可用于细胞分裂或胶囊聚合物合成。表型转换是指在表型状态之间快速转换的能力，是生物体在面对不可预测的环境条件时常用的一种进化生存策略。然而，很少有人知道表型开关如何出现，并在其早期进化阶段的功能。先前的一项研究观察到荧光假单胞菌群体中菌落形态转换的进化出现，以响应波动选择。在这里，我们描述了这种菌落转换的潜在分子基础，提供了实时进化表型转换背后的机制的第一个帐户。我们表明，在这种情况下，菌落转换是在细胞水平上的高频率的开/关表达的可乐酸样胶囊响应于不同水平的代谢产物。生化分析表明，胶囊转换的结果从突变，减少浓度的中间体在一个中央代谢途径-嘧啶生物合成途径。至关重要的是这些代谢资源在聚合物生产（导致包囊化）和细胞分裂（导致非包囊化）之间的分配;这种分叉标志着一个决策点，即代谢物水平低的细胞将资源转移到聚合物生产，增加切换到包囊化状态的可能性。随着更大比例的细胞被包囊化，出现集落转换。这些发现表明，虽然菌落转换相对容易进化，但潜在的分子机制却令人惊讶地复杂。
Phenotype switching is commonly observed in nature. This prevalence has allowed the elucidation of a number of underlying molecular mechanisms. However, little is known about how phenotypic switches arise and function in their early evolutionary stages. The first opportunity to provide empirical insight was delivered by an experiment in which populations of the bacterium Pseudomonas fluorescens SBW25 evolved, de novo, the ability to switch between two colony phenotypes. Here we unravel the molecular mechanism behind colony switching, revealing how a single nucleotide change in a gene enmeshed in central metabolism (carB) generates such a striking phenotype. We show that colony switching is underpinned by ON/OFF expression of capsules consisting of a colanic acid-like polymer. We use molecular genetics, biochemical analyses, and experimental evolution to establish that capsule switching results from perturbation of the pyrimidine biosynthetic pathway. Of central importance is a bifurcation point at which uracil triphosphate is partitioned towards either nucleotide metabolism or polymer production. This bifurcation marks a cell-fate decision point whereby cells with relatively high pyrimidine levels favour nucleotide metabolism (capsule OFF), while cells with lower pyrimidine levels divert resources towards polymer biosynthesis (capsule ON). This decision point is present and functional in the wild-type strain. Finally, we present a simple mathematical model demonstrating that the molecular components of the decision point are capable of producing switching. Despite its simple mutational cause, the connection between genotype and phenotype is complex and multidimensional, offering a rare glimpse of how noise in regulatory networks can provide opportunity for evolution. The molecular basis of an experimentally evolved colony-level phenotype switch is revealed to affect a metabolic bifurcation point where nucleotides can be used for either cell division or capsule polymer synthesis. Phenotype switching—the ability to switch rapidly between phenotypic states—is an evolutionary survival strategy commonly used by organisms in the face of unpredictable environmental conditions. However, little is known about how phenotype switches emerge and function in their early evolutionary stages. A previous study observed the evolutionary emergence of colony morphology switching in Pseudomonas fluorescens populations in response to fluctuating selection. Here we describe the underlying molecular basis of this colony switching, providing the first account of the mechanism behind a real-time evolved phenotype switch. We show that colony switching in this instance is underpinned at the cellular level by high frequency ON/OFF expression of colanic acid-like capsules in response to varying levels of a metabolite. Biochemical assays revealed that capsule switching results from mutations that reduce concentrations of intermediates in a central metabolic pathway—the pyrimidine biosynthetic pathway. Of key importance is the partitioning of these metabolic resources between polymer production (leading to capsulation) and cell division (leading to noncapsulation); this bifurcation marks a decision point whereby cells with low metabolite levels divert resources towards polymer production, increasing the likelihood of switching to the capsulated state. As a greater proportion of cells become capsulated, colony switching emerges. These findings show that, while colony switching evolved with relative ease, the underlying molecular mechanism is surprisingly complex.
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影响因子：
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