原核生物代谢网络进化的遗传模型及其调控
Toolbox model of evolution of prokaryotic metabolic networks and their regulation
摘要：
据报道，在原核基因组中编码的转录因子的数量与它们的基因总数近似成二次方。我们提出了一个概念性的解释这一发现，并说明它使用一个简单的模型，在该模型中，代谢和调控网络的原核生物的共同调节代谢途径的水平基因转移的形状。适应由新的转录因子（例如，学习使用另一种营养素）包括获得新的酶和重新使用一些已经编码在基因组中的酶。随着生物体的酶库（工具箱）变得越来越大，它可以更频繁地重复使用其酶工具，因此需要更少的新工具来掌握每项新任务。从这一观察结果中，逻辑上可以得出，功能任务及其调节因子的数量随着编码酶的基因总数的增加而增加，而不是线性增加。基因组也可以缩小，例如，这是因为环境中的营养素损失，随后其调节因子和所有酶的缺失变得多余。我们提出了几个简单的网络演化模型，阐述了这个工具箱的论点，并再现了经验观察到的二次缩放。在我们的模型中的路径分支的长度分布与大肠杆菌的现实生活中的代谢网络。因此，我们的模型为原核生物中调节子大小的广泛分布提供了定性解释。
英文摘要：
It has been reported that the number of transcription factors encoded in prokaryotic genomes scales approximately quadratically with their total number of genes. We propose a conceptual explanation of this finding and illustrate it using a simple model in which metabolic and regulatory networks of prokaryotes are shaped by horizontal gene transfer of coregulated metabolic pathways. Adapting to a new environmental condition monitored by a new transcription factor (e.g., learning to use another nutrient) involves both acquiring new enzymes and reusing some of the enzymes already encoded in the genome. As the repertoire of enzymes of an organism (its toolbox) grows larger, it can reuse its enzyme tools more often and thus needs to get fewer new ones to master each new task. From this observation, it logically follows that the number of functional tasks and their regulators increases faster than linearly with the total number of genes encoding enzymes. Genomes can also shrink, e.g., because of a loss of a nutrient from the environment, followed by deletion of its regulator and all enzymes that become redundant. We propose several simple models of network evolution elaborating on this toolbox argument and reproducing the empirically observed quadratic scaling. The distribution of lengths of pathway branches in our model agrees with that of the real-life metabolic network of Escherichia coli. Thus, our model provides a qualitative explanation for broad distributions of regulon sizes in prokaryotes.
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