Selection of Early Life Codons by Ultraviolet Light

How life developed in its earliest stages is a central but notoriously difficult question in science. The earliest lifeforms likely used a reduced set of codon sequences that were progressively completed over time, driven by chemical, physical, and combinatorial constraints. However, despite its importance for prebiotic chemistry, UV radiation has not been considered a selection pressure for the evolution of early codon sequences. In this proof-of-principle study, we quantified the UV susceptibility of large pools of DNA protogenomes and tested the timing of evolutionary incorporation of codon sequences using a Monte Carlo method utilizing sequence-context-dependent damage rates previously determined by high throughput sequencing experiments. We traced the UV-radiation selection pressure on early protogenomes comprising a limited number of codon sequences to late protogenomes with access to all codons. The modeling showed that in just minutes under early sunlight, the choice of the first codons determined whether most of the protogenomes remained intact or became damaged entirely. The results correlated with earlier chemical models of the evolution of the genetic code. Our results show how UV could have played a crucial role in the evolution of the early genetic code for a DNA-based genome and provide the concept for future RNA-based studies.生命早期起源与演化的问题是科学界核心且公认的棘手难题。最早的生命形式可能仅使用一套简化的密码子（codon）序列，并在化学、物理及组合约束的驱动下随时间逐步完善该序列集。然而，尽管紫外线辐射对前生命化学研究至关重要，此前学界并未将其视为早期密码子序列演化的选择压力。在本项原理验证研究中，我们对大量DNA原基因组（protogenome）库的紫外线敏感性进行了量化，并借助此前通过高通量测序实验确定的序列上下文依赖性损伤率，采用蒙特卡洛方法测试了密码子序列演化整合的时间节点。我们追踪了紫外线辐射选择压力从仅包含有限密码子序列的早期原基因组，到可使用全部密码子的晚期原基因组的演化轨迹。模型结果显示，在早期阳光照射下仅数分钟内，首批密码子的选择便决定了绝大多数原基因组能否保持完整或完全受损。该研究结果与此前提出的遗传密码演化化学模型相契合。本研究结果揭示了紫外线辐射如何在以DNA为基础的基因组早期遗传密码演化中发挥关键作用，并为未来开展基于RNA的相关研究提供了理论框架。