Department of Plant and Environmental Sciences, Weizmann Institute of Science, Rehovot, 7610001, Israel.
Laboratory of Genetically Encoded Small Molecules, The Rockefeller University, New York, NY 10065, USA.
Nucleic Acids Res. 2020 Jan 24;48(2):761-769. doi: 10.1093/nar/gkz1100.
Identifying the molecular mechanisms that give rise to genetic variation is essential for the understanding of evolutionary processes. Previously, we have used adaptive laboratory evolution to enable biomass synthesis from CO2 in Escherichia coli. Genetic analysis of adapted clones from two independently evolving populations revealed distinct enrichment for insertion and deletion mutational events. Here, we follow these observations to show that mutations in the gene encoding for DNA topoisomerase I (topA) give rise to mutator phenotypes with characteristic mutational spectra. Using genetic assays and mutation accumulation lines, we find that point mutations in topA increase the rate of sequence deletion and duplication events. Interestingly, we observe that a single residue substitution (R168C) results in a high rate of head-to-tail (tandem) short sequence duplications, which are independent of existing sequence repeats. Finally, we show that the unique mutation spectrum of topA mutants enhances the emergence of antibiotic resistance in comparison to mismatch-repair (mutS) mutators, and leads to new resistance genotypes. Our findings highlight a potential link between the catalytic activity of topoisomerases and the fundamental question regarding the emergence of de novo tandem repeats, which are known modulators of bacterial evolution.
确定导致遗传变异的分子机制对于理解进化过程至关重要。此前，我们使用适应性实验室进化使大肠杆菌能够从 CO2 中合成生物量。对来自两个独立进化群体的适应克隆的遗传分析显示，插入和缺失突变事件明显富集。在这里，我们根据这些观察结果表明，编码 DNA 拓扑异构酶 I（topA）的基因突变会导致具有特征突变谱的突变体表型。通过遗传分析和突变积累系，我们发现 topA 中的点突变会增加序列缺失和重复事件的速率。有趣的是，我们观察到单个残基取代（R168C）会导致高频率的头对头（串联）短序列重复，这与现有序列重复无关。最后，我们表明，与错配修复（mutS）突变体相比，topA 突变体的独特突变谱增强了抗生素抗性的出现，并导致新的抗性基因型。我们的研究结果强调了拓扑异构酶的催化活性与关于从头串联重复出现的基本问题之间的潜在联系，已知串联重复是细菌进化的调节剂。
