G-quadruplex (GQ) is formed at various regions of DNA, including telomeres of chromosomes and regulatory regions of oncogenes. Since GQ is important in both gene regulation and genome instability, the biological and medical implications of this abnormal DNA structure have been intensively studied. Its formation mechanisms, however, are not clearly understood yet. We report single-molecule fluorescence experiments to monitor the cotranscriptional GQ formation coupled with R-loop formation using T7 RNA polymerase. The GQ is formed very rarely per single-round transcription. R-loop formation precedes and facilitates GQ formation. Once formed, some GQs are extremely stable, resistant even to RNase H treatment, and accumulate in multiple-round transcription conditions. On the other hand, GQ existing in the non-template strand promotes the R-loop formation in the next rounds of transcription. Our study clearly shows the existence of a positive feedback mechanism of GQ and R-loop formations, which may possibly contribute to gene regulation and genome instability.

中文翻译：

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共转录G四联体形成与R环形成耦合的单分子荧光研究。

G-四链体（GQ）在DNA的各个区域形成，包括染色体的端粒和癌基因的调控区域。由于GQ在基因调控和基因组不稳定性中都非常重要，因此已经对这种异常DNA结构的生物学和医学意义进行了深入研究。但是，其形成机理尚不清楚。我们报告了单分子荧光实验，以监测共转录GQ的形成以及使用T7 RNA聚合酶的R环的形成。每次单轮转录很少形成GQ。R环形成先于并促进GQ形成。一旦形成，一些GQ极其稳定，甚至对RNase H处理也具有抗性，并在多轮转录条件下积累。另一方面，非模板链中存在的GQ在下一轮转录中促进R环形成。我们的研究清楚地表明存在GQ和R环形成的正反馈机制，这可能有助于基因调控和基因组不稳定。