Tippana Ramreddy, Hwang Helen, Opresko Patricia L, Bohr Vilhelm A, Myong Sua
Biophysics Department, The Johns Hopkins University, Baltimore, MD 21218;
Bioengineering Department, University of Illinois, Urbana, IL 61801; Medical Scholars Program, University of Illinois, Urbana, IL 61801;
Proc Natl Acad Sci U S A. 2016 Jul 26;113(30):8448-53. doi: 10.1073/pnas.1603724113. Epub 2016 Jul 12.
G-quadruplex (GQ) is a four stranded DNA secondary structure that arises from a guanine rich sequence. Stable formation of GQ in genomic DNA can be counteracted by the resolving activity of specialized helicases including RNA helicase AU (associated with AU rich elements) (RHAU) (G4 resolvase 1), Bloom helicase (BLM), and Werner helicase (WRN). However, their substrate specificity and the mechanism involved in GQ unfolding remain uncertain. Here, we report that RHAU, BLM, and WRN exhibit distinct GQ conformation specificity, but use a common mechanism of repetitive unfolding that leads to disrupting GQ structure multiple times in succession. Such unfolding activity of RHAU leads to efficient annealing exclusively within the same DNA molecule. The same resolving activity is sufficient to dislodge a stably bound GQ ligand, including BRACO-19, NMM, and Phen-DC3. Our study demonstrates a plausible biological scheme where different helicases are delegated to resolve specific GQ structures by using a common repetitive unfolding mechanism that provides a robust resolving power.
G-四链体（GQ）是一种由富含鸟嘌呤的序列形成的四链DNA二级结构。基因组DNA中GQ的稳定形成可被包括RNA解旋酶AU（与富含AU元件相关）（RHAU）（G4解旋酶1）、布鲁姆解旋酶（BLM）和沃纳解旋酶（WRN）在内的特殊解旋酶的解旋活性所抵消。然而，它们的底物特异性以及GQ解折叠所涉及的机制仍不确定。在此，我们报告RHAU、BLM和WRN表现出不同的GQ构象特异性，但使用一种重复解折叠的共同机制，该机制导致连续多次破坏GQ结构。RHAU的这种解折叠活性导致仅在同一DNA分子内有效退火。相同的解旋活性足以去除稳定结合的GQ配体，包括BRACO-19、NMM和Phen-DC3。我们的研究展示了一种合理的生物学方案，其中不同的解旋酶被委托通过使用一种共同的重复解折叠机制来解析特定的GQ结构，该机制提供了强大的解旋能力。
