Department of Chemistry & Biochemistry, Kent State University, Kent, Ohio 44242, USA.
J Am Chem Soc. 2013 Feb 13;135(6):2235-41. doi: 10.1021/ja309668t. Epub 2013 Jan 31.
Single-stranded guanine (G)-rich sequences at the 3' end of human telomeres provide ample opportunities for physiologically relevant structures, such as G-quadruplexes, to form and interconvert. Population equilibrium in this long sequence is expected to be intricate and beyond the resolution of ensemble-average techniques, such as circular dichroism, NMR, or X-ray crystallography. By combining a force-jump method at the single-molecular level and a statistical population deconvolution at the sub-nanometer resolution, we reveal a complex population network with unprecedented transition dynamics in human telomeric sequences that contain four to eight TTAGGG repeats. Our kinetic data firmly establish that G-triplexes are intermediates to G-quadruplexes while long-loop G-quadruplexes are misfolded population minorities whose formation and disassembly are faster than G-triplexes or regular G-quadruplexes. The existence of misfolded DNA supports the emerging view that structural and kinetic complexities of DNA can rival those of RNA or proteins. While G-quadruplexes are the most prevalent species in all the sequences studied, the abundance of a misfolded G-quadruplex in a particular telomeric sequence decreases with an increase in the loop length or the number of long-loops in the structure. These population patterns support the prediction that in the full-length 3' overhang of human telomeres, G-quadruplexes with shortest TTA loops would be the most dominant species, which justifies the modeling role of regular G-quadruplexes in the investigation of telomeric structures.
人端粒 3'末端的单链鸟嘌呤（G）丰富序列为生理相关结构（如 G-四链体）的形成和相互转化提供了充足的机会。在这个长序列中，种群平衡预计是复杂的，超出了如圆二色性、NMR 或 X 射线晶体学等平均技术的分辨率。通过结合单分子水平的力跳跃方法和亚纳米分辨率的统计群体反卷积，我们揭示了人端粒序列中存在一个复杂的群体网络，该网络具有前所未有的转换动力学，其中包含四到八个 TTAGGG 重复。我们的动力学数据确凿地证明了 G-三链体是 G-四链体的中间体，而长环 G-四链体是错误折叠的少数群体，其形成和解体速度快于 G-三链体或常规 G-四链体。错误折叠的 DNA 的存在支持了一个新兴观点，即 DNA 的结构和动力学复杂性可以与 RNA 或蛋白质相媲美。虽然 G-四链体是所有研究序列中最普遍的物种，但在特定端粒序列中，错误折叠的 G-四链体的丰度随环长度或结构中环的数量的增加而降低。这些群体模式支持了这样的预测，即在人端粒的全长 3'突出端，具有最短 TTA 环的 G-四链体将是最主要的物种，这证明了常规 G-四链体在端粒结构研究中的建模作用。
