连接环、末端碱基及一价阳离子对G-四链体结构的影响

Effects of Loops,Flanking Bases and Monovalent Cations on the Structures of G-Quadruplexes

研究了G-四链体中的连接环(Loop)、末端碱基和一价阳离子对其结构的影响,发现在K+溶液中Loop短的序列易形成平行结构,无末端碱基时容易形成多聚体,而反平行或混合平行/反平行的G-四链体则难以形成多聚体;一价阳离子K+,NH+4和Na+促进形成平行结构及多聚体的能力依次减弱.在平行G-四链体的3'或5'端增加非G碱基,或改变阳离子使其形成非平行结构,均可抑制多聚体的形成.Loop长度影响G-四链体的热稳定性,Loop短的序列可形成很稳定的分子内结构;无末端碱基的G-四链体多聚体的稳定性低于单个G-四链体,且多聚体随着温度升高而变小.结果表明,在K+溶液中,无末端碱基的平行G-四链体序列首先形成分子内结构,然后通过π-π堆积形成多聚体;末端碱基及反平行或混合平行/反平行G-四链体中的Loop可阻碍末端堆积作用,抑制多聚体的形成.本研究为G-四链体的结构与功能研究提供了有用信息.

G-rich tandem repeats sequences are able to fold into structures called G-quadruplexes（G4s）. G4s have complicated topological structures, which closely depend on the sequences and the solution conditions. In this work, the effects of loops, flanking bases and monovalent cations on the structure of G4s were investigated by gel electrophoresis, circular dichroism spectra and UV-Melting. The results showed that G4 forming se-quences with short loops were favorable to form parallel structure in K＋ solution, and the parallel G4s without flanking bases were able to further form G4 multimers, but the anti-parallel or mixed parallel/anti-parallel G4s were not. The monovalent cations had an influence on the adopted structures; K＋ was more favorable for the formation of parallel G4 and G4 multimers than NH＋4 and Na＋. The addition of non-guanine bases to the termi-nals of parallel G4s, or the transformation of the parallel G4s to non-parallel structures by changing the cations, could inhibit the formation of G4 multimers. Furthermore, loop length could affect the thermo-stability of G4s;G4s with short loops had higher thermo-stability. The G4 multimers showed lower thermo-stability than G4 monomer, and their size and amount became smaller with the temperature increase. These results suggest that G4s with short loops and no flanking bases fold into intramolecular parallel structures firstly, and then form multimers through end stacking. The flanking bases and loops located on the G-quartets at 3＆#39;or 5＆#39;termi-nal hinder the stacking interactions, and inhibit the multimer assembly. Our results may offer useful clues for the structure and function studies of G4s.