Lacroix L, Mergny J L, Leroy J L, Hélène C
Laboratoire de Biophysique, Muséum National d'Histoire Naturelle INSERM U201, CNRS UA481, Paris, France.
Biochemistry. 1996 Jul 2;35(26):8715-22. doi: 10.1021/bi960107s.
At slightly acidic or even neutral pH, oligodeoxyribonucleotides which include stretches of cytidines form a tetrameric structure involving C.C+ base pairs in a so-called i-motif. Such a structure, which is very stable at pH 6, is still detectable at neutral pH. This motif, whether intramolecular or intermolecular, was shown to act as an undesired, competing structure for triplex formation. Depending on the sequence and the experimental conditions, triple helix formation was inhibited or completely abolished. Thus, the sequence recognition repertoire of pyrimidine DNA third strands is more restricted than previously admitted; only T.A rich-oligopurine-oligopyrimidine sequences are amenable to triplex formation. On the other hand, cytosine-rich RNA oligoribonucleotides and their 2'-O-methyl derivatives were unable to form a stable autoassociated structure above pH 6. Concomitantly, they were shown to form a triplex with (C.G)-rich targets at pH 6. Thus, not only was the affinity for the duplex increased by DNA to RNA substitution, but the possibility of formation of an inactive form of the third strand was greatly reduced. Thus, i-motif formation is favored for cytosine-rich oligodeoxynucleotides, whereas triplex formation is favored for oligoribonucleotides. These properties make RNA pyrimidine oligonucleotides an attractive choice for triplex formation on a (C.G)-rich target, extending the practical sequence repertoire of pyrimidine triplexes to (C.G*C+)-rich sequences.
在略呈酸性甚至中性的pH值条件下，含有胞嘧啶区段的寡脱氧核糖核苷酸会形成一种四聚体结构，其中包含所谓的i-基序中的C·C⁺碱基对。这种结构在pH值为6时非常稳定，在中性pH值下仍可检测到。已证明，这种基序无论是分子内的还是分子间的，都会作为三链体形成的一种不期望的竞争结构起作用。根据序列和实验条件，三链螺旋的形成会受到抑制或完全消除。因此，嘧啶DNA第三链的序列识别范围比以前公认的更受限制；只有富含T·A的寡嘌呤-寡嘧啶序列适合形成三链体。另一方面，富含胞嘧啶的RNA寡核糖核苷酸及其2'-O-甲基衍生物在pH值高于6时无法形成稳定的自缔合结构。与此同时，它们在pH值为6时能与富含（C·G）的靶标形成三链体。因此，不仅通过DNA到RNA的取代增加了对双链体的亲和力，而且大大降低了第三链形成无活性形式的可能性。因此，富含胞嘧啶的寡脱氧核苷酸有利于形成i-基序，而寡核糖核苷酸有利于形成三链体。这些特性使RNA嘧啶寡核苷酸成为在富含（C·G）的靶标上形成三链体的有吸引力的选择，将嘧啶三链体的实际序列范围扩展到富含（C·G*C⁺）的序列。
