Simonsson T, Pribylova M, Vorlickova M
Department of Molecular Biotechnology, Chalmers University of Technology, SE 405 30 Göteborg, Sweden.
Biochem Biophys Res Commun. 2000 Nov 11;278(1):158-66. doi: 10.1006/bbrc.2000.3783.
Nucleic acid structure-function correlations are pivotal to major biological events like transcription, replication, and recombination. Depending on intracellular conditions in vivo and buffer composition in vitro, DNA appears capable of inexhaustible structure variation. At moderately acidic, or even neutral pH, DNA strands that are rich in cytosine bases can associate both inter- and intramolecularly to form i-tetraplexes. The hemiprotonated cytosine(+)-cytosine base pair constitutes the building block for the formation of i-tetraplexes, and motifs for their formation are frequent in vertebrate genomes. A major control element upstream of the human c-myc gene, which has been shown to interact sequence specifically with several transcription factors, becomes hypersensitive to nucleases upon c-myc expression. The control element is asymmetric inasmuch as that one strand is uncommonly rich in cytosines and exhibits multiple motifs for the formation of i-tetraplexes. To investigate the propensity for their formation we employ circular dichroism (CD) in combination with ultra violet (UV) spectroscopy and native gel electrophoresis. Our results demonstrate the cooperative formation of well-defined i-tetraplex structures. We conclude that i-tetraplex formation occurs in the promoter region of the human c-myc gene in vitro, and discuss implications of possible biological roles for i-tetraplex structures in vivo. Hypothetical formation of intramolecular fold-back i-tetraplexes is important to c-myc transcription, whereas chromosomal translocation events might involve the formation of bimolecular i-tetraplex structures.
核酸的结构与功能的相关性对于转录、复制和重组等重大生物学事件至关重要。根据体内的细胞内条件和体外的缓冲液组成，DNA似乎能够发生无穷无尽的结构变化。在适度酸性甚至中性pH条件下，富含胞嘧啶碱基的DNA链可以在分子间和分子内缔合形成i-四链体。半质子化的胞嘧啶（+）-胞嘧啶碱基对构成了i-四链体形成的基本单元，并且其形成基序在脊椎动物基因组中很常见。人类c-myc基因上游的一个主要控制元件已被证明能与多种转录因子进行序列特异性相互作用，在c-myc表达时对核酸酶变得高度敏感。该控制元件是不对称的，因为其中一条链异常富含胞嘧啶，并呈现出多个形成i-四链体的基序。为了研究它们形成的倾向，我们采用圆二色性（CD）结合紫外（UV）光谱和天然凝胶电泳。我们的结果证明了明确的i-四链体结构的协同形成。我们得出结论，在体外i-四链体形成发生在人类c-myc基因的启动子区域，并讨论了i-四链体结构在体内可能的生物学作用的意义。分子内回折i-四链体的假设形成对c-myc转录很重要，而染色体易位事件可能涉及双分子i-四链体结构的形成。
