Department of Chemical Engineering and Analytical Chemistry, University of Barcelona, Barcelona, Spain.
Department of Food, Environmental and Nutritional Sciences (DEFENS), University of Milan, Milan, Italy.
Sci Rep. 2019 Nov 1;9(1):15807. doi: 10.1038/s41598-019-52311-5.
Some lung and ovarian tumors are connected to the loss of expression of SMARCA4 gene. In its promoter region, a 44-nucleotides long guanine sequence prone to form G-quadruplex structures has been studied by means of spectroscopic techniques (circular dichroism, molecular absorption and nuclear magnetic resonance), size exclusion chromatography and multivariate analysis. The results have shown that the central 21-nucleotides long sequence comprising four guanine tracts of disparate length is able to fold into a pH-dependent ensemble of G-quadruplex structures. Based on acid-base titrations and melting experiments of wild and mutated sequences, the formation of a C·C base pair between cytosine bases present at the two lateral loops is shown to promote a reduction in conformational heterogeneity, as well as an increase in thermal stability. The formation of this base pair is characterized by a pK value of 7.1 ± 0.2 at 20 °C and 150 mM KCl. This value, higher than those usually found in i-motif structures, is related to the additional stability provided by guanine tetrads in the G-quadruplex. To our knowledge, this is the first thermodynamic description of this base pair in loops of antiparallel G-quadruplex structures.
一些肺和卵巢肿瘤与 SMARCA4 基因表达缺失有关。在其启动子区域，通过光谱技术（圆二色性、分子吸收和核磁共振）、尺寸排阻色谱和多元分析研究了一段 44 个核苷酸长的易形成 G-四链体结构的鸟嘌呤序列。结果表明，由四个不同长度的鸟嘌呤链组成的中心 21 个核苷酸长的序列能够折叠成 pH 依赖性的 G-四链体结构。基于野生型和突变型序列的酸碱滴定和熔融实验，表明在两个侧环处存在的胞嘧啶碱基之间形成 C·C 碱基对可促进构象异质性降低，并增加热稳定性。该碱基对的形成特征为在 20°C 和 150 mM KCl 下，pK 值为 7.1±0.2。与通常在 i-motif 结构中发现的值相比，该值较高，这与 G-四链体中鸟嘌呤四联体提供的额外稳定性有关。据我们所知，这是第一个对反平行 G-四链体结构环中这种碱基对的热力学描述。
