信使RNA (mRNA)是一种线性单链分子，通过互补序列片段的碱基配对折叠成复杂的二级结构。核糖体把mRNA翻译成蛋白质的过程需要将这种碱基配对结构破坏。在这项研究中，Ignacio Tinoco及其同事利用一种单分子方法发现，核糖体有两个机制用来打开mRNA的结构化区域；机制的选择取决于碱基配对结合点是处于开启状态还是闭合状态。
核糖体如何打开mRNA结构化区域？
生物探索推荐英文摘要
The ribosome uses two active mechanisms to unwind messenger RNA during translation
Abstract: The ribosome translates the genetic information encoded in messenger RNA into protein. Folded structures in the coding region of an mRNA represent a kinetic barrier that lowers the peptide elongation rate, as the ribosome must disrupt structures it encounters in the mRNA at its entry site to allow translocation to the next codon. Such structures are exploited by the cell to create diverse strategies for translation regulation, such as programmed frameshifting1, 2, the modulation of protein expression levels3, 4, ribosome localization5 and co-translational protein folding6. Although strand separation activity is inherent to the ribosome, requiring no exogenous helicases7, its mechanism is still unknown. Here, using a single-molecule optical tweezers assay on mRNA hairpins, we find that the translation rate of identical codons at the decoding centre is greatly influenced by the GC content of folded structures at the mRNA entry site. Furthermore, force applied to the ends of the hairpin to favour its unfolding significantly speeds translation. Quantitative analysis of the force dependence of its helicase activity reveals that the ribosome, unlike previously studied helicases, uses two distinct active mechanisms to unwind mRNA structure: it destabilizes the helical junction at the mRNA entry site by biasing its thermal fluctuations towards the open state, increasing the probability of the ribosome translocating unhindered; and it mechanically pulls apart the mRNA single strands of the closed junction during the conformational changes that accompany ribosome translocation. The second of these mechanisms ensures a minimal basal rate of translation in the cell; specialized, mechanically stable structures are required to stall the ribosome temporarily1, 2. Our results establish a quantitative mechanical basis for understanding the mechanism of regulation of the elongation rate of translation by structured mRNAs.
