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首页> 外文期刊>Journal of Molecular Biology >EFFECTS OF MG2+, K+, AND H+ ON AN EQUILIBRIUM BETWEEN ALTERNATIVE CONFORMATIONS OF AN RNA PSEUDOKNOT
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EFFECTS OF MG2+, K+, AND H+ ON AN EQUILIBRIUM BETWEEN ALTERNATIVE CONFORMATIONS OF AN RNA PSEUDOKNOT

机译：MG2 +，K +和H +对RNA假想构象交替构象之间平衡的影响

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摘要

A complex pseudoknot structure surrounds the first ribosome initiation site in the Eschericlia coli alpha mRNA and mediates its regulation by ribosomal protein S4. A 112 nt RNA fragment containing this pseudoknot exists in two conformations that are resolvable by gel electrophoresis below room temperature. Between 30 degrees C and 45 degrees C the conformers reach thermodynamic equilibrium on a time scale ranging from one hour to one minute, and the interconversion between conformers is Linked to H+, K+ and Mg2+ concentrations. Mg2+ favors formation of the ''fast'' electrophoretic form: a single Mg2+ is bound in the rate-Limiting step, followed by cooperative binding of similar to 1.7 additional ions. Binding of the latter ions provides most of the favorable free energy for the reaction. However, the ''slow'' form binds about the same number of Mg ions, albeit more weakly, so that saturating Mg2+ concentrations drive the equilibrium to only similar to 70% fast form. A single H+ is taken up in the switch to the ''slow'' conformer, which has apparent pK approximate to 5.9; low pH also stabilizes part of the pseudoknot structure melting at similar to 62 degrees C. Mg2+ and H+ appear to direct alpha mRNA folding by relatively small (10 to 100-fold) differences in their affinities for alternative conformers. K+ has very Little effect on the conformational equilibrium, but at high concentrations accelerates interconversion between the conformers. The alpha mRNA conformational switch is similar in its slow kinetics, large activation energy, and Mg2+ dependence of the equilibrium constant to slow steps in the folding of tRNA, group I introns, and RNase P RNA tertiary structures, though it differs from these in the association of a single Mg2+ with the rate-limiting step. (C) 1997 Academic Press Limited. [References: 43]
机译：复杂的假结结构围绕大肠杆菌αmRNA中的第一个核糖体起始位点，并通过核糖体蛋白S4介导其调控。含有该假结的112 nt RNA片段以两种构象存在，在室温以下可通过凝胶电泳分辨。在30摄氏度至45摄氏度之间，构象异构体在从一小时到一分钟的时间范围内达到热力学平衡，并且构象异构体之间的相互转化与H +，K +和Mg2 +浓度相关。 Mg2 +有利于“快速”电泳形式的形成：单个Mg2 +在限速步骤中结合，然后协同结合约1.7个其他离子。后者离子的结合为反应提供了大多数有利的自由能。但是，“慢速”形式结合的Mg离子数量几乎相同，尽管弱得多，因此饱和的Mg2 +浓度会驱动平衡，仅类似于70％的快速形式。在切换到“慢速”构象异构体的过程中，单个H +被吸收，其表观pK约为5.9;低pH值还可以使假结结构的部分融化稳定在类似于62摄氏度的温度。Mg2+和H +似乎通过相对较小的亲和力差异（10至100倍）来指导αmRNA折叠。 K +对构象平衡的影响很小，但在高浓度下，会加速构象子之间的相互转化。 αmRNA构象转换的相似之处在于其缓慢的动力学，较大的活化能以及平衡常数的Mg2 +依赖于tRNA，I类内含子和RNase P RNA三级结构折叠的缓慢步骤，尽管与之不同。单个Mg2 +与限速步骤的关联。 （C）1997 Academic Press Limited。 [参考：43]

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