Schoemaker H J, Gamble R C
Biochemistry. 1976 Jun 29;15(13):2800-3. doi: 10.1021/bi00658a015.
Purine C-8 tritium-labeling rates have been measured at specific sites in Escherichia coli tRNAIle and tRNA2Tyr. The results are compared with those obtained for yeast tRNAPhe (preceding paper(Gamble et al., 1976)). The tRNAIle and tRNAPhe fall into the same general class of tRNA structures, while tRNA2Tyr is in a differint class; in particular, the latter is characterized by a large extra loop. In each of the three tRNAs the 3'-terminal A has the same labeling rate and, on a relative basis, appears to be the most rapidly labeled site. Bases in cloverleaf helical sections have markedly retarded labeling rates that collectively fall within an approximately threefold range of time constants. At some of the common purines, believed to be essential for the construction of a general system of tertiary interactions, exchange rates for yeast tRNAPhe are significantly different than those for the two Escherichia coli tRNAs. these differences may arise from variations among the tRNAs in the relative stabilities of specific tertiary interactions, or from other factors as well. In the case of tRNA2Tyr, labeling rates for bases in the large variable region are sufficiently retarded to suggest some structural organization for this part of the molecule. In addition, since exchange rates are similar for some of the bases common to Escherichia coli tRNAIle and tRNA2Tyr, it is likely that the large variable loop of tRNA2Tyr does not interact with or perturb these common sites. Finally, for all three tRNAs, structure formation (e.g., base pairing, base stacking) invariably decreases the labeling rate, even though the variety of base environments in the three-dimensional structures of these tRNAs might be expected to affect the acidity of C-8 and other chemical properties in diverse ways. Although these chemical effects no doubt bear influence, in these studies the dominant influence on exchange may be the effect of structure on the accessibility of solvent molecules, i.e. water.
已测定了大肠杆菌异亮氨酸转运核糖核酸（tRNAIle）和酪氨酸转运核糖核酸（tRNA2Tyr）特定位点的嘌呤C-8氚标记率。将结果与酵母苯丙氨酸转运核糖核酸（tRNAPhe）的结果（前文（Gamble等人，1976年））进行了比较。tRNAIle和tRNAPhe属于同一类tRNA结构，而tRNA2Tyr属于不同的类别；特别是，后者的特征是有一个大的额外环。在这三种转运核糖核酸中，3'-末端的A具有相同的标记率，并且相对而言，似乎是标记最快的位点。三叶草叶形螺旋区的碱基标记率明显延迟，其时间常数总体上落在大约三倍的范围内。在一些被认为对构建一般三级相互作用系统至关重要的常见嘌呤处，酵母tRNAPhe的交换率与两种大肠杆菌转运核糖核酸的交换率显著不同。这些差异可能源于转运核糖核酸之间特定三级相互作用相对稳定性的变化，也可能源于其他因素。就tRNA2Tyr而言，大可变区碱基的标记率足够延迟，表明该分子这一部分存在某种结构组织。此外，由于大肠杆菌tRNAIle和tRNA2Tyr共有的一些碱基的交换率相似，tRNA2Tyr的大可变环可能不会与这些共同位点相互作用或干扰它们。最后，对于所有三种转运核糖核酸，结构形成（例如碱基配对、碱基堆积）总是会降低标记率，尽管这些转运核糖核酸三维结构中碱基环境的多样性可能会以不同方式影响C-8的酸度和其他化学性质。尽管这些化学效应无疑会产生影响，但在这些研究中，对交换的主要影响可能是结构对溶剂分子（即水）可及性的影响。
