大肠杆菌苏氨酸-tRNA合成酶通过在一个区域与它结合来调节其自身mRNA的翻译,打电话给接线员,位于核糖体结合位点的前面。操纵子的一级和二级结构类似于几种tRNA(Thr)等受体物种的反密码子臂的结构。我们推断,如果合成酶和它的两个部分类似的配体之间的相互作用,tRNA和mRNA,有一些共同的特点,酶中的单个突变应该以非常相似的方式影响两种相互作用。因此,我们分离了合成酶突变体(称为超级阻遏物),将其mRNA的反式翻译抑制到极端水平,以及其他完全无法进行任何压制的突变体。超级阻遏物,它们被怀疑以高亲和力结合它们的mRNA,显示以增加的亲和力结合tRNA。非压抑的突变体,怀疑它们已经失去了结合mRNA的能力,显示以更小的亲和力结合它们的tRNA。突变酶对其他底物的结合特性,ATP和苏氨酸,是不变的。观察到的调节和氨基酰化缺陷之间的相关性强烈表明,合成酶以类似的方式识别其两个RNA配体的相似部分-mRNA的反密码子样臂和tRNA的真正反密码子臂。
Escherichia coli threonyl-tRNA synthetase regulates the translation of its own mRNA by binding to it in a region, called the operator, located in front of the ribosomal binding site. The primary and secondary structures of the operator resemble those of the
anticodon arm of several tRNA(Thr) isoacceptor species. We reasoned that if the interaction between the synthetase and its two partially analogous ligands, the tRNA and the mRNA, had some common features, single mutations in the enzyme should affect both interactions in a very similar way. We thus isolated synthetase mutants (called super-repressors) that repress the translation of their mRNA in trans to an extreme level, and other mutants that are completely unable to perform any repression. The super-repressors, which are suspected to bind their mRNA with high affinity, are shown to bind the tRNA with an increased affinity. The non-repressing mutants, which are suspected to have lost their capacity to bind the mRNA, are shown to bind their tRNA with less affinity. The binding properties of the mutant enzymes for the other substrates, ATP and threonine, are unchanged. The observed correlation between regulatory and aminoacylation defects strongly suggests that the synthetase recognizes the similar parts of its two RNA ligands--the
anticodon-like arm of the mRNA and the true
anticodon arm of the tRNA--in an analogous way.