Podar M, Chu V T, Pyle A M, Perlman P S
Department of Molecular Biology and Oncology, University of Texas Southwestern Medical Center, Dallas 75235-9148, USA.
Nature. 1998 Feb 26;391(6670):915-8. doi: 10.1038/36142.
Group I, group II and spliceosomal introns splice by two sequential transesterification reactions. For both spliceosomal and group II introns, the first-step reaction occurs by nucleophilic attack on the 5' splice junction by the 2' hydroxyl of an internal adenosine, forming a 2'-5' phosphodiester branch in the intron. The second reaction joins the two exons with a 3'-5' phosphodiester bond and releases intron lariat. In vitro, group II introns can self-splice by an efficient alternative pathway in which the first-step reaction occurs by hydrolysis. The resulting linear splicing intermediate participates in normal second-step reactions, forming spliced exon and linear intron RNAs. Here we show that the group II intron first-step hydrolysis reaction occurs in vivo in place of transesterification in the mitochondria of yeast strains containing branch-site mutations. As expected, the mutations block branching, but surprisingly still allow accurate splicing. This hydrolysis pathway may have been a step in the evolution of splicing mechanisms.
I类、II类和剪接体内含子通过两个连续的转酯反应进行剪接。对于剪接体和II类内含子而言，第一步反应是由内部腺苷的2'羟基对5'剪接位点进行亲核攻击，在内含子中形成2'-5'磷酸二酯分支。第二步反应将两个外显子通过3'-5'磷酸二酯键连接起来，并释放内含子套索。在体外，II类内含子可以通过一种有效的替代途径进行自我剪接，其中第一步反应通过水解发生。产生的线性剪接中间体参与正常的第二步反应，形成剪接后的外显子和线性内含子RNA。在这里，我们表明，在含有分支位点突变的酵母菌株的线粒体中，II类内含子的第一步水解反应在体内发生，取代了转酯反应。正如预期的那样，这些突变阻止了分支形成，但令人惊讶的是，仍然允许准确剪接。这种水解途径可能是剪接机制进化过程中的一个步骤。
