生物谷報導：地球上的生命據信是從一個「RNA世界」演化來的，在這個世界上，RNA分子既催化重要的化學反應，又攜帶遺傳信息。在現代生物學中，蛋白已經成為細胞中完成酶催化作用的主要角色，而核酸則仍舊扮演攜帶遺傳信息的角色。然而，在細胞內，仍然有RNA世界的遺蹟。其中一個遺蹟就是線粒體tyrosyl-tRNA合成酶CYT-18，它來自真菌鏈孢黴，也與一種group I intron核酶結合，並且幫助進行剪接。現在，這一蛋白/核酶複合體的晶體結構已被確定。它們的相互作用界面與CYT-18用來在其酶催化作用中與tRNATyr結合的界面是不同的。研究人員還發現了在非拼接tRNA合成酶中不存在的特定變化，它們可能是RNA-蛋白複合體從只有RNA的酶演化而來的方式。
生物谷推薦英文原文：
Nature 451, 94-97 (3 January 2008) | doi:10.1038/nature06413; Received 26 September 2007; Accepted 24 October 2007
Structure of a tyrosyl-tRNA synthetase splicing factor bound to a group I intron RNA
Paul J. Paukstelis1, Jui-Hui Chen2, Elaine Chase2, Alan M. Lambowitz1,3 & Barbara L. Golden2,3
Correspondence to: Alan M. Lambowitz1,3Barbara L. Golden2,3 Correspondence and requests for materials should be addressed to A.M.L. (Email: lambowitz@mail.utexas.edu) or B.L.G. (Email: barbgolden@purdue.edu).
The 'RNA world' hypothesis holds that during evolution the structural and enzymatic functions initially served by RNA were assumed by proteins, leading to the latter's domination of biological catalysis. This progression can still be seen in modern biology, where ribozymes, such as the ribosome and RNase P, have evolved into protein-dependent RNA catalysts ('RNPzymes'). Similarly, group I introns use RNA-catalysed splicing reactions, but many function as RNPzymes bound to proteins that stabilize their catalytically active RNA structure1, 2. One such protein, the Neurospora crassa mitochondrial tyrosyl-tRNA synthetase (TyrRS; CYT-18), is bifunctional and both aminoacylates mitochondrial tRNATyr and promotes the splicing of mitochondrial group I introns3. Here we determine a 4.5-Å co-crystal structure of the Twort orf142-I2 group I intron ribozyme bound to splicing-active, carboxy-terminally truncated CYT-18. The structure shows that the group I intron binds across the two subunits of the homodimeric protein with a newly evolved RNA-binding surface distinct from that which binds tRNATyr. This RNA binding surface provides an extended scaffold for the phosphodiester backbone of the conserved catalytic core of the intron RNA, allowing the protein to promote the splicing of a wide variety of group I introns. The group I intron-binding surface includes three small insertions and additional structural adaptations relative to non-splicing bacterial TyrRSs, indicating a multistep adaptation for splicing function. The co-crystal structure provides insight into how CYT-18 promotes group I intron splicing, how it evolved to have this function, and how proteins could have incrementally replaced RNA structures during the transition from an RNA world to an RNP world.