美国伊利诺伊大学香槟分校生物化学系教授Raven H. Huang及其同事首次在细菌中发现了RNA修复系统。这是迄今为止发现的第二个RNA修复系统，第一个为噬菌体（可攻击细菌的一种病毒）中的带有2个蛋白的RNA修复系统。相关文章发表在本月的美国《科学》杂志以及《美国国家科学院院刊》上。
此次发现的细菌RNA修复系统的新颖之处在于，在受损的RNA封闭前，一个甲基会附着在该RNA受损点的两个主要羟基之上，使得受损点无法继续开裂，从而达到修复的效果。这一发现对于保护细胞免遭核糖毒素的侵袭具有重要的意义。该毒素能使蛋白质转译涉及的重要RNA发生开裂，从而导致细胞的死亡。
由于新发现的RNA修复系统中对甲基负责的酶是细菌中的Hen1的同系物，因此该发现对理解RNA干涉以及动物、植物和其他真核生物的基因表达同样具有相当重要的意义。
发表在《科学》杂志上的论文主要描述了RNA修复过程的全部机理，而发表在《美国国家科学院院刊》的论文则着重解析了甲基化反应的化学机理，尤其是细菌中的Hen1内起主导作用的转甲基酶的晶体结构。由于真核态的Hen1能产生同样的化学反应，研究应进一步侧重于理解真核生物中的RNA干涉。
Huang表示，Hen1是真核生物RNA干涉中产生小型非编码RNA的三种基础酶之一。虽然Hen1的同系物在细菌中确实存在，但细菌内却没有任何的RNA干涉。因此，研究人员十分好奇，想要破解细菌中的Hen1的具体功能。黄雷文说：“研究表明，细菌中的Hen1与真核生物中的Hen1能产生同样的化学反应。但令我们感到惊讶的是，虽然细菌中的Hen1并不在RNA干涉中发挥作用，却是RNA修复和修正系统中的一部分，它能使修复后的RNA恢复如新，甚至能比全新的RNA发挥更好的功能。”（生物谷Bioon.com）
生物谷推荐原始出处：
Science 9 October 2009:DOI: 10.1126/science.1179480
Reconstituting Bacterial RNA Repair and Modification in Vitro
Chio Mui Chan,1,* Chun Zhou,1,* Raven H. Huang1,2,
Ribotoxins kill cells by endonucleotically cleaving essential RNAs involved in protein translation. We report here that a stable heterotetramer composed of two bacterial proteins, Pnkp and Hen1, was able to repair transfer RNAs cleaved by ribotoxins in vitro. Before the broken RNAs were ligated by the heterotetramer, a methyl group was added to the 2'-OH group that participated in the original RNA cut. Because of the methylation, RNAs repaired by bacterial Pnkp/Hen1 heterotetramer could not be cleaved again by the ribotoxins. Thus, unlike eukaryotic Hen1 involved in RNA interference, the bacterial Hen1 is part of an RNA repair and modification system.
1 Department of Biochemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
2 Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
PNAS October 12, 2009, doi: 10.1073/pnas.0907540106
Structural and biochemical insights into 2′-O-methylation at the 3′-terminal nucleotide of RNA by Hen1
Chio Mui Chana,1, Chun Zhoua,1, Joseph S. Brunzelleb and Raven H. Huanga,c,2
aDepartment of Biochemistry and
cCenter for Biophysics and Computational Biology, University of Illinois at Urbana–Champaign, Urbana, IL 61801; and
bLife Science Collaborative Access Team, Argonne National Laboratory, Argonne, IL 60439
Small RNAs of ≈20–30 nt have diverse and important biological roles in eukaryotic organisms. After being generated by Dicer or Piwi proteins, all small RNAs in plants and a subset of small RNAs in animals are further modified at their 3′-terminal nucleotides via 2′-O-methylation, carried out by the S-adenosylmethionine-dependent methyltransferase (MTase) Hen1. Methylation at the 3′ terminus is vital for biological functions of these small RNAs. Here, we report four crystal structures of the MTase domain of a bacterial homolog of Hen1 from Clostridium thermocellum and Anabaena variabilis, which are enzymatically indistinguishable from the eukaryotic Hen1 in their ability to methylate small single-stranded RNAs. The structures reveal that, in addition to the core fold of the MTase domain shared by other RNA and DNA MTases, the MTase domain of Hen1 possesses a motif and a domain that are highly conserved and are unique to Hen1. The unique motif and domain are likely to be involved in RNA substrate recognition and catalysis. The structures allowed us to construct a docking model of an RNA substrate bound to the MTase domain of bacterial Hen1, which is likely similar to that of the eukaryotic counterpart. The model, supported by mutational studies, provides insight into RNA substrate specificity and catalytic mechanism of Hen1.
