Photosynthesis Research Center, Key Laboratory of Photobiology, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China.
University of Chinese Academy of Sciences, Beijing 100049, China.
Nucleic Acids Res. 2020 Apr 6;48(6):3195-3210. doi: 10.1093/nar/gkaa129.
Methylation of nucleotides in ribosomal RNAs (rRNAs) is a ubiquitous feature that occurs in all living organisms. The formation of methylated nucleotides is performed by a variety of RNA-methyltransferases. Chloroplasts of plant cells result from an endosymbiotic event and possess their own genome and ribosomes. However, enzymes responsible for rRNA methylation and the function of modified nucleotides in chloroplasts remain to be determined. Here, we identified an rRNA methyltransferase, CMAL (Chloroplast MraW-Like), in the Arabidopsis chloroplast and investigated its function. CMAL is the Arabidopsis ortholog of bacterial MraW/ RsmH proteins and accounts to the N4-methylation of C1352 in chloroplast 16S rRNA, indicating that CMAL orthologs and this methyl-modification nucleotide is conserved between bacteria and the endosymbiont-derived eukaryotic organelle. The knockout of CMAL in Arabidopsis impairs the chloroplast ribosome accumulation and accordingly reduced the efficiency of mRNA translation. Interestingly, the loss of CMAL leads not only to defects in chloroplast function, but also to abnormal leaf and root development and overall plant morphology. Further investigation showed that CMAL is involved in the plant development probably by modulating auxin derived signaling pathways. This study uncovered the important role of 16S rRNA methylation mediated by CMAL in chloroplast ribosome biogenesis and plant development.
核苷酸在核糖体 RNA（rRNA）中的甲基化是一种普遍存在的特征，发生在所有生物中。甲基化核苷酸的形成是由各种 RNA-甲基转移酶完成的。植物细胞的叶绿体是内共生事件的结果，拥有自己的基因组和核糖体。然而，负责 rRNA 甲基化的酶以及修饰核苷酸在叶绿体中的功能仍有待确定。在这里，我们在拟南芥叶绿体中鉴定出一种 rRNA 甲基转移酶 CMAL（叶绿体 MraW-Like），并研究了其功能。CMAL 是细菌 MraW/RsmH 蛋白的拟南芥同源物，负责叶绿体 16S rRNA 中 C1352 的 N4-甲基化，表明 CMAL 同源物和这个修饰核苷酸在细菌和内共生的真核细胞器之间是保守的。拟南芥中 CMAL 的敲除会损害叶绿体核糖体的积累，从而降低 mRNA 翻译的效率。有趣的是，CMAL 的缺失不仅导致叶绿体功能缺陷，还导致叶片和根系发育异常以及整个植物形态异常。进一步的研究表明，CMAL 可能通过调节植物发育过程中的生长素衍生信号通路来参与植物发育。本研究揭示了 CMAL 介导的 16S rRNA 甲基化在叶绿体核糖体生物发生和植物发育中的重要作用。
