Kato Masaomi, Miura Asuka, Bender Judith, Jacobsen Steven E, Kakutani Tetsuji
Department of Integrated Genetics, National Institute of Genetics, Mishima, Shizuoka 411-8540, Japan.
Curr Biol. 2003 Mar 4;13(5):421-6. doi: 10.1016/s0960-9822(03)00106-4.
Methylation of cytosine residues in eukaryotic genomes is often associated with repeated sequences including transposons and their derivatives. Methylation has been implicated in control of two potential deleterious effects of these repeats: (1) uncontrolled transcription, which often disturbs proper expression of nearby host genes, and (2) changes in genome structure by transposition and ectopic recombination. Arabidopsis thaliana provides a genetically tractable system to examine these possibilities, since viable mutants in DNA methyltransferases are available. Arabidopsis MET1 (METHYLTRANSFERASE1, ortholog of mammalian DNA methyltransferase Dnmt1) is necessary for maintaining genomic cytosine methylation at 5'-CG-3' sites. Arabidopsis additionally methylates non-CG sites using CHROMOMETHYLASE3 (CMT3). We examined the mobility of endogenous CACTA transposons in met1, cmt3, and cmt3-met1 mutants. High-frequency transposition of CACTA elements was detected in cmt3-met1 double mutants. Single mutants in either met1 or cmt3 were much less effective in mobilization, despite significant induction of CACTA transcript accumulation. These results lead us to conclude that CG and non-CG methylation systems redundantly function for immobilization of transposons. Non-CG methylation in plants may have evolved as an additional epigenetic tag dedicated to transposon control. This view is consistent with the recent finding that CMT3 preferentially methylates transposon-related sequences.
真核生物基因组中胞嘧啶残基的甲基化通常与包括转座子及其衍生物在内的重复序列相关。甲基化与控制这些重复序列的两种潜在有害效应有关：（1）不受控制的转录，这常常干扰附近宿主基因的正常表达；（2）通过转座和异位重组导致基因组结构的改变。拟南芥提供了一个易于进行遗传研究的系统来检验这些可能性，因为存在DNA甲基转移酶的可行突变体。拟南芥MET1（甲基转移酶1，哺乳动物DNA甲基转移酶Dnmt1的直系同源物）对于维持基因组中5'-CG-3'位点的胞嘧啶甲基化是必需的。拟南芥还利用染色质甲基化酶3（CMT3）对非CG位点进行甲基化。我们检测了met1、cmt3和cmt3-met1突变体中内源性CACTA转座子的移动性。在cmt3-met1双突变体中检测到了CACTA元件的高频转座。尽管CACTA转录本积累有显著诱导，但met1或cmt3单突变体在转座方面的效果要差得多。这些结果使我们得出结论，CG和非CG甲基化系统在转座子固定方面具有冗余功能。植物中的非CG甲基化可能已经进化成为一种专门用于转座子控制的额外表观遗传标记。这一观点与最近发现的CMT3优先甲基化转座子相关序列的结果一致。
