Steward Nicolas, Ito Mikako, Yamaguchi Yube, Koizumi Nozomu, Sano Hiroshi
Research and Education Center for Genetic Information, Nara Institute of Science and Technology, Nara 630-0101, Japan.
J Biol Chem. 2002 Oct 4;277(40):37741-6. doi: 10.1074/jbc.M204050200. Epub 2002 Jul 17.
When maize seedlings were exposed to cold stress, a genome-wide demethylation occurred in root tissues. Screening of genomic DNA identified one particular fragment that was demethylated during chilling. This 1.8-kb fragment, designated ZmMI1, contained part of the coding region of a putative protein and part of a retrotransposon-like sequence. ZmMI1 was transcribed only under cold stress. Direct methylation mapping revealed that hypomethylated regions spanning 150 bases alternated with hypermethylated regions spanning 50 bases. Analysis of nuclear DNA digested with micrococcal nuclease indicated that these regions corresponded to nucleosome cores and linkers, respectively. Cold stress induced severe demethylation in core regions but left linker regions relatively intact. Thus, methylation and demethylation were periodic in nucleosomes. The following biological significance is conceivable. First, because DNA methylation in nucleosomes induces alteration of gene expression by changing chromatin structures, vast demethylation may serve as a common switch for many genes that are simultaneously controlled upon environmental cues. Second, because artificial demethylation induces heritable changes in plant phenotype (Sano, H., Kamada, I., Youssefian, S., Katsumi, M., and Wabilko, H. (1990) Mol. Gen. Genet. 220, 441-447), altered DNA methylation may result in epigenetic inheritance, in which gene expression is modified without changing the nucleotide sequence.
当玉米幼苗遭受冷胁迫时，根组织中发生了全基因组去甲基化。对基因组DNA进行筛选，鉴定出一个在冷处理过程中发生去甲基化的特定片段。这个1.8 kb的片段，命名为ZmMI1，包含一个假定蛋白质的部分编码区和部分类反转录转座子序列。ZmMI1仅在冷胁迫下转录。直接甲基化图谱显示，跨度为150个碱基的低甲基化区域与跨度为50个碱基的高甲基化区域交替出现。用微球菌核酸酶消化核DNA的分析表明，这些区域分别对应于核小体核心和连接区。冷胁迫在核心区域诱导了严重的去甲基化，但连接区相对完整。因此，甲基化和去甲基化在核小体中是周期性的。可以想象其具有以下生物学意义。首先，由于核小体中的DNA甲基化通过改变染色质结构诱导基因表达改变，大量去甲基化可能作为许多基因在环境信号作用下同时被调控的共同开关。其次，由于人工去甲基化会诱导植物表型的可遗传变化（佐野，H.，镰田，I.，优素福安，S.，胜见，M.，和瓦比尔科，H.（1990年）《分子与普通遗传学》220卷，441 - 447页），DNA甲基化的改变可能导致表观遗传，即基因表达在不改变核苷酸序列的情况下被修饰。
