Jeddeloh J A, Bender J, Richards E J
Washington University, Department of Biology, St. Louis, Missouri 63130 USA.
Genes Dev. 1998 Jun 1;12(11):1714-25. doi: 10.1101/gad.12.11.1714.
To investigate the relationship between cytosine methylation and gene silencing in Arabidopsis, we constructed strains containing the ddm1 hypomethylation mutation and a methylated and silenced PAI2 tryptophan biosynthetic gene (MePAI2) that results in a blue fluorescent plant phenotype. The ddm1 mutation had both an immediate and a progressive effect on PAI gene silencing. In the first generation, homozygous ddm1 MePAI2 plants displayed a weakly fluorescent phenotype, in contrast to the strongly fluorescent phenotype of the DDM1 MePAI2 parent. After two generations of inbreeding by self-pollination, the ddm1/ddm1 lines became nonfluorescent. The progressive loss of fluorescence correlated with a progressive loss of methylation from the PAI2 gene. These results indicate that methylation is necessary for maintenance of PAI gene silencing and that intermediate levels of DNA methylation are associated with intermediate gene silencing. The results also support our earlier hypothesis that ddm1 homozygotes act as "epigenetic mutators" by accumulating heritable changes in DNA methylation that can lead to changes in gene expression.
为了研究拟南芥中胞嘧啶甲基化与基因沉默之间的关系，我们构建了含有ddm1低甲基化突变以及一个甲基化且沉默的PAI2色氨酸生物合成基因（MePAI2）的菌株，该基因会导致植物呈现蓝色荧光表型。ddm1突变对PAI基因沉默具有即时和渐进性影响。在第一代中，纯合ddm1 MePAI2植株表现出弱荧光表型，这与DDM1 MePAI2亲本的强荧光表型形成对比。经过两代自花授粉近亲繁殖后，ddm1/ddm1株系变得无荧光。荧光的逐渐丧失与PAI2基因甲基化的逐渐丧失相关。这些结果表明甲基化对于维持PAI基因沉默是必要的，并且DNA甲基化的中间水平与中间程度的基因沉默相关。这些结果也支持了我们早期的假设，即ddm1纯合子通过积累DNA甲基化中的可遗传变化（可导致基因表达改变）而充当“表观遗传突变体”。
