Hsieh Ping-Hung, He Shengbo, Buttress Toby, Gao Hongbo, Couchman Matthew, Fischer Robert L, Zilberman Daniel, Feng Xiaoqi
Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720.
Department of Cell and Developmental Biology, John Innes Centre, Norwich NR4 7UH, United Kingdom.
Proc Natl Acad Sci U S A. 2016 Dec 27;113(52):15132-15137. doi: 10.1073/pnas.1619074114. Epub 2016 Dec 12.
Cytosine DNA methylation regulates the expression of eukaryotic genes and transposons. Methylation is copied by methyltransferases after DNA replication, which results in faithful transmission of methylation patterns during cell division and, at least in flowering plants, across generations. Transgenerational inheritance is mediated by a small group of cells that includes gametes and their progenitors. However, methylation is usually analyzed in somatic tissues that do not contribute to the next generation, and the mechanisms of transgenerational inheritance are inferred from such studies. To gain a better understanding of how DNA methylation is inherited, we analyzed purified Arabidopsis thaliana sperm and vegetative cells-the cell types that comprise pollen-with mutations in the DRM, CMT2, and CMT3 methyltransferases. We find that DNA methylation dependency on these enzymes is similar in sperm, vegetative cells, and somatic tissues, although DRM activity extends into heterochromatin in vegetative cells, likely reflecting transcription of heterochromatic transposons in this cell type. We also show that lack of histone H1, which elevates heterochromatic DNA methylation in somatic tissues, does not have this effect in pollen. Instead, levels of CG methylation in wild-type sperm and vegetative cells, as well as in wild-type microspores from which both pollen cell types originate, are substantially higher than in wild-type somatic tissues and similar to those of H1-depleted roots. Our results demonstrate that the mechanisms of methylation maintenance are similar between pollen and somatic cells, but the efficiency of CG methylation is higher in pollen, allowing methylation patterns to be accurately inherited across generations.
胞嘧啶DNA甲基化调控真核基因和转座子的表达。DNA复制后，甲基转移酶会复制甲基化，这导致甲基化模式在细胞分裂过程中，以及至少在开花植物中跨代忠实传递。跨代遗传由一小群细胞介导，包括配子及其祖细胞。然而，甲基化通常在对下一代没有贡献的体细胞组织中进行分析，并且跨代遗传的机制是从此类研究中推断出来的。为了更好地理解DNA甲基化是如何遗传的，我们分析了纯化的拟南芥精子和营养细胞（构成花粉的细胞类型），这些细胞在DRM、CMT2和CMT3甲基转移酶中存在突变。我们发现，精子、营养细胞和体细胞组织中对这些酶的DNA甲基化依赖性相似，尽管DRM活性在营养细胞中延伸到异染色质，这可能反映了这种细胞类型中异染色质转座子的转录。我们还表明，组蛋白H1的缺失会提高体细胞组织中的异染色质DNA甲基化，但在花粉中没有这种作用。相反，野生型精子和营养细胞以及两种花粉细胞类型均起源的野生型小孢子中的CG甲基化水平，显著高于野生型体细胞组织，并且与H1缺失的根中的水平相似。我们的结果表明，花粉和体细胞之间甲基化维持的机制相似，但花粉中CG甲基化的效率更高，从而使甲基化模式能够准确地跨代遗传。
