Two H3K23 histone methyltransferases, SET-32 and SET-21, function synergistically to promote germline nuclear RNAi, proper endogenous siRNA expression, and germline immortality in Caenorhabditis elegans [RNA-seq]

Nuclear RNAi in C. elegans induces a set of transgenerationally heritable marks of H3K9me3, H3K23me3, and H3K27me3 at the target genes. The function of H3K23me3 in the nuclear RNAi pathway is largely unknown due to the limited knowledge of H3K23 histone methyltransferase (HMT). In this study we identified SET-21 as a novel H3K23 HMT. By taking combined genetic, biochemical, and genomic approaches we found that SET-21 functions synergistically with a previously reported H3K23 HMT SET-32 to deposit H3K23me3 at the native targets of germline nuclear RNAi. We identified a subset of nuclear RNAi targets become transcriptionally activated in the set-21;set-32 double mutant. SET-21 and SET-32 are also required for a robust transgenerational gene silencing induced by exogenous dsRNA. The loss of SET-21 and SET-32 leads to a mortal germline phenotype, similar to mutations of the core nuclear RNAi component. Together, these results support a model in which H3K23 HMTs SET-21 and SET-32 ensure the robustness of germline nuclear RNAi and promotes the germline immortality at a high temperature. Overall design: RNA-seq libaries were prepared from from wild type and mutant adult hermaphrodite C. elegans strains.

秀丽隐杆线虫（C. elegans）中的核RNA干扰（nuclear RNAi）会在靶基因处诱导产生一系列可跨代遗传的H3K9me3、H3K23me3及H3K27me3修饰标记。由于当前对H3K23组蛋白甲基转移酶（HMT）的认知较为有限，H3K23me3在核RNA干扰通路中的功能尚不明晰。本研究鉴定出SET-21为一种新型H3K23组蛋白甲基转移酶。通过联合运用遗传学、生物化学与基因组学研究手段，研究发现SET-21与此前已报道的H3K23组蛋白甲基转移酶SET-32协同发挥作用，在生殖系核RNA干扰的内源靶位点沉积H3K23me3修饰。研究团队还鉴定到一类核RNA干扰靶基因，在set-21;set-32双突变体中会出现转录激活现象。SET-21与SET-32同样是外源双链RNA（dsRNA）诱导的高效跨代基因沉默过程所必需的。SET-21与SET-32的缺失会引发生殖系致死表型，该表型与核心核RNA干扰组分的突变表型相似。综上，上述研究结果支持如下模型：H3K23组蛋白甲基转移酶SET-21与SET-32可保障生殖系核RNA干扰的稳健性，并在高温环境下促进生殖系永生。实验整体设计：从野生型及突变型成年雌雄同体秀丽隐杆线虫（C. elegans）菌株中制备RNA测序（RNA-seq）文库。