RNA Therapeutics Institute, University of Massachusetts Medical School, Worcester,United States of America.
Program in Molecular Medicine, University of Massachusetts Medical School, Worcester, United States of America.
PLoS Genet. 2023 Jun 29;19(6):e1010804. doi: 10.1371/journal.pgen.1010804. eCollection 2023 Jun.
Retroviruses and closely related LTR retrotransposons export full-length, unspliced genomic RNA (gRNA) for packaging into virions and to serve as the mRNA encoding GAG and POL polyproteins. Because gRNA often includes splice acceptor and donor sequences used to splice viral mRNAs, retroelements must overcome host mechanisms that retain intron-containing RNAs in the nucleus. Here we examine gRNA expression in Cer1, an LTR retrotransposon in C. elegans which somehow avoids silencing and is highly expressed in germ cells. Newly exported Cer1 gRNA associates rapidly with the Cer1 GAG protein, which has structural similarity with retroviral GAG proteins. gRNA export requires CERV (C. elegans regulator of viral expression), a novel protein encoded by a spliced Cer1 mRNA. CERV phosphorylation at S214 is essential for gRNA export, and phosphorylated CERV colocalizes with nuclear gRNA at presumptive sites of transcription. By electron microscopy, tagged CERV proteins surround clusters of distinct, linear fibrils that likely represent gRNA molecules. Single fibrils, or groups of aligned fibrils, also localize near nuclear pores. During the C. elegans self-fertile period, when hermaphrodites fertilize oocytes with their own sperm, CERV concentrates in two nuclear foci that are coincident with gRNA. However, as hermaphrodites cease self-fertilization, and can only produce cross-progeny, CERV undergoes a remarkable transition to form giant nuclear rods or cylinders that can be up to 5 microns in length. We propose a novel mechanism of rod formation, in which stage-specific changes in the nucleolus induce CERV to localize to the nucleolar periphery in flattened streaks of protein and gRNA; these streaks then roll up into cylinders. The rods are a widespread feature of Cer1 in wild strains of C. elegans, but their function is not known and might be limited to cross-progeny. We speculate that the adaptive strategy Cer1 uses for the identical self-progeny of a host hermaphrodite might differ for heterozygous cross-progeny sired by males. For example, mating introduces male chromosomes which can have different, or no, Cer1 elements.
逆转录病毒和密切相关的 LTR 反转录转座子将全长、未剪接的基因组 RNA (gRNA) 输出，用于包装成病毒粒子，并作为编码 GAG 和 POL 多蛋白的 mRNA。由于 gRNA 通常包含用于剪接病毒 mRNA 的剪接受体和供体序列，因此逆转录元件必须克服宿主将包含内含子的 RNA 保留在核内的机制。在这里，我们研究了 Cer1 中的 gRNA 表达，Cer1 是秀丽隐杆线虫中的 LTR 反转录转座子，它以某种方式避免沉默，并在生殖细胞中高度表达。新输出的 Cer1 gRNA 迅速与 Cer1 GAG 蛋白结合，该蛋白与逆转录病毒 GAG 蛋白具有结构相似性。gRNA 输出需要 CERV（线虫病毒表达调节剂），这是一种由 Cer1 剪接 mRNA 编码的新型蛋白。Cer1 gRNA 输出所必需的 CERV 丝氨酸 214 磷酸化，以及磷酸化的 CERV 与核内 gRNA 在转录假定部位共定位。通过电子显微镜观察，标记的 CERV 蛋白围绕着明显的线性纤维簇，这些纤维簇可能代表 gRNA 分子。单个纤维或排列整齐的纤维簇也定位于核孔附近。在秀丽隐杆线虫自育期，当雌雄同体用自己的精子使卵子受精时，CERV 集中在两个核焦点，与 gRNA 一致。然而，随着雌雄同体停止自育，并只能产生杂交后代，CERV 发生了显著的转变，形成了长达 5 微米的巨大核棒或圆柱体。我们提出了一种新的棒状形成机制，即核仁的阶段特异性变化诱导 CERV 在核仁周围扁平的蛋白质和 gRNA 条纹中定位；这些条纹然后卷成圆柱体。这些棒状物是秀丽隐杆线虫野生菌株中 Cer1 的一个普遍特征，但它们的功能尚不清楚，可能仅限于杂交后代。我们推测，Cer1 对宿主雌雄同体自身后代的适应策略可能因来自雄性的异质杂交后代而异。例如，交配引入了具有不同或没有 Cer1 元件的雄性染色体。
