Research Group Virology, Bacteriology and Phytoplasmology, Plant Protection Department, Agroscope, Nyon, Switzerland.
Virol J. 2024 Jan 4;21(1):6. doi: 10.1186/s12985-023-02257-y.
In cellular organisms, inosine triphosphate pyrophosphatases (ITPases) prevent the incorporation of mutagenic deaminated purines into nucleic acids. These enzymes have also been detected in the genomes of several plant RNA viruses infecting two euphorbia species. In particular, two ipomoviruses produce replicase-associated ITPases to cope with high concentration of non-canonical nucleotides found in cassava tissues.
Using high-throughput RNA sequencing on the wild euphorbia species Mercurialis perennis, two new members of the families Potyviridae and Secoviridae were identified. Both viruses encode for a putative ITPase, and were found in mixed infection with a new partitivirid. Following biological and genomic characterization of these viruses, the origin and function of the phytoviral ITPases were investigated.
While the potyvirid was shown to be pathogenic, the secovirid and partitivirid could not be transmitted. The secovirid was found belonging to a proposed new Comovirinae genus tentatively named "Mercomovirus", which also accommodates other viruses identified through transcriptome mining, and for which an asymptomatic pollen-associated lifestyle is suspected. Homology and phylogenetic analyses inferred that the ITPases encoded by the potyvirid and secovirid were likely acquired through independent horizontal gene transfer events, forming lineages distinct from the enzymes found in cassava ipomoviruses. Possible origins from cellular organisms are discussed for these proteins. In parallel, the endogenous ITPase of M. perennis was predicted to encode for a C-terminal nuclear localization signal, which appears to be conserved among the ITPases of euphorbias but absent in other plant families. This subcellular localization is in line with the idea that nucleic acids remain protected in the nucleus, while deaminated nucleotides accumulate in the cytoplasm where they act as antiviral molecules.
Three new RNA viruses infecting M. perennis are described, two of which encoding for ITPases. These enzymes have distinct origins, and are likely required by viruses to circumvent high level of cytoplasmic non-canonical nucleotides. This putative plant defense mechanism has emerged early in the evolution of euphorbias, and seems to specifically target certain groups of RNA viruses infecting perennial hosts.
在细胞生物中，肌苷三磷酸焦磷酸酶（ITPases）可防止诱变脱氨嘌呤掺入核酸。在感染两种大戟属植物的几种植物 RNA 病毒的基因组中也检测到了这些酶。特别是两种马铃薯 Y 病毒产生复制酶相关的 ITPase，以应对在木薯组织中发现的高浓度非规范核苷酸。
使用高通量 RNA 测序对野生大戟属植物 Mercurialis perennis 进行研究，鉴定出了两个新的马铃薯 Y 病毒科和细环病毒科成员。这两种病毒都编码一个假定的 ITPase，并且与一种新的二分体病毒混合感染。对这些病毒进行生物学和基因组特征分析后，研究了植物病毒 ITPase 的起源和功能。
虽然证明了 potyvirid 是病原性的，但 secovirid 和 partitivirid 不能传播。secovirid 被归类为一个拟议的新 Comovirinae 属，暂命名为“Mercomovirus”，该属还容纳了通过转录组挖掘鉴定的其他病毒，怀疑其具有无症状花粉相关的生活方式。同源性和系统发育分析推断，potyvirid 和 secovirid 编码的 ITPase可能是通过独立的水平基因转移事件获得的，形成了与在木薯马铃薯 Y 病毒中发现的酶不同的谱系。这些蛋白可能起源于细胞生物。同时，预测 M. perennis 的内源性 ITPase 编码一个 C 端核定位信号，该信号似乎在大戟属的 ITPase中保守，但在其他植物科中不存在。这种亚细胞定位与这样一种观点一致，即核酸在核内保持保护，而脱氨核苷酸在细胞质中积累，在细胞质中它们充当抗病毒分子。
描述了感染 M. perennis 的三种新的 RNA 病毒，其中两种编码 ITPase。这些酶具有不同的起源，可能是病毒规避高浓度细胞质非规范核苷酸所必需的。这种假定的植物防御机制在大戟属的进化早期就出现了，似乎专门针对感染多年生宿主的某些 RNA 病毒群。
