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RNA Silencing in Virology

In eukaryotes, RNA silencing controls gene expression to regulate development, genome stability and stress-induced responses. Multicellular organisms possess sophisticated defense systems to confer protection against pathogens; one of them is RNA interference (RNAi). In plants, RNA silencing is also recognized as a major immune system targeted against plant viruses. Small RNA-directed silencing mechanisms play important roles in the regulation of eukaryotic gene expression.
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RNA Silencing in Virology

From: Recent Advances in Plant Virology | RNA Interference and Viruses | RNA and the Regulation of Gene Expression

The term RNA silencing refers to several pathways present in eukaryotic organisms that lead to the sequence specific elimination or functional blocking of RNAs with homology to double stranded RNAs (dsRNAs) that have previously triggered the mechanism. Besides playing important roles in developmental control, RNA silencing forms part of the defence against viruses, acting as a potent antiviral mechanism.

RNA Silencing and the Interplay Between Plants and Viruses
from Lourdes Fernandez-Calvino, Livia Donaire and Cesar Llave writing in Recent Advances in Plant Virology
In eukaryotes, RNA silencing controls gene expression to regulate development, genome stability and stress-induced responses. In plants, this process is also recognized as a major immune system targeted against plant viruses. Plant viruses stimulate RNA silencing responses though formation of viral RNA with double-stranded features that are subsequently processed into functional small RNAs (sRNAs). Recent studies highlight the complexity of the viral sRNA populations and their potential to associate with multiple silencing effector complexes. This fact has profound implications in the cross-talk interactions between plants and viruses since both virus genomes and host genes are putative targets of viral sRNAs. The concept of RNA silencing is an elegant natural antiviral mechanism in plants. Viral sRNA-mediated regulation of gene expression is important in the frame of compatible interactions between plants and viruses.

RNA Silencing in Plants and the Role of Viral Suppressors
from Ana Giner, Juan Jose Lopez-Moya and Lorant Lakatos writing in RNA Interference and Viruses
The term RNA silencing refers to several pathways present in eukaryotic organisms that lead to the sequence specific elimination or functional blocking of RNAs with homology to double stranded RNAs (dsRNAs) that have previously triggered the mechanism. Besides playing important roles in developmental control, RNA silencing forms part of the defence against viruses in plants, acting as a potent antiviral mechanism. To escape from the RNA silencing-based defence, most plant viruses make use of different strategies, the most common relying in the action of viral proteins with the capacity to suppress RNA silencing. The characterization of these viral suppressors is providing useful insights to understand how RNA silencing works, revealing components and steps in the silencing pathways.

Virus-encoded Suppressors of RNA Silencing and the Role of Cellular miRNAs in Mammalian Antiviral Immune Responses
from Joost Haasnoot and Ben Berkhout writing in RNA Interference and Viruses
Small RNA-directed silencing mechanisms play important roles in the regulation of eukaryotic gene expression. In plants, insects, nematodes and fungi RNA silencing mechanisms are also involved in innate antiviral defence responses. To counter antiviral RNA silencing, viruses from plants, insects and fungi encode RNA silencing suppressors (RSSs). Recent studies suggest that RNA silencing in mammals, or RNA interference (RNAi), is also involved in antiviral responses. In particular, there is increasing evidence that cellular regulatory microRNAs (miRNAs) have a function in restricting virus replication in mammalian cells. Similar to plant and insect viruses, several mammalian viruses encode RSS factors that inhibit the RNAi mechanism. Several of these suppressors are multifunctional proteins that were previously shown to block innate antiviral immune responses involving the interferon (IFN) pathway.

Mechanism of Action of Viral Suppressors of RNA Silencing
from Jozsef Burgyan writing in Recent Advances in Plant Virology
RNA silencing is an evolutionarily conserved sequence-specific gene-inactivation system that also functions as an antiviral mechanism in higher plants and insects. To overcome this defence system, viruses encode suppressors of RNA silencing, which can counteract the host silencing-based antiviral process. More than 50 individual viral suppressors have been identified from almost all plant virus genera, underlining their crucial role in successful virus infection. Viral suppressors are considered to be of recent evolution, and they are surprisingly diverse within and across kingdoms, exhibiting no obvious sequence similarity. Virus-encoded silencing suppressors can target several key components in the silencing machinery, such as silencing-related RNA structures and essential effector proteins and complexes. There has been much recent progress in our understanding of the mechanism and function of viral suppressors of antiviral RNA silencing in plants.