Viroids: an Ariadne's thread into the RNA labyrinth.

Instituto de Biología Molecular y Celular de Plantas (CSIC-UPV), Universidad Politécnica de Valencia, Avenida de los Naranjos s/n, 46022 Valencia, Spain.
EMBO Reports (Impact Factor: 7.19). 07/2006; 7(6):593-8. DOI: 10.1038/sj.embor.7400706
Source: PubMed

ABSTRACT Viroids are structurally, functionally and evolutionarily different from viruses. Despite their small, non-protein-encoding, single-stranded circular RNA genome, viroids can infect higher plants and cause certain diseases. Members of the two viroid families, Pospiviroidae and Avsunviroidae, have evolved to usurp the transcriptional machinery of their host nuclei and chloroplasts, respectively, in which replication proceeds through a rolling-circle mechanism involving RNA polymerization, cleavage and ligation. Remarkably, viroids subvert certain DNA-dependent RNA polymerases to transcribe RNA templates, and, in the family Avsunviroidae, post-transcriptional cleavage is catalysed by hammerhead ribozymes. Viroids are models for studying RNA evolution and for analysing RNA transport in plants, because they can move intracellularly, intercellularly through plasmodesmata and to distal parts of the plant through the vascular system. Viroids elicit RNA-silencing phenomena, which might mediate some of their biological properties, including pathogenesis. As some viroids behave as catalytic RNAs, they are regarded as remnants of the RNA world.

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    ABSTRACT: Intracellular RNA-trafficking and specific subcellular compartmentalization are key regulatory processes controlling gene expression. Viroids are plant-pathogenic non-coding RNAs that specifically replicate in the nucleus (family Pospiviroidae) or chloroplasts (Avsunviroidae) of infected cells. Subcellular compartmentalization is a critical step in guaranteeing their access to the machineries involved in their RNA transcription and processing. However, how the members of the Avsunviroidae are imported into chloroplasts is a poorly understood process. Elucidating the mechanism used by viroids to enter chloroplasts will enhance our understanding of viroid infection and reveal significant aspects of intracellular RNA trafficking. Here, we report the use of a combined approach involving the cytoplasmic and nuclear expression of monomeric viroid-RNAs (vd-RNA) to study in vivo aspects of the intracellular movement of the Avsunviroidae. We observed that, in leaves infected with a Potato virus X-(PVX)/Green fluorescence protein-(GFP) construct interrupted by an intron carrying a full-length Eggplant latent viroid-(ELVd) sequence, the vd-RNA mediates in cis the nuclear import of the chimeric GFP-mRNA expressed in the cytoplasm, indicating that the vd-RNA has a nuclear import signal. Subsequently, it was demonstrated that a linear monomeric form of the vd-RNA mediates the selective and functional trafficking of GFP-mRNA from the nucleus to chloroplasts. These findings support the idea that during the initial phase of their life-cycle the Avsunviroidae are first transported from the cytoplasm into the nucleus and are then specifically delivered from the nucleus to the chloroplasts, where they can replicate, revealing a novel and unexpected route that could explains their selective subcellular compartmentalization.
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    Revista de Protección Vegetal. 01/2008; 23:01-10.

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