A pathogenic non coding RNA that replicates and accumulates in chloroplasts traffics to this organelle through a nuclear-dependent step

Abstract

Viroids belonging to the family Avsunviroidae are the only functional RNAs known to traffic selectively into chloroplasts. Subcellular targeting is a critical step in guaranteeing their access to the machineries involved in their replication. However, the host mechanisms exploited by these non coding pathogenic RNAs to be selectively imported into chloroplasts are poorly understood. Recently, we provide evidence supporting the idea that the Avsunviroidae have evolved to subvert a signaling mechanism between the nucleus and chloroplasts to regulate their differential compartmentalization into the chloroplast of infected cells. Here, we discuss our model and previous observations that provide biological relevance to our hypothesis.

Figure 1. Proposed model to explain the specific compartmentalization of members of the family Avsunviroidae into chloroplasts. In this transport pathway (likely mediated by a RNA domain localized in the left terminal region), after the viroid invades the cell cytoplasm, it is imported into the nucleus, by means of an unknown host-dependent mechanism. The viroid then uses this organelle as a sub-cellular shuttle for delivery into the chloroplast, where replication takes place. The possibility that the Avsunviroidae can directly traffic from the cytoplasm to the chloroplast cannot be excluded.

Figure 2. A common RNA sequence and/or structure mediates both nuclear and chloroplastic localization. Using partially deleted constructs we demonstrate that the nuclear and chloroplastic signals are restricted to an overlapping RNA region limited to a small area in the left part of the predicted ELVd circular form. Numbering refers to ELVd sequence variant with GenBank accession number AJ536613.