RNA Structural Features Responsible for Potato Spindle Tuber Viroid Pathogenicity

Abstract

The native structure of potato spindle tuber viroid (PSTVd) contains a series of short double helices and small internal loops that are organized into five structural domains. Nucleotides within the pathogenicity domain are known to play a critical role in modulating PSTVd symptom expression, and it has been suggested that disruption of a comparatively unstable "premelting region" within the pathogenicity domain may be required for disease induction. We have used a combination of quantitative bioassays, temperature gradient gel electrophoresis of circularized RNA transcripts, and thermodynamic calculations to compare the biological and structural properties of 12 representative PSTVd sequence variants. Certain mutations appeared to act indirectly, downregulating pathogenicity by suppressing the rate of PSTVd replication/accumulation. The effects of other mutations appeared to be more direct, but there was no consistent correlation between symptom severity and melting temperature. Taking into account the three-dimensional shape of RNA helices, comparison of the optimal secondary structures for these variants point to major differences in the geometry of their pathogenicity domains; i.e., variants producing intermediate symptoms possess a linear arrangement of three consecutive helices, whereas for variants producing mild or severe symptoms this domain is bent in opposing directions. Such alterations in RNA structure together with concomitant alterations in RNA-protein interaction(s) may be the primary cause of viroid pathogenicity.

On the role of RNA silencing in the pathogenicity and evolution of viroids and viral satellites

“…Previous studies have shown that the pathogenicity of viroids and viral satellites is generally determined by the nucleotide sequences within particular small (Ϸ20-nt) regions of their RNA genomes (3,7,32), such as the previously defined virulence modulating region of PSTVd (3). Interestingly, a BLAST search (33) with the full-length sequence of PSTVd-RG1 revealed numerous sequences from several plant species that have 19-to 20-nt identities with the PSTVd sequence.…”

Estudios de Patogenicidad de Viroides del Género Apscaviroide y Hostuviroide en cítricos.

“…Se ha sugerido que la inducción de síntomas se debe a modificaciones en la capacidad de unión del viroide con proteínas del huésped ya que estos cambios nucleotídicos provocan alteraciones en la geometría de la molécula (Owens et al 1996, Schmitz y Riesner, 1998. También se ha propuesto la hipótesis de que la molécula viroidal vea alterada su estabilidad termodinámica provocando nuevas conformaciones estructurales que sean las responsables de la patogénesis (Schnölzer et al 1985).…”

“…También se ha propuesto la hipótesis de que la molécula viroidal vea alterada su estabilidad termodinámica provocando nuevas conformaciones estructurales que sean las responsables de la patogénesis (Schnölzer et al 1985). Sin embargo existen datos que evidencian que la conformación de varilla de las variantes más agresivas es termodinámicamente más estable que la de las variantes más suaves (Owens et al 1996).…”

“…Most studies on viroid pathogenesis have been undertaken with PSTVd, the type species of the family Pospiviroidae, or with Peach latent mosaic viroid (Malfitano et al, 2003) and Chrysanthemum chlorotic viroid (De la as representative members of the family Avsunviroidae. In PSTVd, sequence variations related to symptom severity have been essentially mapped to the P domain (Schnölzer et al, 1985;Owens et al, 1991Owens et al, , 1995Owens et al, , 1996, but more recent reports have shown that the left and right terminal domains, as well as a single substitution in the loop E within the C domain, also contribute to symptom expression . The results of the present study confirm that in HSVd, the "cachexia expression motif" located in the V domain, plays a major role in inciting cachexia symptoms, and those subtle changes within this motif affect symptom severity and may even suppress symptom expression.…”

Resistencia a viroides inducida por ribozimas de cabeza de martillo y RNAs interferentes

“…La translocación de los viroides por el floema está mediada muy probablemente por proteínas del huésped, con las que formarían complejos capaces de interaccionar con los plasmodesmos y aumentar su límite de exclusión (Gómez y Pallás, 2001Owens et al, 2001;Ding e Itaya, 2007). …”