Suppression of potato spindle tuber viroid replication and symptom expression by mutations which stabilize the pathogenicity domain.

Nucleotides within the pathogenicity domain of potato spindle tuber viroid (PSTVd) are known to play an important role in regulating symptom expression, but the underlying molecular mechanism is unknown. In order to determine more precisely how structural features within the pathogenicity domain regulate symptom expression, we have characterized a series of mutations that progressively stabilize premelting region 1 and the rest of the "virulence modulating" region. The structural effects of these mutations were monitored by temperature gradient gel electrophoresis of circularized RNA transcripts, and their biological effects were assessed by quantitative bioassays in tomato. Closure of a 4-nucleotide loop within the premelting region 1 virtually abolished PSTVd infectivity, especially when a nearby 2-nucleotide loop was also closed. Although RNA transcripts containing less stabilizing mutations were readily infectious, none of the four single and one double substitutions examined were stably maintained in vivo. The pattern of spontaneous, apparently compensatory sequence changes observed in the progeny suggests that PSTVd variants with less stable secondary structures enjoy a selective advantage. Mutations which stabilize the pathogenicity domain of PSTVd in vitro also suppressed symptom expression, but at least one other mutation having no obvious structural effects was associated with a similar phenotype. Conformational stability appears to be only one of several factors regulating PSTVd replication and pathogenicity.