ABSTRACT
This chapter shows that viroids provide a minimal genetic and biological system ideally suited to detailed analysis of the interrelationship between RNA structure and function. Viroids are small covalently circular RNAs that can be isolated from certain higher plant species afflicted with specific diseases. They replicate autonomously after introduction into healthy individuals of the same species and cause the appearance of the characteristic disease syndrome. A potential connection between symptom severity and viroid secondary structure has been noted by Schnolzer et al., who recognized that increasing symptom severity can be correlated with the decreasing thermodynamic stability of a portion of the potato spindle tuber viroid pathogenicity domain. Cleavage/ligation of greater-than-unit-length RNA transcripts was the first step in viroid replication to be studied by site-specific mutagenesis techniques. The location of the mismatch is determined by gel electrophoretic analysis of the resulting RNA cleavage products.
