Is the conformation of viroids involved in their pathogenicity?

Viroids and the nature of viroid diseases

In its methodology, the unexpected discovery of the viroid in 1971 resembles that of the virus by Beijerinck some 70 years earlier. In either case, a novel type of plant pathogen was recognized by its ability to penetrate through a medium with pores small enough to exclude even the smallest previously known pathogen: bacteria as compared with the tobacco mosaic agent; viruses as compared with the potato spindle tuber agent. Interestingly, one of the two methods used by Beijerinck, diffusion of the tobacco mosaic agent into agar gels, is conceptually similar to one method used to establish the size of the potato spindle tuber agent, namely polyacrylamide gel electrophoresis. Further work demonstrated that neither agent is an unusually small conventional pathogen (a microbe in the case of the tobacco mosaic agent; a virus in the case of the potato spindle tuber agent), but that either agent represents the prototype of a fundamentally distinct class of pathogen, the viruses and the viroids, respectively. With the viroids, this distinction became evident once their unique molecular structure, lack of mRNA activity, and autonomous replication had become elucidated. Functionally, viroids rely to a far greater extent than viruses on their host's biosynthetic systems: Whereas translation of viral genetic information is essential for virus replication, viroids are totally dependent on their hosts' transcriptional system and, in contrast to viruses, no viroid-coded proteins are involved. Because of the viroids' simplicity and extremely small size they approach more closely even than viruses Beijerinck's concept of a contagium vivum fluidum.

Analysis of the virulence modulating region of potato spindle tuber viroid (PSTVd) by site-directed mutagenesis

A series of nucleotide substitutions (G46----C; C47----A; C315----U; U317----C) were introduced into the virulence modulating region of the intermediate strain of potato spindle tuber viroid (PSTVd) in order to examine their effect upon viroid infectivity and pathogenicity with the presence of all four mutations resulting in the sequence of a previously reported severe strain of PSTVd. Eight of the resulting mutant cDNAs were characterized for infectivity and symptom induction in tomato, and the secondary structure of their corresponding RNAs was examined. The combined results of infectivity, computer analysis, and chemical mapping data imply that a previously proposed correlation between thermodynamic stability and PSTVd pathogenicity does not hold true in all cases and suggest that conformation and/or sequence-specific interactions with host factors play a role in symptom development.