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.