A Single Nucleotide Substitution Converts Potato Spindle Tuber Viroid (PSTVd) from a Noninfectious to an Infectious RNA for Nicotiana tabacum

Abstract Mechanical inoculation of Nicotiana tabacum with the PSTVd isolate KF 440-2 from the host plant tomato resulted in the de novo emergence, replication, and accumulation of a new “tobacco variant,” designated PSTVd NT. It produces no symptoms in tobacco but, like PSTV KF 440-2, severe ones in tomato. The sequence analysis of PSTVd NT revealed a single nucleotide substitution from C → U at position 259. Autonomous viroid replication was also induced in tobacco by genomic integration of oligomeric cDNA copies of PSTVd KF 440-2. Although these cDNAs contained the original tomato-specific C 259 , the circular PSTVd RNA subsequently accumulating in tobacco also exhibited the C 259 → U 259 substitution. In the secondary structure of PSTVd, nucleotide 259 is part of an internal loop analogous to loop E of eukaryotic 5S rRNA and presumed to be the only bulged extrahelical nucleotide of this loop. The C 259 in PSTVd KF 440-2 and in practically all other isolates and the U 259 in PSTVd NT of the loop E-like structure might be involved in protein binding and in viroid processing. The new variant PSTVd NT is genetically stable in both tobacco and tomato.