A single nucleotide substitution converts potato spindle tuber viroid (PSTVd) from a noninfectious to an infectious RNA for Nicotiana tabacum

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 bath tobacco and tomato. (C) 1996 Academic Press, Inc.