Evidence for a single infectious species of potato spindle tuber viroid

Potato spindle tuber viroid (PSTV) has been separated into two electrophoretic species on polyacrylamide gels containing 8 M urea at 60 degrees. Only the slower-migrating electrophoretic form was infectious and is presumably the circular form of the molecule. Both forms had similar base compositions with a 55-58% GC content, similar to the citrus exocortis viroid. Labeling studies in vivo suggest that the viroid is not methylated.