Potato spindle tuber "virus"

Earlier results had indicated that the spindle tuber disease of potato is incited by free RNA, and that neither conventional virions nor proteins that could be construed as viral coat proteins are synthesized in infected plants. By a combination of density-gradient centrifugation and polyacrylamide gel electrophoresis, using internal marker RNAs, it is now shown that the infectious RNA occurs in the form of several species with molecular weights ranging from 2.5 × 104 to 1.1 × 105 daltons. No evidence for the presence in uninoculated plants of a latent helper virus was found. Thus, potato spindle tuber “virus” RNA, which is too small to contain the genetic information necessary for self-replication, must rely for its replication mainly on biosynthetic systems already operative in the uninoculated plant. Several possible mechanisms are discussed. The term “viroid” is proposed to designate potato spindle tuber “virus” RNA and other RNAs with similar properties.