Viroid replication is inhibited by α-amanitin

VIROIDS are infectious single-stranded circular RNA molecules1 which cause disease in higher Plants. Although the complete molecular structure of one viroid has recently been established2, the mechanism of viroid replication and patho-genesis is still unknown. The viroid molecule carries insufficient genetic information to code for a complete viroid-specified replicase. Replication must therefore depend on host cell enzymes. We report here the inhibition by α-amanitin, of 3H-uridine incorporation into viroid RNA in infected tomato protoplasts. The inhibitory intracellular concentrations of α-amanitin which we determined suggest the involvement of the nuclear DNA-dependent RNA polymerase II in viroid replication.