A revised replication cycle for viroids: the role of longer than unit length RNA in viroid replication.

Longer than unit length plus and minus strand RNAs were detected in hop stunt viroid (HSV) infected cucumber leaf tissues by Northern blot hybridization analysis using strand-specific probes. To elucidate the role of these longer than unit length RNAs in the viroid replication cycle, we synthesized tandemly repeated plus and minus strand HSV RNAs in vitro from cloned HSV cDNA and assayed their infectivities. Two and four unit tandemly repeated plus strand RNAs were infectious, but one unit plus, and one, two and four unit minus strands were noninfectious. Taking these data into consideration, we propose a revised rolling circle model for viroid replication.