Viroid-specific polymerase activity was detected in preparations rich in nuclei from infected with citrus exocortis viroid (CEV). The polymerase catalysed the synthesis of several RNAs, shown to be viroid-specific since they could not be observed in control experiments with healthy plants, and they contained CEV-specific sequences most of which were of the same polarity as the viroid RNA. The synthesis of the CEV-specific RNA species was greatly reduced in the presence of 1 µ-α-amanitin, suggesting the involvement of RNA polymerase II in this process. The structure of the viroid-specific RNA species was studied by chromatography on non-ionic cellulose, digestion with RNase under low and high ionic strength conditions, and analysis by polyacrylamide gel electrophoresis in non-denaturing and denaturing systems. The results showed that these RNAs synthesized contain unit and longer than unit length linear viroid strands forming multistranded complexes with single- and double-stranded regions. The RNAs therefore have the same structural properties as deduced for RNAs isolated from viroid-infected tissues which are the presumed replicative intermediates of the rolling circle mechanism proposed for viroid synthesis. A soluble fraction containing the polymerase-template complex responsible for the synthesis of the CEV-specific RNAs was isolated by treatment of the nuclei-rich preparation with heparin and DNase. This soluble fraction could be of interest in further studies to characterize the components of the polymerase-template complex involved in CEV replication.


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