RNA-DIRECTED DE-NOVO METHYLATION OF GENOMIC SEQUENCES IN PLANTS

One monomeric and three oligomeric potato spindle tuber viroid (PSTVd) cDNA units were introduced into the tobacco genome via the Agrobacter ium-mediated leaf-disc transformation. Southern analysis of the integr ates revealed that only their PSTVd-specific sequences become fully me thylated, whereas the flanking T-DNA and the genomic plant DNA remain unaltered. Viroid cDNA methylation could only be observed after autono mous viroid RNA-RNA replication had taken place in these plants. These findings demonstrate that a mechanism of de novo methylation of genes might exist that can be induced and targeted in a sequence-specific m anner by their own mRNA.