Abstract
Recombinant plasmids carrying more than two units of the hop stunt viroid (HSV) cDNA sequence linked in tandem were highly infectious. On the other hand, those carrying one unit were not. To determine the length of the HSV cDNA sequence necessary for the infectivity, we constructed series of plasmids carrying more than one but less than two units of the HSV sequence and investigated their infectivities. The results showed that cDNA clones were infectious if about 60 nucleotides of the internal part of the HSV sequence (region A) were undeleted and remained with the same arrangement as in the infectious two-unit clone. The region A can be folded into a stable secondary structure. Viroid replication is thought to involve a site-specific endonucleolytic cleavage of plus strand RNA multimers produced by a rolling circle mechanism to yield unit length molecules. Our results suggest that the region A contains the sequence or structure recognized by the putative endonuclease.
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