Synthesis of infectious viroids and other circular RNAs.

Viroids are small autonomously replicating RNAs that share structural features with other subviral circular single-stranded RNAs of plants. Viroids and other circular single-stranded RNAs can be synthesised in vitro by a PCR-based procedure using a simple set of reactions. Two end-to-end primers are selected from a desired region of the viroid, one for the synthesis of the first strand cDNA and another for the production of the second strand DNA. The second primer contains an 18 nucleotide T7 promoter at its 5' end, and is selected such that the G nucleotide at the transcription start site represents a G in the viroid. Linked reverse transcription-PCR results in linear double-stranded DNA consisting of the viroid sequence and the T7 promoter. Run-off transcription of the PCR product allows the synthesis of exact-length linear viroid RNA which can be circularised by T4 RNA ligase following an enzymic modification of the 5' triphosphate to a monophosphate. This procedure results in authentic viroid molecules and obviates the need for construction and cloning of DNA in the form of tandem repeats for infectivity tests. It also allows PCR-based manipulation of circular RNAs, thus greatly simplifying structure-function analyses of viroid molecules.