RNA progeny of an infectious two-base deletion cDNA mutant of potato spindle tuber viroid (PSTV) acquire two nucleotides in Planta.

Deletion mutations were generated in four structural domains of a potato spindle tuber viroid (PSTV) complementary DNA (cDNA) clone. Deletions of 3 to 5 nucleotides at the central conserved domain (CCGGG, positions 94 to 98), variable domain (GCCG, positions 146 to 149) and pathogenicity domain (CGA, positions 286 to 288) abolished infectivity of dimeric or trimeric cDNA constructs, or their in vitro transcripts. By contrast, a clone (St4) with a deletion of two nucleotides (UU, positions 339 and 340), located at the left terminal domain, retained infectivity when DNA or in vitro transcribed (+)RNA was used as inoculum. Sequencing of four cDNA clones of such viroid progeny demonstrated that two nucleotides were added at the deletion site. Two of the viroid progeny contained a CG addition. A third clone possessed a GU addition, whereas the fourth clone had a UU addition which represents a true reversion to full-length wild-type PSTV RNA. Ribonuclease protection assay of viroid progeny from St4-infected tomato plants suggested that only a negligible proportion of the St4 progeny were true revertants.