Mutational analysis of potato spindle tuber viroid reveals complex relationships between structure and infectivity.

Viroids are single-stranded, covalently closed circular RNA pathogens that can be isolated from certain higher plants afflicted with specific diseases. Their small size (246-375 nucleotides; M(r) 0.8-1.3 x 10(5)) and ability to replicate autonomously make viroids a unique model system in which to study the relationships between the structure of an RNA and its biological function. The demonstrated infectivity of certain cloned viroid cDNAs allows the use of site-specific mutagenesis techniques to probe structure-function relationships suggested by comparative sequence analysis. Several site-specific mutations that disrupt base pairing in either the native structure or secondary hairpin I destroyed the ability of potato spindle tuber viroid cDNA to initiate infection. Alterations in the terminal loops of the native structure also abolished cDNA infectivity. One pseudorevertant, a mutant cDNA containing compensating changes that restore base pairing in the native structure, was marginally infectious; a second pseudorevertant in which base pairing was restored within the stem of secondary hairpin I was not infectious. The behavior of these mutants dramatically demonstrates the effect of remarkably small structural changes on viroid infectivity and emphasizes the importance of the conserved rod-like native structure for viroid function.