Characterization of viroid-like RNAs associated with the citrus exocortis syndrome.

Three additional viroid-like RNA species (RNA-I, -II, and -III), smaller than the 371-nucleotide citrus exocortis viroid (CEV), have been identified in citron (Citrus medica) trees which display symptoms of the exocortis disease. The three RNAs migrate on polyacrylamide gels under denaturing conditions in the region between CEV and avocado sunblotch viroid circular molecules, indicating a size range of about 311-335 nucleotide residues in both circular and linear molecular forms. RNA-II is removed from the preparations when chromatographed on CF-11 cellulose. All three RNAs fail to replicate when introduced into Gynura aurantiaca, a herbaceous host of CEV, yet RNA-I and RNA-III can be independently transmitted to citron. Dot-blot hybridization with CEV-cDNA indicates no significant homology between CEV and RNA-I, RNA-II, or RNA-III. The relationship among CEV-RNA and the three viroid-like RNA species in the segregation of expression and intensity of the exocortis disease reaction is discussed.