Viroids are single-stranded covalently closed circular RNA molecules existing as highly base-paired rod-like structures.

Abstract

Viroids are uncoated infectious RNA molecules pathogenic to certain higher plants. Four different highly purified viroids were studied. By ultracentrifugation, thermal denaturation, electron microscopy, and end group analysis the following features were established: (i) the molecular weight of cucumber pale fruit viroid from tomato is 110,000, of citrus exocortis viroid from Gynura 119,000, of citrus exocortis viroid from tomato 119,000, and of potato spindle tuber viroid from tomato 127,000. (ii) Viroids are single-stranded molecules. (iii) Viroids exhibit high thermal stability, cooperativity, and selfcomplementarity resulting in a rod-like native structure. (iv) Viroids are covalently closed circular RNA molecules. Viroids exert biological functions like viruses since they are infectious and pathogenic. In contrast to viruses, however, all known viroids seem to exist only as uncoated RNA molecules and their genetic information is not sufficient to code for a protein with a molecular weight larger than 10,000. They are pathogens of certain higher plants. Little is understood, as yet, about their mechanisms of replication and pathogenicity. Moreover, the available information is too limited to lead to a concise structural model of viroids (1). Previous data on molecular weights of viroids were obtained by indirect methods and the determinations were based on particular assumptions about their unknown conformation. Analysis in sucrose gradients, in polyacrylamide gels, and in the electron microscope resulted in molecular weight values ranging between 25,000 and 150,000 for both the viroid of the spindle tuber disease of potato (PSTV) (e.g. refs. 2-4) and the viroid of the exocortis disease of citrus (CEV) (e.g., refs. 5-7). The sedimentation coefficients reported range from 4 S to 25 S (2, 6, 7). A considerable number of structural models have been proposed (e.g. refs. 4, 7, and 8); single strands with hairpins or double strands with incomplete base pairing have been considered as the most probable structure of viroids.The availability of highly purified viroid RNA in sufficient quantities enabled us to investigate the structure in detail.

Common structural features of different viroids: serial arrangement of double helical sections and internal loops

“…The origin, propagation and purification of CEV, PSTV, ChSTV, and CPFV [6J was as previously described [2,5,8]. The original culture of ChCMV 061 was kindly provided by Dr. M. Hollings, Littlehampton, and this viroid was propagated in the Chrysanthemum morifolium cultivar "Yellow Delaware".…”

“…Branching would result in much lower cooperativity than observed in viroids which follows from a comparison of experimental [10] and theoretical [14] results of tRNA. Furthermore a highly extended structure of viroids was deduced from electron microscopy and hydrodynamic studies on viroids [2]. Structures of the following type are discussed:…”

“…Little is known about their mechanism of replication and pathogenicity; their genetic information is not sufficient to code for a protein with a molecular weight larger than 10,000 if they were translated. The availability of sufficient amounts of purified viroid material of electrophoretic homogeneity for optical absorption studies has led to several biochemical and physico-chemical investigations of their molecular Nucleic Acids Research structure [2][3][4][5]. It has been shown that viroids are singlestranded, covalently linked, circular RNA molecules.…”

“…It has been shown that viroids are singlestranded, covalently linked, circular RNA molecules. They are the first such RNA natural occurring structures ever discovered in nature [2]. Under native conditions viroids exist in a highly base-paired rod-like conformation.…”

Sequence‐specific priming of the in vitro synthesis of DNA complementary to citrus exocortis viroid

“…Viroid RNA was purified to homogeneity by PAGE in the presence of 8 M urea [8]. Synthesis of the oligodeoxyribonucleotide primer p3 d(TTCTTT-TTTCTTTTC), p3-primed reverse transcription, and DNA sequencing have been described previously [7].…”