Hybridization of potato spindle tuber viroid to cellular DNA of normal plants

Are viroids escaped introns?

A complex of considerable stability is possible between the 5' end of U1 RNA and a specific nucleotide sequence of the potato spindle tuber viroid complement. Because base-pairing between the 5' end of U1 RNA and the ends of introns is believed by some to be responsible for the precise alignment and correct excision of introns, the U1-related sequence may represent the two ends of a presumed intron ancestor of the viroid complement after circularization.

RNA intermediates in potato spindle tuber viroid replication

Two double-stranded RNA intermediates of viroid replication have been isolated from potato spindle tuber viroid (PSTV)-infected tomato tissue and characterized by polyacrylamide gel electrophoresis and DNA-RNA hybridization techniques. These replicative intermediates contain monomeric circular or linear PSTV strands complexed with a multimeric complementary RNA strand. Synchronous synthesis of single-stranded PSTV is accompained by a simultaneous marked increase in double-stranded PSTV RNA; thus, in vivo precursors of the characterized double-stranded PSTV RNAs seem to be involved in PSTV replication. A "rolling circle" model for viroid replication on a circular PSTV template can accommodate the double-stranded PSTV RNA species characterized.

RNA sequences complementary to citrus exocortis viroid in nucleic acid preparations from infected Gynura aurantiaca

Molecular hybridization with (125)I-labeled citrus exocortis viroid RNA has been used to survey nucleic acid preparations from Gynura aurantiaca for viroid complementary molecules. A differential hybridization effect was detected between nucleic acid extracts from healthy and infected tissue in which significant RNase-resistant (125)I-labeled citrus exocortis viroid resulted in hybridization studies with the infected tissue extracts. Subsequent characterization indicated that RNA from infected tissue was involved in the formation of a duplex molecule with citrus exocortis viroid RNA and had properties of an RNA.RNA hybrid. Subcellular fractionation of infected tissue indicates that the complementary RNA is present in nuclear and soluble RNA fractions. This RNA may represent an intermediate molecule in the replication of the viroid or a pathogenic expression and may have a regulatory role in the host cell.

Potato spindle tuber and citrus exocortis viroids undergo no major sequence changes during replication in two different hosts

Potato spindle tuber viroid and citrus exocortis viroid, each purified from tomato (Lycopersicon esculentum) and from Gynura aurantiaca, were iodinated in vitro with (125)I, digested with ribonuclease T1, and subjected to two-dimensional RNA fingerprinting analysis. With the exception of minor variations, each viroid retained its distinctive fingerprint pattern irrespective of the host species from which it was isolated. We conclude that the nucleotide sequences of these viroids are principally determined by the infecting viroid and not by the host.

Potato spindle tuber viroid: the simplicity paradox resolved?

TAXONOMY

Potato spindle tuber viroid (PSTVd) is the type species of the genus Posipiviroid, family Pospiviroidae. An absence of hammerhead ribozymes and the presence of a 'central conserved region' distinguish PSTVd and related viroids from members of a second viroid family, the Avsunviroidae.

PHYSICAL PROPERTIES

Viroids are small, unencapsidated, circular, single-stranded RNA molecules which replicate autonomously when inoculated into host plants. Because viroids are non-protein-coding RNAs, designation of the more abundant, highly infectious polarity strand as the positive strand is arbitrary. PSTVd assumes a rod-like, highly structured conformation that is resistant to nuclease degradation in vitro. Naturally occurring sequence variants of PSTVd range in size from 356 to 361 nt. HOSTS AND SYMPTOMS: The natural host range of PSTVd-cultivated potato, certain other Solanum spp., and avocado-appears to be quite limited. Foliar symptoms in potato are often obscure, and the severity of tuber symptoms (elongation with the appearance of prominent bud scales/eyebrows and growth cracks) depends on both temperature and length of infection. PSTVd has a broad experimental host range, especially among solanaceous species, and strains are classified as mild, intermediate or severe based upon the symptoms observed in sensitive tomato cultivars. These symptoms include shortening of internodes, petioles and mid-ribs, severe epinasty and wrinkling of the leaves, and necrosis of mid-ribs, petioles and stems.

Subviral pathogens of plants: the viroids

Research during the last 15 years has conclusively shown that viroids are not only fundamentally different from viruses at the molecular level, but that they are most likely not directly related to viruses in an evolutionary sense. Today, viroids are among the most thoroughly studied biological macromolecules. Their molecular structures have been elucidated to a large extent, but much needs to be learned regarding the correlation between molecular structure and biological function. The availability of the tools of recombinant DNA technology in viroid research promises rapid progress in these areas of inquiry.

Nuclear DNA from uninfected or potato spindle tuber viroid-infected tomato plants contains no detectable sequences complementary to cloned double-stranded viroid cDNA

High molecular weight tomato nuclear DNA was isolated from uninfected and potato spindle tuber viroid (PSTV)-infected tomato leaves. Restriction digests were fractionated on agarose gels, denatured and transferred to diazobenzyloxymethylpaper, and hybridized to 32P-labeled cloned double-stranded PSTV cDNA. No hybridization to DNA from either uninfected or infected tissue could be detected under conditions that permitted detection of cloned double-stranded PSTV cDNA at a concentration equivalent to one-fifth copy of PSTV-related DNA per haploid tomato genome. Hybridization of tomato DNA to 32P-labeled cloned soybean 18S and 28S ribosomal DNA sequences showed that the restricted nuclear DNA was suitable for hybridization to probes containing homologous sequences. Our results indicate that neither PSTV nor its complementary strand is transcribed from nuclear DNA but do not rule out the possibility of sequence homology between host DNA and a small portion of PSTV or its complement.

DNA-dependent RNA polymerase II of plant origin transcribes viroid RNA into full-length copies

DNA-dependent RNA polymerase II purified from healthy plant tissue is capable of synthesizing linear (-)-viroid RNA copies of full length from (+)-viroid RNA templates in vitro. Together with the specific alpha-amanitin sensitivity of viroid replication observed in vivo, these findings suggest that viroids replicate by an entirely novel mechanism in which infecting viroid RNA molecules are copied by the host enzyme which is normally responsible for the synthesis of nuclear precursors to messenger RNA.

Detection of complementary RNA intermediates of viroid replication by Northern blot hybridization

Molecular hybridization by the Northern blot technique in combination with 125I-labeled PSTV (+) RNA and 32P-labeled PSTV cDNA as probes has been applied to detect viroid-specific sequences in healthy and viroid(PSTV)-infected tomato plants. Conditions are described which allow differentiation of (+) and (-) viroid sequences on the basis of the different thermostabilities of the corresponding hybrid molecules. By this experimental approach, it is documented that no viroid-specific DNA sequences can be detected and that viroid replication proceeds via complementary RNA intermediates. Out of the seven (-) RNA species found, six are apparently larger than the circular viroid (+) RNA and one is about the same size as the linear (+) RNA molecule.

Viroids: structure and function

Viroids are nucleic acid species of relatively low molecular weight and unique structure that cause several important diseases of cultivated plants. Similar nucleic acid species may be responsible for certain diseases of animals and humans. Viroids are the smallest known agents of infectious disease. Unlike viral nucleic acids, viroids are not encapsidated. Despite their small size, viroids replicate autonomously in cells of susceptible plant species. Known viroids are single-stranded, covalently closed circular, as well as linear, RNA molecules with extensive regions of intramolecular complementarity; they exist in their native state as highly base-paired rods.