In the early 1970s, American scholar Diener and his colleagues observed that the pathogen had no virus particles and antigenicity, was insensitive to organic solvents such as phenol, was heat-resistant (70℃ ~75℃), was stable to high-speed centrifugation (indicating its low molecular weight), and was sensitive to RNase. All these characteristics show that the pathogen is not a virus, but a free small molecular RNA. Thus, a new concept-Viroid is proposed. Before this concept was put forward, people always thought that the system consisting of two biopolymers, protein and nucleic acid, was a primitive life system, and never doubted that virus was the lowest limit of a complex life system.

Viroid is a kind of single-stranded closed-loop RNA molecule that can infect some plants and cause diseases. Viroid genome is small, and its molecular weight is 1 × 10 to the fifth power. There are more than 100 virus-like variants that have been sequenced, and their RNA molecules are rod-shaped, which are composed of some base-paired double-stranded regions and unpaired single-stranded circular regions alternately. A common feature of them is that there is a conserved region in the center of secondary structure molecules. Viroid is usually 246~399 nucleotides. For example, potato spindle tuber viroid (pstvd, Vd is used to distinguish it from viruses) is a * * * valence closed circular RNA molecule composed of 359 nucleotide units, and its length is about 50A~70nm.

All virus-like RNA has no mRNA activity and does not encode any polypeptide. Its replication is the direct transcription from RNA to RNA in the nucleus with the help of RNA polymerase II of the host.

Viroid can cause infection independently, and there are virus-like strains with different virulence in nature. The weak strain of PSTVd can only reduce the yield by about 10%, while the strong strain can reduce the yield by 70%~80%.

All viroids can be transmitted through mechanical damage, and mechanical transmission through contact with farming tools is the main way to spread this disease in nature. Some viroids, such as PSTVd, can also spread directly through seeds and pollen. There is no obvious difference in symptoms between viroid and viroid, and most typical symptoms of viroid can also be caused by viroid. Viroid infection has a long incubation period and persistent infection. ?

Different viroids have different host ranges. For example, there are hundreds of host plants that are sensitive to PSTVd. Besides Solanaceae, there are also Lithocarpaceae, Campanulaceae, Dianthus, Compositae and so on. The host range of citrus exocortis viroid (CEVD) is narrower than that of PSTVd, but it can also infect 50 kinds of plants such as Citraceae, Compositae, Solanaceae and Cucurbitaceae.

It is the smallest infectious pathogenic factor known at present, which is simpler than ordinary viruses. Viroid is a free * * * valence closed circular single-stranded RNA molecule without the protection of protein shell, which invades the host cell and replicates itself, causing the host to become ill or die. The molecular weight of viroid is 0.5 ~1.2×10.5, which is 1/4 of the smallest known RNA satellite ring-dead virus. The potato spindle tuber disease virus (PSTV) first reported in 197 1 year contains only 359 nucleotides, the smallest grass dwarf virus (HSV) contains only 290 ~ 300 nucleotides, and the larger citrus peel disease virus (CEV) contains only 37 1 nucleotide. The mechanism of virus-like infection and replication is still unclear.

It was first discovered by T. O. Diener et al. (1969) on the diseased plant of potato fiber tuber disease. Under the microscope, it can be seen that this RNA molecule is a rod-shaped molecule with a length of 50nm, and it has 359 base pairs, and it is confirmed to be free RNA, so it is officially named as viroid. It is usually transmitted by juice in the nucleus of the host, and its molecular weight is 75000 ~130000, which is one eightieth of the smallest virus. Subsequently, low molecular weight pathogenic RNA was isolated from diseased plants such as chrysanthemum dwarf disease, chrysanthenum chlorotic mottle and citrus excortis. It is speculated that it may also exist in other plants, animals and even human bodies. Most viroids have the same structural characteristics: (1) has a highly conserved sequence in the center of the rod structure; (2) There is a poly purine region near the left side of this conservative central region; (3) The left sequence of rod-like structure is highly conservative and the right sequence is highly variable. It may be caused by direct interaction with host cells and interference with cell metabolism through changes in nucleotide sequence or structure. S. B. Prusiner, an American scholar, won the Nobel Prize in Physiology and Medicine in 1997 for discovering prion, the pathogenic factor of scrapie in sheep. Prion, also known as prion (protein infectious agents), is an infectious protein molecule which is smaller than virus and contains only hydrophobic.

The purified infection factor is called Prion protein (PrP). Pathogenic prion is represented by PrP SC, which has the ability to resist proteolysis by protease K, and can specifically appear in infected brain tissue, and exists in amyloid form.

Many fatal degenerative diseases of the central nervous system in mammals are related to prions, such as human kuru disease, Creutzfeldt-Jakob Disease, CJD, Fatal Familiar Insomnia, FFI) and Scrapie in animals, spongiform encephalopathy in cattle (BSE or mad cow disease), spongiform encephalopathy in cats (FSE), etc.

There is a gene encoding PrP in normal human and animal cell DNA, and its expression product is expressed by PrP c, with a relative molecular weight of 33~35kDa. The PrP c expressed in normal cells and PrP sc of sheep pruritus are isomers. PrP C and PrP SC have the same amino acid sequence. PrP C has 43% α helix and 34% β sheet, while PrP SC has about 34% α helix and 43% β sheet. Multiple folds reduce the solubility of PrP SC and increase the resistance to protease.

Since PrP SC is a kind of protein and does not contain any nucleic acid, how does it replicate and spread in humans or animals? Prusiner et al. put forward the hypothesis of heterodimerization mechanism, that is, PrP SC single molecule is an infectious agent, and the conformation of PrP SC single molecule is slowly changed from PrP C single molecule to form PRP SC single molecule, and then it passes through PrP C-PrP SC heterodimer in the middle, and then it is transformed into PrP SC -PrP C. In this process, an unknown protein X may play a role in regulating the transformation of PrP C or maintaining the form of PrP SC. This dimer dissociates and releases new PrP SC, so it keeps "copying". Since the 1980s, four new plant viruses have been found in Australia, including villous tobacco, alfalfa, scopolamine and underground clover. The protein capsids of these viruses all contain two kinds of RNA molecules, one is linear RNA 1.5 × 10 6 Da, and the other is virus-like circular RNA 2 with molecular weight of 10 5 Da, which is called Virusoid. Pseudovirus has two molecular structures, one is circular RNA 2, and the other is linear RNA 3. RNA 2 and RNA 3 are two different configurations presented by the same RNA molecule, in which RNA 3 may be the precursor of RNA 2, that is, RNA 2 is formed by cyclization of RNA 3.