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The ''Avsunviroidae'' are a family of viroids. There are five species in three genera ('' Avsunviroid'', '' Elaviroid'' and '' Pelamoviroid''). They consist of RNA genomes between 246 and 375
nucleotide Nucleotides are Organic compound, organic molecules composed of a nitrogenous base, a pentose sugar and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both o ...
s in length. They are single-stranded covalent circles and have intramolecular base pairing. All members lack a central conserved region.


Replication

Replication occurs in the chloroplasts of plant cells. Key features of replication include no helper virus required and no proteins are encoded for. Unlike the other family of viroids, '' Pospiviroidae'', ''Avsunviroidae'' are thought to replicate via a symmetrical rolling mechanism. It is thought the positive RNA strand acts as a template to form negative strands with the help of an enzyme thought to be
RNA polymerase In molecular biology, RNA polymerase (abbreviated RNAP or RNApol), or more specifically DNA-directed/dependent RNA polymerase (DdRP), is an enzyme that catalyzes the chemical reactions that synthesize RNA from a DNA template. Using the e ...
plus 3 II. The negative RNA strands are then cleaved by
ribozyme Ribozymes (ribonucleic acid enzymes) are RNA molecules that have the ability to Catalysis, catalyze specific biochemical reactions, including RNA splicing in gene expression, similar to the action of protein enzymes. The 1982 discovery of ribozy ...
activity and circularises. A second rolling circle mechanism forms a positive strand which is also cleaved by ribozyme activity and then ligated to become circular. The site of replication is unknown but it is thought to be in the chloroplast and in the presence of Mg2+ ions.


Structure

Predictions of structure have suggested that they exist either as rod-shaped molecules with regions of base pairing causing formation of some hairpin loops or have branched configurations. The family has four stretches of conserved nucleotides, , , , from 5' to 3', plus their Watson-Crick pairings on the other end of the loop. This is part of its
hammerhead ribozyme The hammerhead ribozyme is an RNA Sequence motif, motif that catalyzes reversible cleavage and Ligation (molecular biology), ligation reactions at a specific site within an RNA molecule. It is one of several catalytic RNAs (ribozymes) known to occ ...
. Otherwise there is little structural similarity in the family.p.120, "Apart from the core nucleotides conserved in their hammerhead structures, no extensive sequence similarities exist between them" They do not have the conserved CCH, TCR, or TCH motifs, which is one of the features defining their separation from the Pospiviroidae.p.118-120, "The other four viroids, ''Avocado sunblotch viroid'' (ASBVd) (47), ''Peach latent mosaic viroid'' (PLMVd) (45), ''Chrysanthemum chlorotic mottle viroid'' (CChMVd) (65), and ''Eggplant latent viroid'' (ELVd) (30), do not have the conserved CCR, TCR, and TCH motifs but, remarkably, both their polarity strands self-cleave through hammerhead ribozymes; they form the second family, Avsunviroidae (reviewed in 33), whose type species is ASBVd (formal inclusion of ELVd in this family is pending ICTV approval) (Figure 1b)."


Classification

The family has three genera, with a total of five species. *Family Avsunviroidae **Genus '' Avsunviroid'';p.120, "ASBVd, the only viroid with a high A + U content (62%) (47), forms a monospecific genus" ***Species: '' Avocado sunblotch viroid'' (ASBVd, acc. J02020, gen. len. 247nt)p.119, TABLE 1 **Genus '' Elaviroid'';p.120, "and ELVd, whose properties fall between those of the members of the other two genera, has been proposed to constitute its own genus (30). ***Species: '' Eggplant latent viroid'' (ELVd, acc. AJ536613, gen. len. 333nt) **Genus '' Pelamoviroid'';p.120, "but PLMVd and CChMVd are grouped in one genus because of their branched secondary structure (21, 45, 65), which is stabilized by a pseudoknot (10; S. Gago, M. De la Peňa & R. Flores, unpublished results) (Figure 1b), and their insolubility in2M LiCl (65)." ***Species: '' Appler hammerhead viroid'', '' Chrysanthemum chlorotic mottle viroid'' (CChMVd, acc. Y14700, gen. len. 399nt) and '' Peach latent mosaic viroid'' (PLMVd, acc. M83545, gen. len. 337nt)


Detection

The lack of a long, central conserved region makes Avsunviroidae harder to identify than Pospiviroidae. A method to detect them is to use their circularity: a computer can piece together many overlapping reads that appear to form repeats when placed linearly.


References

*


External links


ICTV Online (10th) Report; Avsunviroidae
* * * {{Taxonbar, from=Q1952007 Viroids