The ''Avsunviroidae'' are a family of
viroids
Viroids are small single-stranded, circular RNAs that are infectious pathogens. Unlike viruses, they have no protein coating. All known viroids are inhabitants of angiosperms (flowering plants), and most cause diseases, whose respective economi ...
. There are four species in three genera (''
Avsunviroid'', ''
Elaviroid
The ''Avsunviroidae'' are a family of viroids. There are four species in three genera ('' Avsunviroid'', '' Elaviroid'' and '' Pelamoviroid''). They consist of RNA genomes between 246 and 375 nucleotides in length. They are single-stranded coval ...
'' and ''
Pelamoviroid'').
They consist of RNA genomes between 246 and 375
nucleotide
Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecule ...
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
A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...
of plant cells. Key features of replication include no helper virus required and no proteins are encoded for. Unlike the other family of
viroids
Viroids are small single-stranded, circular RNAs that are infectious pathogens. Unlike viruses, they have no protein coating. All known viroids are inhabitants of angiosperms (flowering plants), and most cause diseases, whose respective economi ...
, ''
Pospiviroidae
The Pospiviroidae are a family of viroids, including the first viroid to be discovered, PSTVd. Their secondary structure is key to their biological activity. The classification of this family is based on differences in the conserved central re ...
'', ''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 plus 3 II. The negative RNA strands are then cleaved by
ribozyme
Ribozymes (ribonucleic acid enzymes) are RNA molecules that have the ability to catalyze specific biochemical reactions, including RNA splicing in gene expression, similar to the action of protein enzymes. The 1982 discovery of ribozymes demons ...
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 Mg
2+ 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 motif that catalyzes reversible cleavage and ligation reactions at a specific site within an RNA molecule. It is one of several catalytic RNAs (ribozymes) known to occur in nature. It serves as a model system for ...
.[ 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
The ''Avsunviroidae'' are a family of viroids. There are four species in three genera ('' Avsunviroid'', '' Elaviroid'' and '' Pelamoviroid''). They consist of RNA genomes between 246 and 375 nucleotides in length. They are single-stranded coval ...
'';[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
''Peach latent mosaic viroid'' is a species of the genus ''Pelamoviroid'', which belongs to the family ''Avsunviroidae''. This family is characterized as having chloroplastic viroids with hammerhead ribozymes. Peach latent mosaic viroid is a 336-3 ...
'' (PLMVd, acc. M83545, gen. len. 337nt)
Detection
The lack of a long, central conserved region makes Avsunviroidae harder to identify than Pospiviroidae
The Pospiviroidae are a family of viroids, including the first viroid to be discovered, PSTVd. Their secondary structure is key to their biological activity. The classification of this family is based on differences in the conserved central re ...
. 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
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{{Taxonbar, from=Q1952007
Viroids