Alphasatellite

Alphasatellite
Virus classification
Group:	Group II (ssDNA)
Species
Begomovirus group Ageratum leaf curl Cameroon alphasatellite Ageratum leaf curl Pakistan alphasatellite Ageratum yellow vein alphasatellite Ageratum yellow vein India alphasatellite Ageratum yellow vein Kenya alphasatellite Ageratum yellow vein Pakistan alphasatellite Ageratum yellow vein Singapore alphasatellite Chilli leaf curl Multan alphasatellite Cleome leaf crumple alphasatellite Cotton leaf curl alphasatellite Cotton leaf curl Burewala alphasatellite Cotton leaf curl Dabwali alphasatellite Cotton leaf curl Gezira alphasatellite Cotton leaf curl India alphasatellite Cotton leaf curl Multan alphasatellite Cotton leaf curl Pakistan alphasatellite Croton yellow vein mosaic alphasatellite Euphorbia yellow mosaic alphasatellite Gossypium darwinii symptomless alphasatellite Gossypium mustelinium symptomless alphasatellite Hollyhock crumple alphasatellite Hollyhock yellow vein virus-associated symptomless alphasatellite Hibiscus leaf curl alphasatellite Malvastrum yellow mosaic alphasatellite Malvastrum yellow mosaic Cameroon alphasatellite Malvastrum yellow mosaic Hainan alphasatellite Milk vetch dwarf alphasatellite Melon chlorotic mosaic alphasatellite Mimosa yellow leaf curl alphasatellite Okra leaf curl alphasatellite Okra leaf curl Barombi alphasatellite Okra leaf curl Burkina Faso alphasatellite Okra leaf curl Mali alphasatellite Okra leaf curl Tiko alphasatellite Potato leaf curl alphasatellite Sida leaf curl alphasatellite Sida yellow vein alphasatellite Sida yellow vein Vietnam alphasatellite Tobacco curly shoot alphasatellite Tobacco curly shoot China alphasatellite Tomato leaf curl alphasatellite Tomato leaf curl Cameroon alphasatellite Tobacco leaf curl China alphasatellite Tobacco leaf curl Pakistan alphasatellite Tomato yellow leaf curl China alphasatellite Tomato yellow leaf curl DNA2 Tomato yellow leaf curl Thailand alphasatellite Tomato yellow leaf curl Yunnan alphasatellite Verbesina encelioides leaf curl alphasatellite Vernonia yellow vein Fujian alphasatellite Nanoviridae group Banana bunchy top alphasatellite Banana bunchy top Taiwan alphasatellite Banana bunchy top S1 alphasatellite Banana bunchy top Vietnam alphasatellite Banana bunchy top Y alphasatellite Coconut foliar decay alphasatellite Faba bean necrotic yellows alphasatellite Milk vetch dwarf alphasatellite Subterranean clover stunt alphasatellite

Alphasatellites are single stranded satellite DNA that are dependent on a virus for transmission. The genome is a single circular single strand DNA molecule. The first alphasatellites were described in 1999 and were associated with cotton leaf curl disease and Ageratum yellow vein disease. As begomoviruses are being characterised at the molecular level an increasing number of alphasatellites are being described.

These viruses were earlier known as DNA 1 components.

These viruses are generally found in the Old World. A number have been isolated from the New World but their association with their host viruses is still being studied.

Genome

The genome is between 1300 and 1400 nucleotides in length and has three conserved features: a hairpin structure, a single open reading frame (ORF) and an adenine rich region.

The hairpin structure has a loop that includes the nonanucleotide, TAGTATTAC, which is common to nanoviruses and differs from the TAATATTAC sequence of geminiviruses by one nucleotide. In both geminiviruses and nanoviruses this sequence contains the origin of replication (ori) and is nicked by the rolling circle replication initiator protein to initiate viral DNA replication. On the basis of the hairpin structures alphasatellites can be divided into 5 clades.

The open reading frame encodes a rolling circle replication initiator protein (Rep) similar to that found in the nanoviruses. The encoded protein is 32–37 kiloDalton in molecular weight with ~320 amino acids. It is highly conserved with 86.3–100.0% amino acid sequence identy between isolates.

The adenine rich region is immediately downstream of the rep gene and is approximately 153–169 nucleotides in length with an adenine content of between 52.3–58.4%. Phylogenectic analysis of this region shows that they can be divided into three clades which correspond to those found on phylogenetic analysis of the entire genome. This portion of the genome appears to be redundant.

A putative second ORF in the genome of an alphasatellite virus has been described. The significance of this finding (if any) is not known.

Recombination occurs between alphasatellites.

Virology

There are no distinctive virons because the viral genomes are encapsidated within the coat protein of the helper virus.

Alphasatellites associated with the begomoviruses require a begomovirus for movement in plants and insect transmission but are capable of self replication in host plants. They do not appear to cause disease in plants or to alter the course of infection by the begomovirus. They may be able to reduce the severity of an infection by the begomoviruses.

Alphasatellites have also been described in association with the Nanoviridae. These tend to be slightly shorter (1100–1300 nucleotides) but to encode proteins in addition to the rep gene. Because of the multiple component genome of the Nanoviridae these were not initially recognised as distinct genomes.

Alphasatellites may be the target of RNA silencing.

Taxonomy

There is no formal type member.

At present alphasatellites are not organised into genera or higher taxa. A division between those associated with the Begomoviruses and those with associated with Nanoviridae seems logical at present.

It is recommended that strains with 80%+ identity be classified into a species. Proposals for their consistent naming have also been proposed.

Evolution

Given the similarities between the rep proteins of the alphasatellites and the nanoviruses, it is likely that the alphasatellites evolved from the nanoviruses. Further work in this area is needed to clarify this.

Uses

These viruses have been used in the development of viral gene silencing studies.

See also

• Betasatellite