Virus Famili Parvoviridae

The Parvoviridae family includes the smallest known viruses, and some of the most environmentally resistant. They were discovered during the 1960s and affect vertebrates and insects. Parvoviridae have a genome consisting of single-stranded DNA and an icosahedral capsid.

Parvovirus B19 was the first human parvovirus to be discovered and is best known for causing a childhood exanthem called "fifth disease" (erythema infectiosum), although it is also associated with other diseases including arthritis.

Parvovirus RA-1 had originally also been associated with rheumatoid arthritis, but this is now thought to have been an error due to laboratory contamination.

Virology
The viruses in this family are small (18-26 nanometers in diameter) and non enveloped. The viron is isosahedral with triangulation number (T) = 1. There are 60 copies of the coat protein in the viron.

The genome is 4-6 kilobases in length and usually encodes two open reading frames. The 5' open reading frame encodes two nonstructural proteins (NS-1 and NS-2) and the 3' open reading frame encodes two or three capsid proteins (VP1, VP2, VP3). Both the 5' and 3' termini have hairpin loops. In the genus Bocavirus there is a third open reading frame between the non structural and structural open reading frames.

The NS-1 protein has a superfamily 3 DNA helicase motif. These motifs are common in DNA viruses. The proteins that contain these motifs bind to the origin of replication and unwind the viral genome allowing access by the host's proteins to the viral genome for replication and transcription.

The genome is replicated by a unique rolling hairpin mechanism.

Taxonomy
The family is divided into two subfamilies - Parvovirinae - which infect vertebrates and - Densovirinae - which infect invertebrates. Each subfamily has been subdivided into several genera. The classification of the subfamily Parvovirinae may need revision.

Subfamily Parvovirinae:

    Genus Amdovirus; type species: Aleutian mink disease virus
    Genus Bocavirus; type species: Bovine parvovirus
    Genus Dependovirus; type species: Adeno-associated virus 2
    Genus Erythrovirus; type species: B19 virus
    Genus Partetravirus;
    Genus Parvovirus; type species: Murine minute virus

Subfamily Densovirinae:
Genus Brevidensovirus; type species: Aedes aegypti densovirus
Genus Densovirus; type species: Junonia coenia densovirus
Genus Iteravirus; type species: Bombyx mori densovirus
Genus Pefudensovirus; type species: Periplanta fuliginosa densovirus

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Virus Famili Nanoviridae

The Nanoviridae are a family of viruses, including the following genera:

Genus Nanovirus; type species: Subterranean clover stunt virus
Genus Babuvirus; type species: Banana bunchy top virus
Virus structure and genome

Nanoviruses have a genome composed of a single strand of DNA which is arranged in a circular fashion. They have a multipartite genome, with between 6 and 11 circular ssDNAs, each of ~1kb. Their name is derived from the Greek word “nano” meaning dwarf, because of their small genome and also the stunting effect on infected plants.

Virions of this family have a simple composition: they have a capsid but no surrounding envelope. The capsid is round in shape, has a diameter of between 18-20 nm and displays icosahedral symmetry.
Replication

After infection of a host cell, the small DNA molecules that have become encapsidated with the genomic ssDNA act as primers. They bind to complementary regions and help in initiation of DNA synthesis by host polymerases. On completion of synthesis, there will be a double stranded intermediate that is transcribed unidirectionally. Most individual nanovirus particles only encode for a single protein.

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Virus Famili Geminiviridae

Geminiviruses are plant viruses which have single-stranded circular DNA genomes encoding genes that diverge in both directions from a virion strand origin of replication (i.e. geminivirus genomes are ambisense). According to the Baltimore classification they are considered class II viruses.
Purified Maize streak virus (MSV) particles stained with uranyl acetate. Size bar indicates 50 nm. Picture courtesy of Kassie Kasdorf.

Whereas mastreviruses and curtoviruses transmission is via various leafhopper species (e.g. maize streak virus and other African streak viruses are transmitted by Cicadulina mbila), begomoviruses are transmitted by the whitefly species, (Bemisia tabaci and the only described topocuvirus species, Tomato pseudo-curley top virus is transmitted by the treehopper Micrutalis malleifera.

These viruses are responsible for a significant amount of crop damage worldwide. Epidemics of geminivirus diseases have arisen due to a number of factors, including the recombination of different geminiviruses coinfecting a plant, which enables novel, possibly virulent viruses to be developed. Other contributing factors include the transport of infected plant material to new locations, expansion of agriculture into new growing areas, and the expansion and migration of vectors that can spread the virus from one plant to another.

Virology
The genome can either be a single component between 2500-3100 nucleotides, or, in the case of some begomoviruses, two similar-sized components each between 2600 and 2800 nucleotides. They have elongated, geminate capsids with two incomplete T=1 icosahedra joined at the missing vertex. The capsids range in size from 18-20 nm in diameter with a length of about 30 nm. Begomoviruses with two component (i.e. bipartite) genomes have these components separated into two different particles both of which must usually be transmitted together to initiate a new infection within a suitable host cell.

Replication
Geminivirus genomes encode only a few proteins; thus, they are dependent on host cell factors for replication: these include factors such as DNA polymerase - and probably repair polymerases - in order to amplify their genomes, as well as transcription factors. Geminiviruses replicate via a rolling circle mechanism like bacteriophages such as M13, and many plasmids. Replication occurs within the nucleus of an infected plant cell. First the single-stranded circular DNA is converted to a double-stranded circular intermediate. This step involves the use of cellular DNA repair enzymes to produce a complementary negative-sense strand, using the viral genomic or plus-sense DNA strand as a template. The next step is the rolling circle phase, where the viral strand is cleaved at a specific site situated within the origin of replication by the viral Rep protein in order to initiate replication. This process in a eukaryotic nucleus can give rise to concatemeric double-stranded forms of replicative intermediate genomes, although double-stranded unit circles can be isolated from infected plants and cells. New single-stranded DNA forms of the virus genome (plus-sense) are probably formed by interaction of the coat protein with replicating DNA intermediates, as genomes lacking a CP gene do not form ssDNA. The ssDNA is packaged into germinate particles in the nucleus. It is not clear if these particles can then leave the nucleus and be transmitted to surrounding cells as virions, or whether ssDNA associated with coat protein and a movement protein is the form of the genome that gets trafficked from cell to cell via the plasmodesmata.

These viruses tend to be introduced into and initially infect differentiated plant cells, via the piercing mouthparts of the vector insect: however, these cells generally lack the host enzymes necessary for DNA replication, making it difficult for the virus to replicate. To overcome this block geminiviruses can induce plant cells to reenter the cell cycle from a quiescent state so that viral replication can occur.

Taxonomy
The Geminiviridae include the following genera:

Genus Mastrevirus; type species: Maize streak virus
Genus Curtovirus; type species: Beet curly top virus
Genus Begomovirus; type species: Bean golden mosaic virus
 Genus Topocuvirus; type species: Tomato pseudo-curly top virus

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Virus Famili Circoviridae

Circoviridae Classification

The family's name, "circo," is derived from the Greek root "gyro," which means ring or cirular.  This refers to the characteristic circular shape of the circovirus genome.  The family contains the circovirus genus, the gyrovirus genus, and the anellovirus genus.  The type species of the circovirus genus is porcine circovirus, type 1.  Genus classification is made based on phylogenetic differences in the highly conserved CP genomic coding sequence, with viral species with complete genomic sequences with less than 75% similarity, and CP sequences of less than 70% similarity, classified in a different genus.

Virion Morphology
Viruses in the circoviridae family are small, non-enveloped viruses with icosahedral nucleocapsids.  The nucleocapsid displays icosahedral symmetry of T = 13, and is 17-22 nm in diameter.  It consists of 32 capsomers.
The image above shows a diagram of the T = 13 symmetry of the viral nucleocapsid, as well as circoviridae's scanning electron micrograph appearance.
Genomic Characteristics

The genome of circoviruses is non-segmented and single-stranded.  Of particular note is the fact that the single-strand of genomic DNA is ambisense, and is present in the virion in the shape of a covalently-closed circle.  The genomic DNA is made up of 1800-2000 nucleotides.

The human circoviruses show greatest similarity to chicken anemia virus (CAV), another circovirus, and seem to have a worldwide distribution in human populations.  TTV and TLMV show great similarity in their genomic organization.  Their genomic DNA each encodes at least 2 overlapping open reading frames and at least 1 non-coding region. 


Host Range
While much of the circoviridae replication strategy is still unknown, members of this family are known to infect a variety of hosts.  Circoviruses are known to infect pigs and other porcine species, geese, canaries, ducks, finches, chickens and gulls.  The first human circovirus was discovered by Nishizawa et al. in 1997.  The human circoviruses include Torque Teno virus (TTV) and Torque Teno-like mini virus (TTMV), both members of the anellovirus genus.


Replication
While the replication of human circoviruses remains largely unknown, TTV and TLMV have been found to be hepatotropic.  They have also been isolated in leukocytes.  Despite theories that infection with human circoviruses cause liver disease, no definitive links have identified TTV or TLMV as an etiologic agent of liver cirrhosis or disease.


  

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Virus Famili Anelloviridae

he Anelloviridae are a recently discovered family of viruses . They are classified as a vertebrate viruses and have a non-enveloped capsid, which is round with isometric, icosahedral symmetry.

The genome is not segmented and contains a single molecule of circular, negative-sense, single-stranded DNA. The complete genome is 3000-4000 nucleotides long.a

The type species is torque teno virus. Anellovirus species are highly prevalent and genetically diverse. They cause chronic human viral infections that have not yet been associated with diseaseAncient human remains are potential sources of biological information including traces of past infections, since previous studies have demonstrated the effective detection of several bacterial agents or host-integrated viruses in old biological remnants like tissues or teeth. Studies of skeletal dental pulp samples are of particular interest since this location is potentially exposed to bloodborne agents during life through its vascularization, and could be considered as well preserved from environment after death of the host. DNA viruses belonging to the family Anelloviridae are highly present in human populations where they harbor an extreme genetic diversity but a yet undefined implication in hosts' health. We hypothesized that anelloviruses would be detected in ancient remains and that they may also serve as tracer viruses for the study of other viral agents. We analyzed 200-year-old dental pulp samples from remains of soldiers of Napoleon's Great Army during the Russian Retreat. Successful detection of Anelloviridae DNA by PCR was obtained for 1/21 ancient samples tested. The sequence identified showed 23% nucleotide divergence with the closest group of modern isolates (genus Gammatorquevirus), and was confirmed as phylogenetically distinct from those identified in saliva samples from the two investigators in charge of the study (genera Alphatorquevirus and Betatorquevirus). PCR directed toward the human beta globin gene was also performed. Negative controls were negative. Our results demonstrate that an ubiquitary, non-integrated, DNA virus is detectable from ancient biological material, with potential developments in terms of evolution studies or subsequent molecular investigations involving further viral agents.

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