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68-559: Riboviria was established in 2018 to accommodate all RdRp-encoding RNA viruses and was expanded a year later to also include RdDp-encoding retroviruses . These two groups of viruses are assigned to two separate kingdoms: Orthornavirae for RdRp-encoding RNA viruses, and Pararnavirae for RdDp-encoding viruses, i.e. all reverse-transcribing viruses. While the realm has few prokaryotic viruses, it includes most eukaryotic viruses, including most human, animal, and plant viruses, however, metagenomic studies are changing this perspective. Many of

136-440: A " kissing stem-loop ". Although virions of different retroviruses do not have the same morphology or biology, all the virion components are very similar. The main virion components are: The retroviral genome is packaged as viral particles. These viral particles are dimers of single-stranded, positive-sense, linear RNA molecules. Retroviruses (and orterviruses in general) follow a layout of 5'– gag – pro – pol – env –3' in

204-459: A DNA copy of its RNA genome into the DNA of a host cell that it invades, thus changing the genome of that cell. After invading a host cell's cytoplasm , the virus uses its own reverse transcriptase enzyme to produce DNA from its RNA genome, the reverse of the usual pattern, thus retro (backward). The new DNA is then incorporated into the host cell genome by an integrase enzyme, at which point

272-514: A body into the environment, where the virus may infect another. Vaccine shedding is a form of viral shedding which can occur in instances of infection caused by some attenuated (or "live virus") vaccines . "Budding" through the cell envelope —in effect, borrowing from the cell membrane to create the virus' own viral envelope — into extracellular space is most effective for viruses that require their own envelope. These include such viruses as HIV , HSV , SARS or smallpox . When beginning

340-453: A concern that insertional mutagenesis due to integration into the host genome might lead to cancer or leukemia. This is unlike Lentivirus , a genus of Retroviridae , which are able to integrate their RNA into the genome of non-dividing host cells. Two RNA genomes are packaged into each retrovirus particle, but, after an infection, each virus generates only one provirus . After infection, reverse transcription occurs and this process

408-939: A newly introduced fish virus genus are retroviruses classified as complex. These viruses have genes called accessory genes, in addition to gag, pro, pol and env genes. Accessory genes are located between pol and env, downstream from the env, including the U3 region of LTR, or in the env and overlapping portions. While accessory genes have auxiliary roles, they also coordinate and regulate viral gene expression. In addition, some retroviruses may carry genes called oncogenes or onc genes from another class. Retroviruses with these genes (also called transforming viruses) are known for their ability to quickly cause tumors in animals and transform cells in culture into an oncogenic state. The polyproteins are cleaved into smaller proteins each with their own function. The nucleotides encoding them are known as subgenes . When retroviruses have integrated their own genome into

476-635: A result of that, it is estimated that about 6.5–8% of the human genome originates from these viruses. Riboviria is a portmanteau of ribo , referencing ribonucleic acid, and the suffix - viria , which is the suffix used for virus realms. All members of Riboviria contain a gene that encodes for an RNA-dependent polymerase, also called RNA-directed polymerase. There are two types of RNA-dependent polymerases: RNA-dependent RNA polymerase (RdRp), also called RNA replicase, which synthesizes RNA from RNA, and RNA-dependent DNA polymerase (RdDp), also called reverse transcriptase (RT), which synthesizes DNA from RNA. In

544-457: A typical virus particle, called a virion, the RNA-dependent polymerase is bound to the viral genome in some manner and begins transcription of the viral genome after entering a cell . As part of a virus's life cycle , the RNA-dependent polymerase also synthesizes copies of the viral genome as part of the process of creating new viruses. Viruses that replicate via RdRp belong to three groups in

612-481: A virus first became endogenized into the host's genome as well as the rate of evolution for the viruses since endogenization first occurred. Diseases caused by viruses in Riboviria have been known for much of recorded history, though their cause was only discovered in modern times. Tobacco mosaic virus was discovered in 1898 and was the first virus to be discovered. Viruses transmitted by arthropods have been central in

680-415: Is a good way for a virus to get into macrophages either to infect them or simply travel to other tissues in the body. Although this process is primarily used by non-enveloped viruses, enveloped viruses may also use this. HIV is an example of an enveloped virus that exploits this process for the infection of macrophages. Viruses that have envelopes that come from nuclear or endosomal membranes can leave

748-427: Is a retrovirus. This virus passes to newborn mice through mammary milk. When they are 6 months old, the mice carrying the virus get mammary cancer because of the retrovirus. In addition, leukemia virus I (HTLV-1), found in human T cell, has been found in humans for many years. It is estimated that this retrovirus causes leukemia in the ages of 40 and 50. It has a replicable structure that can induce cancer. In addition to

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816-450: Is accompanied by recombination . Recombination involves template strand switching between the two genome copies (copy choice recombination) during reverse transcription. From 5 to 14 recombination events per genome occur at each replication cycle. Genetic recombination appears to be necessary for maintaining genome integrity and as a repair mechanism for salvaging damaged genomes. The DNA formed after reverse transcription (the provirus)

884-430: Is degraded and later replaced by RdDp with a +DNA strand to synthesize a linear dsDNA copy of the viral genome. This genome is then integrated into the host cell's DNA. For dsDNA-RT viruses, a pregenomic +RNA strand is transcribed from the relaxed circular DNA (rcDNA), which is in turn used by RdDp to transcribe a -cDNA strand. The +RNA strand is degraded and replaced in a similar manner as +ssRNA-RT viruses to synthesize

952-406: Is estimated to cause more than US$ 1 billion in damages annually, affecting more than 800 plant species including chrysanthemum, lettuce, peanut, pepper, and tomato. Cucumber mosaic virus infects more than 1,200 plant species and likewise causes significant crop losses. Potato virus Y causes significant reductions in yield and quality for pepper, potato, tobacco, and tomato, and Plum pox virus

1020-500: Is longer than the RNA genome because each of the terminals have the U3 - R - U5 sequences called long terminal repeat (LTR). Thus, 5' terminal has the extra U3 sequence, while the other terminal has the U5 sequence. LTRs are able to send signals for vital tasks to be carried out such as initiation of RNA production or management of the rate of transcription. This way, LTRs can control replication, hence,

1088-534: Is needed to know their exact placement in higher taxa. Metagenomic studies have suggested the existence of six new phyla not in the ICTV: Arctiviricota , Taraviricota , Pomiviricota , Paraxenoviricota , Wamoviricota and Artimaviricota . Riboviria partially merges Baltimore classification with virus taxonomy, including the Baltimore groups for RNA viruses and reverse transcribing viruses in

1156-509: Is now commonly used to describe a cancer-causing virus. This family now includes the following genera: Note that according to ICTV 2017, genus Spumavirus has been divided into five genera, and its former type species Simian foamy virus is now upgraded to genus Simiispumavirus with not less than 14 species, including new type species Eastern chimpanzee simian foamy virus . Both families in Group VII have DNA genomes contained within

1224-423: Is the expulsion and release of virus progeny following successful reproduction during a host cell infection. Once replication has been completed and the host cell is exhausted of all resources in making viral progeny, the viruses may begin to leave the cell by several methods . The term is variously used to refer to viral particles shedding from a single cell, from one part of the body into another, and from

1292-465: Is the most important virus among stone fruit crops. Brome mosaic virus , while not causing significant economic losses, is found throughout much of the world and primarily infects grasses, including cereals. Many reverse transcribing viruses, called retroviruses, in Riboviria are able to become integrated into the DNA of their host. These viruses become endogenized as part of their replication cycle. Namely,

1360-407: Is the use of recycling viral particle receptors in the enveloped varicella-zoster virus . A human with a viral disease can be contagious if they are shedding virus particles, even if they are unaware of doing so. Some viruses such as HSV-2 (which produces genital herpes ) can cause asymptomatic shedding and therefore spread undetected from person to person, as no fever or other hints reveal

1428-537: Is translated into molecules of the capsid protein, and for the pol gene; it is translated into molecules of reverse transcriptase. Retroviruses need a lot more of the Gag proteins than the Pol proteins and have developed advanced systems to synthesize the required amount of each. As an example, after Gag synthesis nearly 95 percent of the ribosomes terminate translation, while other ribosomes continue translation to synthesize Gag–Pol. In

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1496-522: The Flavivirus and Phlebovirus genera are numerous and often transmitted to humans. Coronaviruses and influenza viruses cause disease in various vertebrates, including bats, birds, and pigs. The family Retroviridae contains many viruses that cause leukemia , immunodeficiency , and other cancers and immune system-related diseases in animals. Plant viruses in the realm are numerous and infect many economically important crops. Tomato spotted wilt virus

1564-507: The Baltimore classification system, which groups viruses together based on their manner of messenger RNA synthesis, they are classified into two groups: Group VI: single-stranded RNA viruses with a DNA intermediate in their life cycle, and Group VII: double-stranded DNA viruses with an RNA intermediate in their life cycle. All members of Group VI use virally encoded reverse transcriptase , an RNA-dependent DNA polymerase, to produce DNA from

1632-427: The germ line , their genome is passed on to a following generation. These endogenous retroviruses (ERVs), contrasted with exogenous ones, now make up 5–8% of the human genome. Most insertions have no known function and are often referred to as " junk DNA ". However, many endogenous retroviruses play important roles in host biology, such as control of gene transcription, cell fusion during placental development in

1700-599: The Baltimore classification system, all of which are in the kingdom Orthornavirae : single-stranded RNA (ssRNA) viruses, which are either positive (+) or negative (-) sense , and double-stranded RNA viruses (dsRNA). +ssRNA viruses have genomes that can functionally act as mRNA, and a negative sense strand can also be created to form dsRNA from which mRNA is transcribed from the negative strand. The genomes of -ssRNA viruses and dsRNA viruses act as templates from which RdRp creates mRNA. Viruses that replicate via reverse transcription belong to two Baltimore groups, both of which are in

1768-532: The HIV life cycle . Combination of several (typically three or four) antiretroviral drugs is known as highly active antiretroviral therapy (HAART). Feline leukemia virus and Feline immunodeficiency virus infections are treated with biologics , including the only immunomodulator currently licensed for sale in the United States, Lymphocyte T-Cell Immune Modulator (LTCI). Viral shedding Viral shedding

1836-482: The RNA genome. gag and pol encode polyproteins, each managing the capsid and replication. The pol region encodes enzymes necessary for viral replication, such as reverse transcriptase, protease and integrase. Depending on the virus, the genes may overlap or fuse into larger polyprotein chains. Some viruses contain additional genes. The lentivirus genus, the spumavirus genus, the HTLV / bovine leukemia virus (BLV) genus, and

1904-679: The RNA to DNA transcription processes used by retroviruses may have first caused DNA to be used as genetic material. In this model, the RNA world hypothesis , cellular organisms adopted the more chemically stable DNA when retroviruses evolved to create DNA from the RNA templates. An estimate of the date of evolution of the foamy-like endogenous retroviruses placed the time of the most recent common ancestor at > 450  million years ago . Gammaretroviral and lentiviral vectors for gene therapy have been developed that mediate stable genetic modification of treated cells by chromosomal integration of

1972-405: The budding process, the viral nucleocapsid cooperates with a certain region of the host cell membrane. During this interaction, the glycosylated viral envelope protein inserts itself into the cell membrane. In order to successfully bud from the host cell, the nucleocapsid of the virus must form a connection with the cytoplasmic tails of envelope proteins. Though budding does not immediately destroy

2040-451: The cccDNA is retrotranscribed into new dsDNA genomes. For +ssRNA-RT viruses, the genome is replicated from the integrated genome. After replication and translation, the genome and the viral proteins are assembled into complete virions, which then leave the host cell . Both kingdoms in Riboviria show a relation to the reverse transcriptases of group II introns that encode RTs and retrotransposons , which are self-replicating DNA sequences,

2108-444: The cell to undergo apoptosis or cell suicide, release of progeny into the extracellular space is possible. However, apoptosis does not necessarily result in the cell simply popping open and spilling its contents into the extracellular space. Rather, apoptosis is usually controlled and results in the cell's genome being chopped up, before apoptotic bodies of dead cell material clump off the cell to be absorbed by macrophages . This

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2176-407: The cell via exocytosis , in which the host cell is not destroyed. Viral progeny are synthesized within the cell, and the host cell's transport system is used to enclose them in vesicles ; the vesicles of virus progeny are carried to the cell membrane and then released into the extracellular space. This is used primarily by non-enveloped viruses, although enveloped viruses display this too. An example

2244-399: The course of the germination of an embryo , and resistance to exogenous retroviral infection. Endogenous retroviruses have also received special attention in the research of immunology -related pathologies, such as autoimmune diseases like multiple sclerosis , although endogenous retroviruses have not yet been proven to play any causal role in this class of disease. While transcription

2312-469: The development of vector control , which often aims to prevent viral infections. In modern history, numerous disease outbreaks have been caused by various members of the realm, including coronaviruses, ebola, and influenza. HIV especially has had dramatic effects on society as it causes a sharp decline in life expectancy and significant stigma for infected persons. For a long time, the relation between many viruses in Riboviria could not be established due to

2380-409: The disruption of cellular proto-oncogenes. Rous sarcoma virus contains the src gene that triggers tumor formation. Later it was found that a similar gene in cells is involved in cell signaling, which was most likely excised with the proviral DNA. Nontransforming viruses can randomly insert their DNA into proto-oncogenes, disrupting the expression of proteins that regulate the cell cycle. The promoter of

2448-411: The emergence of eukaryotes or originated before the last universal common ancestor (LUCA) being descendants of the ancient RNA world and that they preceded the retroelement reverse transcriptases. A larger study (2022) where new lineages (phyla) were described, was in favor of the hypothesis that RNA viruses descend from the RNA world, suggesting that retroelements originated from an ancestor related to

2516-419: The entire progress of the viral cycle. Although located in the nucleus, the non-integrated retroviral cDNA is a very weak substrate for transcription. For this reason, an integrated provirus is a necessary for permanent and an effective expression of retroviral genes. This DNA can be incorporated into host genome as a provirus that can be passed on to progeny cells. The retrovirus DNA is inserted at random into

2584-471: The established knowledge on this topic. However, although later research disproved some of the claims made about retroviruses, there are several controversial figures who continue to make claims which overall are considered to not have any valid basis or consensus in support of these claims. Antiretroviral drugs are medications for the treatment of infection by retroviruses, primarily HIV . Different classes of antiretroviral drugs act on different stages of

2652-404: The fact that the gene sequences that code for the protease and the reverse transcriptase quickly mutate. These changes in bases cause specific codons and sites with the enzymes to change and thereby avoid drug targeting by losing the sites that the drug actually targets. Because reverse transcription lacks the usual proofreading of DNA replication, a retrovirus mutates very often. This enables

2720-435: The form of independent particles of retroviruses, consist of enveloped particles about 100  nm in diameter. The outer lipid envelope consists of glycoprotein. The virions also contain two identical single-stranded RNA molecules 7–10 kilobases in length. The two molecules are present as a dimer, formed by base pairing between complementary sequences. Interaction sites between the two RNA molecules have been identified as

2788-412: The generation and insertion of new copies of retrotransposons into the host genome. These inserts are transcribed by enzymes of the host into new RNA molecules that enter the cytosol. Next, some of these RNA molecules are translated into viral proteins. The proteins encoded by the gag and pol genes are translated from genome-length mRNAs into Gag and Gag–Pol polyproteins. In example, for the gag gene; it

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2856-411: The high amount of genetic divergence among RNA viruses. With the development of viral metagenomics , many additional RNA viruses were identified, helping to fill in the gaps of their relations. This led to the establishment of Riboviria in 2018 to accommodate all RdRp-encoding RNA viruses based on phylogenetic analysis that they were related. A year later, all reverse transcribing viruses were added to

2924-471: The highest level of taxonomy used for viruses and Riboviria is one of four, the other three being Duplodnaviria , Monodnaviria , and Varidnaviria . Most identified eukaryotic viruses are RNA viruses, and for that reason most eukaryotic viruses belong to Riboviria , including most human, animal, and plant viruses. Other major branches of eukaryotic viruses include herpesviruses in Duplodnaviria ,

2992-568: The host cell, this process will slowly use up the cell membrane and eventually lead to the cell's demise. This is also how antiviral responses are able to detect virus-infected cells. Budding has been most extensively studied for viruses of eukaryotes . However, it has been demonstrated that viruses infecting prokaryotes of the domain Archaea also employ this mechanism of virion release. Animal cells are programmed to self-destruct when they are under viral attack or damaged in some other way. By forcing

3060-496: The host genome. Because of this, it can be inserted into oncogenes . In this way some retroviruses can convert normal cells into cancer cells. Some provirus remains latent in the cell for a long period of time before it is activated by the change in cell environment. Studies of retroviruses led to the first demonstrated synthesis of DNA from RNA templates, a fundamental mode for transferring genetic material that occurs in both eukaryotes and prokaryotes . It has been speculated that

3128-430: The initial virion RNA genome. This DNA is often integrated into the host genome, as in the case of retroviruses and pseudoviruses , where it is replicated and transcribed by the host. Group VI includes: The family Retroviridae was previously divided into three subfamilies ( Oncovirinae , Lentivirinae , and Spumavirinae ), but are now divided into two: Orthoretrovirinae and Spumaretrovirinae . The term oncovirus

3196-513: The invading virus particles. The DNA genome is transcribed into both mRNA, for use as a transcript in protein synthesis, and pre-genomic RNA, for use as the template during genome replication. Virally encoded reverse transcriptase uses the pre-genomic RNA as a template for the creation of genomic DNA. Group VII includes: The latter family is closely related to the newly proposed whilst families Belpaoviridae , Metaviridae , Pseudoviridae , Retroviridae , and Caulimoviridae constitute

3264-510: The kingdom Shotokuvirae in Monodnaviria , and many viruses in Varidnaviria . In contrast, only three groups of prokaryotic RNA viruses have been identified: the class Leviviricetes , the family Cystoviridae and the phylum Artimaviricota . They also suggest that families Picobirnaviridae and Partitiviridae previously associated with eukaryotes also infect prokaryotes and also

3332-502: The kingdom Pararnavirae : single-stranded RNA (ssRNA-RT) viruses, all of which belong to the order Ortervirales , and double-stranded DNA (dsDNA-RT) viruses, which belong to the family Caulimoviridae , also in Ortervirales , and the family Hepadnaviridae of the order Blubervirales . ssRNA-RT viruses have their positive-sense genome transcribed by RdDp to synthesize a negative sense complementary DNA (-cDNA) strand. The +RNA strand

3400-478: The latter of which self-replicate via reverse transcription and integrate themselves into other parts of the same DNA molecule. Reverse transcribing viruses, assigned to Pararnavirae , appear to have evolved from a retrotransposon on a single occasion. The origin of the RdRps of Orthornavirae is less clear due to a lack of information, that they originate from a reverse transcriptase from bacterial group II intron before

3468-447: The most widely known viral diseases are caused by viruses in Riboviria , which includes coronaviruses , ebola virus , HIV , influenza viruses , and the rabies virus . These viruses and others have been prominent throughout history, including Tobacco mosaic virus , which was the first virus to be discovered. Many reverse transcribing viruses notably become integrated into the genome of their host as part of their replication cycle. As

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3536-562: The most widely known viral diseases. Notable disease-causing viruses in the realm include: Animal viruses in Riboviria include orbiviruses , which cause various diseases in ruminants and horses, including Bluetongue virus , African horse sickness virus , Equine encephalosis virus , and epizootic hemorrhagic disease virus . The vesicular stomatitis virus causes disease in cattle, horses, and pigs. Bats harbor many viruses including ebolaviruses and henipaviruses , which also can cause disease in humans. Similarly, arthropod viruses in

3604-424: The order Ortervirales . Endogenous retroviruses are not formally included in this classification system, and are broadly classified into three classes, on the basis of relatedness to exogenous genera: Retroviruses have been the focus of several recent claims and assertions which have been largely discredited by the science community. An initial study in 2009 seemed to make new findings which might change some of

3672-521: The phylum Lenarviricota and that members of a newly discovered Taraviricota lineage (phylum) would be the ancestors of all RNA viruses. Riboviria contains two kingdoms: Orthornavirae and Pararnavirae . Orthornavirae contains multiple phyla and unassigned taxa, whereas Pararnavirae is monotypic down to the rank of class. This taxonomy can be visualized hereafter. Additionally, Riboviria contains two incertae sedis families and four incertae sedis genera. Additional information about them

3740-400: The phylum Taraviricota . Studies of metagenomic samples have uncovered new prokaryotic RNA virus taxa including two new phyla that infect only prokaryotes, suggesting that their diversity is greater than previously thought and challenging the traditional view that RNA viruses only infect mostly eukaryotes. Viruses in Riboviria are associated with a wide range of diseases, including many of

3808-537: The plasma membrane after further glycosylation. It is important to note that a retrovirus must "bring" its own reverse transcriptase in its capsid , otherwise it is unable to utilize the enzymes of the infected cell to carry out the task, due to the unusual nature of producing DNA from RNA. Industrial drugs that are designed as protease and reverse-transcriptase inhibitors are made such that they target specific sites and sequences within their respective enzymes. However these drugs can quickly become ineffective due to

3876-437: The provirus DNA can also cause over expression of regulatory genes. Retroviruses can cause diseases such as cancer and immunodeficiency. If viral DNA is integrated into host chromosomes, it can lead to permanent infections. It is therefore important to discover the body's response to retroviruses. Exogenous retroviruses are especially associated with pathogenic diseases. For example, mice have mouse mammary tumor virus (MMTV), which

3944-471: The rcDNA. The rcDNA genome is later repaired by the host cell's DNA repair mechanisms to synthesize a covalently closed circular DNA (cccDNA) genome. The integrated genome of +ssRNA-RT viruses and the cccDNA of dsDNA-RT viruses are then transcribed into mRNA by the host cell enzyme RNA polymerase II . Viral mRNA is translated by the host cell's ribosomes to produce viral proteins. In order to produce more viruses, viral RNA-dependent polymerases use copies of

4012-416: The realm. Baltimore classification is a classification system used for viruses based on their manner of mRNA production, often used alongside standard virus taxonomy, which is based on evolutionary history. All members of five Baltimore groups belong to Riboviria : Group III: dsRNA viruses, Group IV: +ssRNA viruses, Group V: -ssRNA viruses, Group VI: ssRNA-RT viruses, and Group VII: dsDNA-RT viruses. Realms are

4080-416: The realm. The kingdoms were also established in 2019, separating the two RNA-dependent polymerase branches. When the realm was founded, it mistakenly included two viroid families, Avsunviroidae and Pospiviroidae , and the genus Deltavirus , which were promptly removed in 2019 because they use host cell enzymes for replication. Retrovirus A retrovirus is a type of virus that inserts

4148-785: The retroviral DNA is referred to as a provirus . The host cell then treats the viral DNA as part of its own genome, transcribing and translating the viral genes along with the cell's own genes, producing the proteins required to assemble new copies of the virus. Many retroviruses cause serious diseases in humans, other mammals, and birds. Retroviruses have many subfamilies in three basic groups. The specialized DNA-infiltration enzymes in retroviruses make them valuable research tools in molecular biology, and they have been used successfully in gene delivery systems. Evidence from endogenous retroviruses (inherited provirus DNA in animal genomes) suggests that retroviruses have been infecting vertebrates for at least 450 million years. Virions , viruses in

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4216-479: The rough endoplasmic reticulum glycosylation begins and the env gene is translated from spliced mRNAs in the rough endoplasmic reticulum, into molecules of the envelope protein. When the envelope protein molecules are carried to the Golgi complex, they are divided into surface glycoprotein and transmembrane glycoprotein by a host protease. These two glycoprotein products stay in close affiliation, and they are transported to

4284-783: The transferred vector genomes. This technology is of use, not only for research purposes, but also for clinical gene therapy aiming at the long-term correction of genetic defects, e.g., in stem and progenitor cells. Retroviral vector particles with tropism for various target cells have been designed. Gammaretroviral and lentiviral vectors have so far been used in more than 300 clinical trials, addressing treatment options for various diseases. Retroviral mutations can be developed to make transgenic mouse models to study various cancers and their metastatic models . Retroviruses that cause tumor growth include Rous sarcoma virus and mouse mammary tumor virus . Cancer can be triggered by proto-oncogenes that were mistakenly incorporated into proviral DNA or by

4352-405: The usual gene sequence of retroviruses, HTLV-1 contains a fourth region, PX. This region encodes Tax, Rex, p12, p13 and p30 regulatory proteins. The Tax protein initiates the leukemic process and organizes the transcription of all viral genes in the integrated HTLV proviral DNA. Exogenous retroviruses are infectious RNA- or DNA-containing viruses that are transmitted from one organism to another. In

4420-441: The viral genome as templates to replicate the viral genome. For +ssRNA viruses, an intermediate dsRNA genome is created from which +ssRNA is synthesized from the negative strand. For -ssRNA viruses, genomes are synthesized from complementary positive sense strands. dsRNA viruses replicate their genomes from mRNA by synthesizing a complementary negative sense strand to form genomic dsRNA. For dsDNA-RT viruses, pregenomic RNA created from

4488-415: The viral genome is integrated into the host genome by the retroviral enzyme integrase, and viral mRNA is produced from that DNA. Endogenization is a form of horizontal gene transfer between unrelated organisms, and it is estimated that about 7–8% of the human genome consists of retroviral DNA. Endogenization can also be used to study the evolutionary history of viruses, showing an approximate time period when

4556-481: The virus to grow resistant to antiviral pharmaceuticals quickly, and impedes the development of effective vaccines and inhibitors for the retrovirus. One difficulty faced with some retroviruses, such as the Moloney retrovirus, involves the requirement for cells to be actively dividing for transduction. As a result, cells such as neurons are very resistant to infection and transduction by retroviruses. This gives rise to

4624-549: Was classically thought to occur only from DNA to RNA, reverse transcriptase transcribes RNA into DNA. The term "retro" in retrovirus refers to this reversal (making DNA from RNA) of the usual direction of transcription. It still obeys the central dogma of molecular biology , which states that information can be transferred from nucleic acid to nucleic acid but cannot be transferred back from protein to either protein or nucleic acid. Reverse transcriptase activity outside of retroviruses has been found in almost all eukaryotes , enabling

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