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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Baltimore classification | 4/7 | https://en.wikipedia.org/wiki/Baltimore_classification | reference | science, encyclopedia | 2026-05-05T09:06:57.533227+00:00 | kb-cron |
The sixth Baltimore group contains viruses that have a (positive-sense) single-stranded RNA genome with a DNA intermediate ((+)ssRNA-RT) in their replication cycle. ssRNA-RT viruses are transcribed in the same manner as DNA viruses, but their genomes are first converted to a dsDNA form through a process called reverse transcription (RT). The viral reverse transcriptase enzyme then synthesizes a DNA strand from the +ssRNA strand, and the RNA strand is degraded and replaced with a DNA strand to create a dsDNA copy of the genome. The viral enzyme integrase then integrates the dsDNA molecule into the DNA of the host cell, where it is now called a provirus. The host cell's RNA polymerase II then transcribes RNA in the nucleus from the proviral DNA. Some of this RNA becomes mRNA whereas other strands become copies of the viral genome for replication. ssRNA-RT viruses are all included in the class Revtraviricetes, the sole class in the kingdom Pararnavirae, realm Riboviria. Excluding the family Caulimoviridae, which belongs to group VII, all members of the Revtraviricetes order Ortervirales are ssRNA-RT viruses. ssRNA-RT viruses are sometimes called retroviruses, a term shared with members of the ssRNA-RT family Retroviridae.
=== Group VII: double-stranded DNA viruses with an RNA intermediate ===
The seventh Baltimore group contains viruses that have a double-stranded DNA genome with an RNA intermediate (dsDNA-RT) in their replication cycle. dsDNA-RT viruses have gaps in their circular genomes so that parts of the genome are ssDNA. These gaps are repaired to create a complete, covalently-closed circular dsDNA genome before transcription. The host cell's RNA polymerase II then transcribes RNA strands from the genome in the nucleus. The viral enzyme reverse transcriptase then produces dsDNA from pregenomic RNA (pgRNA) strands by the same general mechanism as ssRNA-RT viruses, but with replication occurring in a loop around the circular genome. Replication occurs after pgRNA is packaged into capsids and before capsids bud from the cell. dsDNA-RT viruses are, like ssRNA-RT viruses, all included in the class Revtraviricetes. Two families of dsDNA-RT viruses are recognized: Caulimoviridae, which belongs to the order Ortervirales, and Hepadnaviridae, which is the sole family in the order Blubervirales. The provisional family Nudnaviridae is considered to be a sister family to hepadnavirids. dsDNA-RT viruses are often called pararetroviruses.
== Multi-group viruses == Some viruses can be classified into two Baltimore groups. Pleolipoviruses, for example, encapsidate either ssDNA or dsDNA genomes. For betapleolipoviruses, an encapsidated genome contains ssDNA regions and dsDNA regions. Similarly, bacilladnaviruses are ssDNA viruses with short dsDNA regions in their genomes. Therefore, both pleolipoviruses and bacilladnaviruses can be considered both dsDNA and ssDNA viruses. Ambisense RNA viruses also exist: certain bunyaviruses, such as arenaviruses, contain segmented genomes in which one segment is partly positive sense and partly negative sense. Furthermore, RNA viruses of the phylum Ambiviricota have non-segmented genomes with at least two open reading frames (ORFs). One is encoded on the positive-sense strand and the other on the negative-sense strand. These RNA viruses may constitute a new Baltimore group, or they can be considered both +ssRNA and –ssRNA viruses.
== Correlates to Baltimore groups ==
Many characteristics of viruses do not define which Baltimore group a virus belongs to but still correspond to specific Baltimore groups. This includes the use of RNA editing, alternative splicing during transcription, whether the virus's genome is segmented, the size and structure of the virus's genome, the host range of viruses, whether the virus packages replication and transcription machinery into virions, and unorthodox methods of translating mRNA.
=== RNA editing === RNA editing is used by various ssRNA viruses to produce different proteins from a single gene. This can be done by polymerase slippage during transcription or by post-transcriptional editing. During polymerase slippage, the RNA polymerase slips one nucleotide back during transcription, which adds a nucleotide not included in the template strand to the mRNA strand. Editing of a genomic template would impair gene expression, so RNA editing is only done during and after transcription. For ebola viruses, RNA editing is used to express three different proteins from a single gene, which increases their ability to adapt to their hosts.
=== Alternative splicing === Alternative splicing is a mechanism by which different proteins can be produced from a single gene by using alternative splicing sites to produce different mRNA strands. It is used by various DNA, –ssRNA, and reverse transcribing viruses. Viruses may make use of alternative splicing solely to produce multiple proteins from a single pre-mRNA strand or for other specific purposes. For some viruses, such as papillomaviruses, alternative splicing acts as a way to regulate early and late gene expression during different stages of infection. Herpesviruses use it as a potential anti-host defense mechanism to prevent synthesis of specific antiviral proteins. Alternative splicing differs from RNA editing in that alternative splicing does not change the mRNA sequence like RNA editing but instead changes the coding capacity of an mRNA sequence as a result of alternative splicing sites. The two processes otherwise have the same result: multiple proteins are expressed from a single gene.
=== Genome segmentation ===
Viral genomes can exist as a single (monopartite) segment, a segmented genome, or a multipartite genome. For monopartite viruses, all genes are on a single genome segment. For segmented viruses, the genome is separated into at least two molecules that are packaged together into one virion. Multipartite viruses are segmented viruses that package their genome segments into separate virions. Monopartite and segmented viruses are found in all cellular life, whereas multipartite viruses mainly infect plants and fungi. By Baltimore group, dsDNA and RT viruses are non-segmented, ssDNA and +ssRNA viruses are mostly non-segmented, dsRNA viruses are mostly segmented, and around half of –ssRNA viruses are segmented. Pleolipoviruses vary as some have monopartite ssDNA genomes while others are bipartite with one ssDNA segment and one dsDNA segment. Viruses in the ssDNA plant virus family Geminiviridae likewise vary between being monopartite and bipartite.