Baltimore classification
Baltimore classification is a system used to classify viruses based on their manner of messenger RNA (mRNA) synthesis.
Since then, it has become common among virologists to use Baltimore classification alongside standard virus taxonomy, which is based on evolutionary history.
In 2018 and 2019, Baltimore classification was partially integrated into virus taxonomy based on evidence that certain groups were descended from common ancestors.
[1] Baltimore classification is chiefly based on the transcription of the viral genome, and viruses within each group typically share the manners by which the mRNA synthesis occurs.
[2][3] Because of this, Baltimore classification provides insights into both the transcription and replication parts of the viral life cycle.
Structural characteristics of a virus particle, called a virion, such as the shape of the viral capsid and the presence of a viral envelope, a lipid membrane that surrounds the capsid, have no direct relation to Baltimore groups, nor do the groups necessarily show genetic relation based on evolutionary history.
Third, the RNA polymerase terminates transcription upon reaching a specific signal, such as a polyadenylation site.
[7] A rolling circle mechanism that produces linear strands while progressing in a loop around the circular genome is also common.
Because the genome is single-stranded, however, it is first made into a double-stranded form by a DNA polymerase upon entering a host cell.
The double-stranded form of ssDNA viruses may be produced either directly after entry into a cell or as a consequence of replication of the viral genome.
[18] Parvoviruses, as previously mentioned, may package either the positive or negative sense strand into virions.
After entering a host cell, the dsRNA genome is transcribed to mRNA from the negative strand by the viral RNA-dependent RNA polymerase (RdRp).
Endornaviruses are also unusual in that unlike other RNA viruses, they possess a single, long open reading frame (ORF), or translatable portion, and a site-specific nick in the 5′ region of the positive strand.
Typically, subgenomic RNA (sgRNA) strands are used for translation of structural and movement proteins needed during intermediate and late stages of infection.
[27][28][29] Because the process of replicating the viral genome produces intermediate dsRNA molecules, +ssRNA viruses can be targeted by the host cell's immune system.
[26][27] +ssRNA viruses can be subdivided between those that have polycistronic mRNA, which encodes a polyprotein that is cleaved to form multiple mature proteins, and those that produce subgenomic mRNAs and therefore undergo two or more rounds of translation.
The first process for -ssRNA transcription involves RdRp binding to a leader sequence on the 3′ end of the genome, transcribing a 5′ triphosphate-leader RNA that is capped, then stopping and restarting on a transcription signal which is capped, continuing until a stop signal is reached.
The manner of producing the polyA tail may be via polymerase stuttering, during which RdRp transcribes an adenine from uracil and then moves back in the RNA sequence with the mRNA to transcribe it again, continuing this process numerous times until hundreds of adenines have been added to the 3′-end of the mRNA.
The sixth Baltimore group contains viruses that have a (positive-sense) single-stranded RNA genome that has a DNA intermediate ((+)ssRNA-RT) in its replication cycle.
[37][39][40][41] ssRNA-RT viruses are all included in the class Revtraviricetes, phylum Arterviricota, kingdom Pararnavirae of the realm Riboviria.
Excluding Caulimoviridae, which belongs to Group VII, all members of the Revtraviricetes order Ortervirales are ssRNA-RT viruses.
[25][42] The seventh Baltimore group contains viruses that have a double-stranded DNA genome that has an RNA intermediate (dsDNA-RT) in its replication cycle.
dsDNA-RT viruses have a gap in one strand, which is repaired to create a complete dsDNA genome prior to transcription.
Viruses may make use of alternative splicing solely to produce multiple proteins from a single pre-mRNA strand or for other specific purposes.
For certain viruses, including the families Orthomyxoviridae and Papillomaviridae, 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.
[44][45] Viral genomes can exist in a single, or monopartite, segment, or they may be split into more than one molecule, called multipartite.
[6][49][54] RNA editing is used by various ssRNA viruses to produce different proteins from a single gene.
[69] From the 1990s to the 2010s, virus taxonomy used a 5-rank system ranging from order to species with Baltimore classification used in conjunction.
Outside of the ICTV's official framework, various supergroups of viruses joining different families and orders were created over time based on increasing evidence of deeper evolutionary relations.
