Baltimore classification

Baltimore classification is a system used to classify viruses by their routes of transferring genetic information from the genome to messenger RNA (mRNA). Seven Baltimore groups, or classes, exist and are designated by the Roman numerals I to VII. Groups are defined by whether the viral genome is composed of deoxyribonucleic acid (DNA) or ribonucleic acid (RNA); whether the genome is single- or double-stranded; and whether the virus makes DNA from RNA (reverse transcription (RT)). Viruses with single-stranded genomes are further differentiated by whether these genomes are positive-sense (+) or negative-sense (–). Viruses within each of the Baltimore groups typically share a common replication method. Additional characteristics, such as virion structure, are not directly related to Baltimore classification and may not be shared by viruses within each group. The seven Baltimore groups are for double-stranded DNA (dsDNA) viruses, single-stranded DNA (ssDNA) viruses, double-stranded RNA (dsRNA) viruses, positive-sense single-stranded RNA (+ssRNA) viruses, negative-sense single-stranded RNA (–ssRNA) viruses, ssRNA viruses that have a DNA intermediate in their life cycle (ssRNA-RT), and dsDNA viruses that have an RNA intermediate in their life cycle (dsDNA-RT). Only one class exists for ssDNA viruses because their genomes are converted to dsDNA before transcription regardless of sense. Some viruses belong to more than one Baltimore group, such as DNA viruses that have either dsDNA or ssDNA as their genome. Many virus characteristics do not define which Baltimore group they belong to but do correlate to specific Baltimore groups. This includes the use of RNA editing and alternative splicing, 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 into proteins. Furthermore, while Baltimore groups were not established based on evolutionary relationships, research in the 21st century has found that certain groups, such as dsRNA, +ssRNA, and many –ssRNA viruses, share common ancestry. Baltimore classification was created in 1971 by virologist David Baltimore and initially only included the first six groups. It was later expanded to include group VII after the discovery of dsDNA-RT viruses. Since then, it has become common among virologists to use Baltimore classification alongside virus taxonomy due to its utility. In 2018 and 2019, Baltimore classification was partially integrated into virus taxonomy based on evidence that certain groups were descended from common ancestors. Various taxa now correspond to specific Baltimore groups. An extension of Baltimore classification has been proposed by virologist Vadim Agol to encompass all possible routes of genetic information transmission.

Overview Baltimore classification groups viruses together by their routes of transferring genetic information from the genome to messenger RNA (mRNA). Characteristics that determine the Baltimore group of a virus include whether the genome is made of deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), the strandedness of the genome, which can be either single- or double-stranded, the sense of a single-stranded RNA genome, which can be either positive (+) or negative (–), and whether the virus synthesizes DNA from RNA (reverse transcription (RT)). There are seven Baltimore groups or classes, numbered with Roman numerals, listed hereafter.

Group I: double-stranded DNA viruses (dsDNA) Group II: single-stranded DNA viruses (ssDNA) Group III: double-stranded RNA viruses (dsRNA) Group IV: positive-sense single-stranded RNA viruses (+ssRNA) Group V: negative-sense single-stranded RNA viruses (–ssRNA) Group VI: single-stranded RNA viruses with a DNA intermediate in their life cycle (ssRNA-RT) Group VII: double-stranded DNA viruses with an RNA intermediate in their life cycle (dsDNA-RT) Baltimore classification is chiefly based on the path toward transcription of the viral genome, and viruses within each group usually share the manner by which the mRNA synthesis occurs. While not the direct focus of Baltimore classification, groups are organized in such a manner that viruses in each group also typically have the same mechanisms of replicating the viral genome. Structural characteristics of the extracellular virus particle, called a virion, such as the shape of the viral capsid, which stores the genome, 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.

Baltimore groups

Group I: double-stranded DNA viruses

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