| MICROBIOLOGY AND IMMUNOLOGY ON-LINE |
Dr Dorian McIlroy
Universite de Nantes
In an article entitled "Expression of animal virus genomes" published in 1971, David Baltimore (Nobel Prize in Physiology or Medicine 1975) proposed a classification scheme for viruses based on the nucleic acid composition of the viral genome, and the major steps in viral genome replication and expression.
For example, in the Baltimore scheme, retroviruses are considered distinct from RNA viruses because, even though the virion contains (+) strand RNA, retrovirus replication involves a DNA intermediate in the infected host cell.
The different virus classes according to the Baltimore scheme are:
|
Baltimore Class |
Virus Genome |
Characteristics |
Virion contains viral polymerase ? |
|
I |
ds DNA |
|
Yes/No |
|
II |
ss DNA |
|
No |
|
III |
ds RNA |
|
Yes |
|
IV |
(+) strand RNA |
Virus genome codes directly for virus proteins, similar to mRNA |
No |
|
V |
(-) strand RNA |
Virus genome does not code directly for virus proteins |
Yes |
|
VI |
RNA in virion, but replication involves a DNA intermediate in the infected host cell |
Reverse transcription (RNA sequence copied into DNA) is required during virus replication |
Yes |
|
VII |
DNA in virion, but replication involves a RNA intermediate in the infected host cell |
Reverse transcription (RNA sequence copied into DNA) is required during virus replication |
Yes |
Class VI defines the retroviruses. Together with class VII, they constitute the reverse-transcribing viruses.
A single Baltimore class may contain several different virus Families and Orders. However, it is important to understand that this does not imply a phylogenetic link between viruses in the same class.
That is to say, that even though viruses within the same taxonomic Family (for example, the Picornaviridae) appear to be derived from a common ancestor, it is not the case that different Families of (+) strand RNA viruses (for example, Picornaviridae and Caliciviridae) have a common ancestor.
This is an important difference between the Baltimore scheme and the international system of virus taxonomy.