Medical Pharmacology Chapter 37:  Antiviral Drugs

• Viral Gene Expression and Viral Replication:  Baltimore Classification System

  • Viral genomes can be categorized as belonging to one of seven groups, dependent on pathways leading to mRNA and directed protein synthesis.¹ 

    • This grouping is the Baltimore classification system.¹   

      • The system highlights differing RNA polymerases required by viruses associated with each group.

        • Sometimes the polymerase enzyme is available natively in the cell and in other circumstances must be made available by the virus.

      • Single-stranded RNA viral genomes are subcategorized into two groups.

        • Viruses in one group package the "sense" RNA strand which  support direct translation into viral proteins.

          • This strand, as disucssed earlier, is described as the positive (or plus) strand.

        • However, other viruses package the "antisense" strand which only provides a complimentary copy code for viral proteins and is therefore not directly transcribable.

          • This latter group is described therefore as a negative (or minus) strand. On this basis the details of the Baltimore classification system can be described.¹ 

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      Baltimore Classification System²

      Splettstoesser T Virus:  Baltimore Classification based on the method of viral mRNA synthesis http://en.wikipedia.org/wiki/Baltimore_classification#mediaviewer/File:VirusBaltimoreClassification.svg I. Double stranded (dsDNA) DNA viruses such as Adenoviruses, Herpesviruses and Poxiviruses2,3 II.  Single stranded (ssDNA) DNA viruses (+ strand or sense) such as Parvoviruses III. Double stranded (dsRNA) RNA viruses such as Reoviruses IV. Single stranded ((+) ssRNA viruses) (+ strand or sense) RNA viruses such as Picornaviruses, Togaviruses. V.  Single stranded (-)ssRNA viruses  ((- strand or antisense) RNA such as Orthomyxoviruses, Rhabdovirsuses VI. Single stranded ssRNA-RT viruses (+ strand or sense, RT = reverse transcriptase) RNA with DNA intermediate in life-cycle such as Retroviruses VII Double stranded dsDNA-RT viruses such as Hepadnaviruses.2,3

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      Group I:  Double Stranded DNA Viruses-Baltimore Classification^2,3,4

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      Group II:  Single Stranded DNA Viruses-Baltimore Classication^2,3,4

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      Double Stranded RNA Viruses-Baltimore Classification^2,3,4

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      Positive Sense Single Stranded RNA Viruses-Baltimore Classification^2,3,4

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      Negative Sense Single Stranded RNA Viruses-Baltimore Classification^2,3,4

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      Positive Sense Single Stranded RNA Viruses Replicating through a DNA Intermediate^2,3,4

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      Double Stranded DNA Viruses Replicating through a Single Stranded RNA Intermediate^2,3,4