Classifying reverse transcribing elements: a proposal and a challenge to the ICTV
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Virology Division News
Classifying reverse transcribing elements: a proposal and a challenge to the ICTV* R. Hull John Innes Centre, Norwich Research Park, Colney, Norwich, U.K.
The replication of most nucleic acids is either from DNA to DNA (chromosomal and viral nucleic acids) or from RNA to RNA (viruses and some cytoplasmic nucleic acids). However, an increasing number of nucleic acids are being found whose replication involves reverse transcription of RNA to produce DNA. This replication is driven by the enzyme reverse transcriptase (RT), which was first recognised over 30 years ago [1, 21]. Nucleic acids that replicate by reverse transcription are termed retroelements [10, 20] and this form of replication is employed by elements in higher plants, higher animals, fungi, insects and bacteria. Retroelements have been grouped into viral retroelements, eukaryotic chromosomal non-viral retroelements and bacterial chromosomal retroelements (Table 1) [9]. Retrotransposons are also known as LTR (long terminal repeat) retrotransposons and retroposons as non-LTR or poly (A) retrotransposons. There are various and separate classification systems for the viral and non-viral elements but, as these elements have many features in common, a universal classification for all retroelements should be considered. Viral retroelements have been formally classified by the International Committee on Taxonomy of Viruses (ICTV) over several years. Recently retrotransposons have been included in the classification scheme to give a total of five families (Table 2) [17, 18, 23]. The inclusion of retrotransposons in virus classification is based primarily on similarities in their genome organizations with those of viral retroelements and on phylogenetic relationships among the reverse transcriptases. It is the encoding of a reverse transcriptase and the mechanism of replication that differentiate retroelements from all other viruses and cellular elements. Thus consideration should be given to the inclusion of other reverse transcribing elements in the classification. The suggestion being put forward here is that there is a case for classifying all these elements together. As well as the basic RNA to DNA replication, retroelements have several other features in common. The enzyme complex of active retroelements comprises reverse * Editor’s footnote: This paper is a revised up-dated version of a paper published previously (Hull R, 1999, Arch Virol 144: 209–214). The ideas proposed by the author are under discussion by an ad hoc subcommittee of the Executive Committee of ICTV, who would welcome comments from any interested virologists on the proposals and the questions posed by Roger Hull. Please send comments to the Chair of the subcommittee (Mike Mayo) at [email protected].
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Table 1. Viral and non-viral reverse transcribing elements* Viral retroelements I. Retroviruses (RNA in virions)
II. Pararetroviruses (DNA in virions)
Eukaryotic non-viral retroelements LTRa RT INT Examples
I. Retrotransposon + + +
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