Arch Virol 146/11 (2001)
Virology Division News
Classifying reverse transcribing elements:
a proposal and a challenge to the ICTV
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 chromo-
somal non-viral retroelements and bacterial chromosomal retroelements (Table 1) .
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 relation-
ships 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@example.com.