Keeping up with virus taxonomy: reverse-transcribing viruses
Posted on December 14, 2022 by Charlotte Hartley
In the second instalment of the relaunched ‘Keeping up with Virus Taxonomy’ blog series, we look at viruses that use reverse transcription to replicate.
Reverse transcription is a process by which viruses generate complementary DNA (cDNA) from an RNA template, using enzymes called reverse transcriptases. This is in contrast to the typical process of transcription, which forms RNA from a DNA template. Some viruses use reverse transcription to replicate and then integrate their own genetic information into their host’s genome.
Retroviridae
The ‘retro’ in Retroviridae refers to reversing the usual direction of transcription. In fact, the reverse transcriptase enzymes that catalyse this process were first discovered in a type of retrovirus (then known as RNA tumour viruses). Along with Metaviridae, Belpaoviridae, Psuedoviridae and Caulimoviridae, this family is part of the order Ortervirales.
Retrovirus virions are spherical and enveloped, with glycoprotein surface projections. Viruses in this family are found in a wide range of vertebrate hosts, where they invade their host cells’ DNA to create what is known as a provirus. Retroviruses can cause a range of diseases in humans and other animals, including leukaemia and other types of cancer, autoimmune diseases, motor neuron diseases and, perhaps most famously, AIDS caused by human immunodeficiency virus (HIV).
Metaviridae
Metaviridae is a family of reverse-transcribing viruses with little known about their morphologies. They exist within their host’s DNA as transposable elements of genetic material, meaning they can move to different locations within the host’s genome.
Metaviridae viruses are closely related to retroviruses and therefore share some genomic traits, such as long terminal repeats, which are long, repeating sequences of RNA base pairs that surround either side of the main genetic code. Well-known examples of this virus family include Saccharomyces cerevisiae Ty3 virus, which infects yeast cells, and the Gypsy virus, which infects Drosophila melanogaster fruit flies.
Belpaoviridae
Like the other families in the order Ortervirales, the family Belpaoviridae comprises reverse-transcribing viruses with long terminal repeats, and their replication involves the formation of virus-like particles. Little else is known about the structure of Belpaoviridae virions.
This family was previously included within the Metaviridae family, but an improved understanding of the ancestry of these viruses led to its separation. The only genus in the family is Semotivirus, which contains several species that infect a range of hosts, from vertebrates to insects and nematode worms. For example, Anopheles gambiaea Moose virus infects the species of mosquito that transmits malaria.
Caulimoviridae
Viruses in the Caulimoviridae family are characterised by three main features: double-stranded DNA, reverse transcription as part of their lifecycle and a tendency to infect plants. There are 11 genera comprised of 94 species in this family and all of them are non-enveloped. The name is a reference to cauliflower mosaic virus, which causes lesions and stunted growth in cauliflowers and turnips.
Caulimoviridae viruses target the nucleus of host cells and, using reverse transcription of RNA, integrate cDNA into the host genome. In doing so, some species in the Caulimoviridae family cause economically important diseases in tropical and subtropical crops, and transmission sometimes occurs through insect vectors. For example, Banana streak MY virus is spread by mealy bugs and causes banana streak disease in Australia, India and Tonga.
Hepadnaviridae
Hepadnaviridae is a family of small, spherical, enveloped viruses with a partially double-stranded DNA genome. All viruses in the family produce three sets of proteins called preC/C, polymerases and preS/S, and their replication involves reverse transcription. This family contains 18 species belonging to five genera.
As the name suggests, members of the Hepadnaviridae family are hepatotrophic, meaning they primarily affect the liver of their hosts. Infections can be transient or persistent. Perhaps the best known example is hepatitis B virus, which causes overproduction of surface proteins that are secreted into the blood of its host, causing liver infection, and can be spread from person to person by contact with blood, semen or other body fluids.
The International Committee on Taxonomy of Viruses (ICTV) is responsible for developing and maintaining a universal virus taxonomy. Known viruses are categorised into a classification scheme taking into consideration their physical and biological properties in combination with their phylogenetic relationships.
These two-page summaries of each chapter of the ICTV Report (a free resource published by the ICTV which provides an up-to-date description of virus taxonomy) are freely available in the Journal of General Virology, and are supported by the Microbiology Society. These summaries are known as ICTV Virus Taxonomy Profiles and describe the structure, replication and taxonomy of each virus order and family.