A retrovirus is an enveloped, single‑stranded RNA virus that replicates through reverse transcription of its RNA genome into DNA, which integrates into the host genome and directs the production of new viral particles.
Replication and Biology
Retroviruses belong to the family Retroviridae and have a genome consisting of two copies of positive‑sense single‑stranded RNA enclosed within an icosahedral capsid and lipid envelope derived from the host cell membrane. At the core is a virus‑encoded reverse transcriptase and integrase. After entering a host cell, the reverse transcriptase copies the viral RNA into a complementary DNA (cDNA) that is converted into double‑stranded DNA. The viral integrase inserts this DNA into the host genome as a provirus. Host RNA polymerase II then transcribes the proviral DNA to produce viral mRNA and genomic RNA, while the host’s translational machinery synthesizes viral proteins. Assembly of capsid proteins around the genomic RNA and budding from the plasma membrane complete the replication cycle. Retroviruses are divided into Orthoretrovirinae, which includes gammaretroviruses, deltaretroviruses, lentiviruses and spumaviruses, and the Spumaretrovirinae. Their ability to integrate into host DNA allows persistent infection and vertical transmission. Oncogenic retroviruses carry or activate host proto‑oncogenes, causing tumors in animals, while lentiviruses, such as human immunodeficiency virus, cause chronic immune deficiency.
Notable Members and Implications
Human immunodeficiency virus type 1 is the most familiar retrovirus; it infects CD4+ T lymphocytes and leads to progressive immune suppression. Human T‑lymphotropic viruses cause T‑cell leukemias and neurologic disorders. Animal retroviruses include feline leukemia virus and bovine leukemia virus. Retroviruses were first discovered in chickens as Rous sarcoma virus. Because retroviral genomes integrate into host DNA, they are used as vectors for gene therapy to introduce therapeutic genes into hematopoietic stem cells. However, insertional activation of host oncogenes is a potential risk and modern self‑inactivating vectors incorporate safety modifications.
Retroviruses replicate through an RNA‑to‑DNA step and integrate into host genomes, enabling persistent infection and genetic transmission. They include important human pathogens and have served as tools in molecular biology and gene therapy. Understanding retrovirus biology has informed treatments that target reverse transcription and integration.
Related Terms: Reverse Transcriptase, Lentivirus, HIV, Provirus, Oncovirus