Final Exam - Retroviridae Flashcards
Retroviridae
Causative agents of some types of cancer, immunosuppression or immunodeficiency disease. May exists as stable components in the host gene. Retro = reverse/backward. Uses the RNA genome to produce DNA intermediates by means of RNA dependent DNA polymerase.
Retroviridae Implication
They integrate into the genome of the host by means of an enzyme called integrase. They may alter or acquire host genome sequences, may activate or inactivate particular host genes located near the integration site.
Morphology - Type A virus
Form extracellular structures with a characteristically thick shell with a hollow electron lucent ceter
Morphology - Type B virus
Show a round but eccentrically positioned inner core
Morphology - Type C virus
Assemble at plasma membrane and contain a central symmetrically placed spherical inner core
Morphology - Type D virus
Assemble in the cytoplasm via an A type intermediate and budding, show a distinct cylindrical core
Virion properties - gag
gag gene encodes the virus core proteins. Capsid, nucleocapsid, matrix
Virion properties - pol
pol gene encodes: reverse transcriptase and integrase
Virion properties - env
env gene encodes: viral envelop proteins, surface, and transmembrane
Virion properties - pro
gene encodes the protease
Alpharetroviruses Characteristics
Simple retroviruses. Represented by avian leukosis and sarcoma virus (ALSV). Contains gag, pro, pol and env. Mostly endogenous and exogenous viruses of chickens.
Exogenous retroviruses
Those viruses transmitted horizontally and are rarely transmitted via in-utero or germline infection.
Endogenous retroviruses
(retroviral elements) are those viruses included in the genome of most if not all animals.
Betaretroviruses
Simple retroviruses. Have type B morphology with round eccentric core or type D with cylindrical core. The genomes contain gag, pro, pol, and env genes. Mouse Mammaroy Tumor Virus (MMTV) has a sag gene that encodes superantigen.
Gammaretroviruses
Simple retroviruses with a C type morphology. Has the largest number of members e.g., murine leukemia virus, feline leukemia virus. Genome contains gag, pro, pol, and env. Contain both endogenous and exogenous retroviruses of mammals and reptiles.
Deltaretroviruses
Complex retroviruses with a C-type morphology. Human T-lymphotrophic virus (HTLV), bovine leukemia virus. Genome contains gag, pro, pol, env, and regulatory genes, rex and tax. No known endogenous deltaretroviruses. Exogenous retroviruses are found in a few mammals.
Rex and Tax Genes
Expressed from an alternatively spliced mRNS. Control synthesis and processing of viral RNA.
Epsilonretroviruses
Complex retroviruses with a type C morphology. Walleye dermal sarcoma virus. Contain gag, pro, pol, and env. Also genes ORF A, B, C. Only endogenous retroviruses are found in fish and reptiles.
ORFa
Is a viral homologue of the host cyclin D and may regulate host cell cycle
Lentiviruses
Complex viruses with a cylindrical or conical shaped core. Human immunodeficiency virus (HIV) and caprine arthritis-encephalitis virus and visna virus. Contains gag, pro, pol, env. ALso contains accessory genes. Exogenous retroviruses are found in many mammals.
Accessory genes
vif, vpr, vpo, tat, rev, and nef. Control transcription, RNA processing, virion assembly, host genes expression.
tat gene
in lentiviruses, it encodes a transactivator that enhances the efficiency of cellular RNA polymerase
rev gene
in lentiviruses, encodes a protein that facilitates transport of non-spliced viral RNA from the nucleus to the cytoplasm.
nef gene
in lentiviruses, encodes Nef protein that: is essential for replication in target hosts but no in cell culture in vitro. May down regulate expression of the CD4 molecule or alter the activation status of target cells.
vif gene
encodes a protein vif which enhances replication in lymphocytes
Spumaviruses
Complex viruses with a unique virion morphology containing spikes on the surface. Human foamy virus. Contains gag, pro, pol, env. Accessory genes (tas/bel-1, and bet). Exogenous viruses
tas gene
Transcriptional activator
Retrovirus Receptor Binding
To enter a cell and initiate infection, retroviruses require interaction between a cell surface molecule (receptor) and the envelope proteins on the virion surface.