Retroviridae
Baltimore Classification
Higher order taxa
Viruses; Retro-transcribing viruses; Reoviridae
Genera
Alpharetrovirus, Betaretrovirus, Spumavirus (examples)
Description and Significance
Genome Structure
The genome of retroviridae is dimeric, unsegmented and contains a single molecule of linear. The genome is -RT and a positive-sense, single-stranded RNA. Minor species of non-genomic nucleic acid are also found in virions. The encapsidated nucleic acid is mainly of genomic origin but virions may also contain nucleic acid of host origin, including host RNA and fragments of host DNA believed to be incidental inclusions. The complete genome of one monomer is 700-11000 nucleotides long. The 5'-end of the genome has a methylated nucleotide cap with a cap sequence type 1 m7G5ppp5'GmpNp. The 3'-terminus of each monomer has a poly (A) tract and the terminus has a tRNA-like structure. (source: ICTVdB Descriptions)
Virion Structure of a Retroviridae
The virions of a retroviridae consist of an encelope, a nucleocapsid and a nucleoid. The virus capsid is enveloped. The virions are spherical to pleomorphic and measure 80-100 nm in diameter. The surface projections are small or distinctive glycoprotein spikes that cover the surface evenly. The projections are densely dispersed and 8 nm long. The nucleoid is concentric or eccentric while the core is spherical. (source: ICTVdB Descriptions)
Reproduction Cycle of a Retroviridae in a Host Cell
The SU envelope glycoprotein binds to a specific receptor on the surface of the host targer cell to initiate the infection. The specificity of this interaction does much to determine the cell-tropism and pathogenesis of different retroviruses and even different isolates of the same virus. Murine reroviruses (MLVs) are sub-divided to three categories on the basis of receptor-determined host species specificity: ecotropic, xenotropic, and amphotropic. Ecotropic MLVs infect only mouse cells, xenotropic MLVs infect only non-mouse cells like rat and hamster, and amphotropic MLVs infect both mouse and non-mouse cells.
Interference between an exogenous virus and an endogenous virus of the same receptor specificity results in interference groups of viruses, as exemplified by ALVs. A number of retrovirus receptor molecules have been identified in recent years.
There is a possibility that receptor binding results in conformational changes in the glycoprotein spike, revealing the fusion domain in the TM protein and resulting in the fusion of the virus envelope with the cell membrane. Very little is known about penetration and uncoating but it is known that uncoating is only partial. resulting eventually in a core particle within the cytoplasm. Reverse transcription occurs inside the ordered structure of this core particle. Reverse transcription is initiated but cannot be completed with the reactants free in solution, and aborts soon after.
The d/s DNA product formed is known as provirus and differs from the vRNA in being longer by one U3,R,U5 sequence. There is a direct repear of this sequence present at each end of the provirus genome as a result, and these are known as the long terminal repeats (LTRs)Three forms of provirus DNA are found in all infected cells.
Integration is a highly specific reaction with respect to the provirus, but random with respect to the host cell DNA. It is believed that the linear form, probably the direct product of reverse transcription, is the substrate used.
