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Togaviruses
(Arboviruses) |
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Togaviruses
(Arboviruses)
Togaviruses are spherical, enveloped viruses with 50-70 nm capsid and infectious (+) ssRNA genome. These viruses were formerly called arboviruses, a shortened form of 'arthropods-borne' viruses. It was, however, later shown that viruses from different biochemical classes can be carried by arthropods, and hence the term 'arboviruses' was discarded.
The togaviruses, which constitute a major biochemical group of the arboviruses, are divided into two serological subgroups, A and B, now called alphaviruses and flaviviruses, respectively. The two best studied togaviruses are the alphaviruses Sindbis and Semliki Forest (SFV). The alphaviruses cause encephalitis in humans and are mosquito borne
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The flaviviruses include the viruses causing dengue, yellow fever and St. Louis encephalitis. Some flaviviruses are mosquito borne and some tick-borne. The rubella virus and the lactic dehydrogenase-elevating virus are non-arthropod-borne togaviruses.
The envelope is derived from the plasma membrane of the infected host cell and consists of a lipid bilayer in which is embedded glycoproteins
The envelope glycoproteins are of three types, EI (MW 50,000), E2 (MW 50,000) and E3 (MW 10,000). The nucleocapsid is icosahedral, with T= 4 surface topology. Its protein component, C, (MW 30,000) is non-glycosylated. The M protein, which is present in rhabdoviruses, myxoviruses and paramyxoviruses, is absent in togaviruses
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The genome of togaviruses consists of a linear ssRNA strand which is itself infectious (i e. serves as mRNA), and is hence called a positive (+) strand. The 3' end of the RNA is polyadenylated and the 5' end is capped. In these respects togavirus RNA resembles mammalian RNA. It differs, however, in that only the terminals' guanosine has the methyl group, and that the adenosine at the other end of the 5'.5' triphosphate bond is not methylated.
The genome RNA is 42S, but the major intracellular RNA in. infected cells is subgenomic 26S RNA. 26S RNA nucleotide sequences are contained within the 42S RNA, and it represents the 3' end of the latter. Both these RNAs appear to serve as mRNAs in infected cells. 42S RNA has two initiation codons far protein synthesis, AUG-1 and AUG-2. Only AUG-1, however, serves as an initiator for protein synthesis.
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The internal initiation site, AUG-2, thus appears to be inactive or cryptic. This is true for all eukaryotic mRNAs characterized so far. A polyprotein is synthesized, which is cleaved to form the viral enzymes . There maybe four of these, of which two appear to be involved in an RNA replicase activity. 268 RNA appears to act as an mRNA, initiating protein synthesis at AUG-2.
It directs the synthesis of a polyprotein which is distinct from that synthesized by 42S-RNA, The polyprotein is cleaved into the capsid protein (C) and the envelope protein a (EI, E2, E3). In SFV the polypeptide synthesized by 428 RNA begins with fMet. Ala, white that synthesized by 428 RNA begins with fMet, AsN. Two other RNA species, 388 and 338, can be resolved between 42S and 268 RNAs. 388 RNA is a conformational variation of 428 RNA, and 338 RNA of 26 RNA
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