MOLECULAR BIOLOGY OF THE ALPHAVIRUS
Alphaviruses consist of an enveloped nucleocapsid containing a single-stranded RNA genome of the positive sense, some 12 kb in length. The viral non-structural proteins, nsP1, nsP2, nsP3 and nsP4 are encoded on the left two-thirds of the genome and are the first viral products to be translated following viral entry. They are initially translated as a polyprotein that is cleaved into four proteins, which form the viral replicase. Upon assembly, the replicase binds to an untranslated region in the 5’ end of the viral genome, mediating the synthesis of the complementary negative strand of RNA. The viral replicase then binds to the negative strand, which serves as the template for the synthesis of multiple copies of the positive strand genome. Moreover, the negative strand also acts as the template for the structural genes located on the right one-third of the genome (see diagram one).
Diagram One: Alphavirus Genome
After activation of the sub-genomic promoter, the viral replicase mediates synthesis of the 26S RNA species that serves as the message encoding the structural proteins. The RNA genome is subsequently packaged into the nucleocapsid and new viral particles bud from the cell by a similar mechanism adopted by the retroviruses. Given that alphaviruses are able to amplify RNA in the cytoplasm, independently of the nucleus, they have been extensively explored as potential gene transfer agents.
