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Retroviral Genome

The RNA genome in the retrovirion is diploid, consisting of two copies of a 7- to 10-kb single-stranded (ss) RNA that is capped and polyadenylated. The two copies of the genome are normally identical, but during mixed infection hybrid genomes can result. They are joined near their 5’ ends, and perhaps in other regions as well, by hydrogen bonds.

All Retroviruses encode the four genes called gag, pro, pol, and env, which are always found in his order in the genome. The name gag comes from group-specific antigen, because the proteins encoded in this gene are more highly conserved, and therefore more widely cross reactive immunologically, than are the envelope proteins of the virion. The peptides derived from the Gag polyprotein, the precursor polyprotein encoded in the gag gene, form the capsid of the retrovirion. The pro gene encodes a protease (PR) that is required for the processing of Gag, and pol encodes three activities, RT,RNase H, and integrase (IN). RNase H forms a separate domain in the RT-RNase H protein, but functions as an integral component of RT. IN is required for the integration of the dsDNA copy of the retroviral genome into the host chromosome. The fourth gene, env, encodes the envelope glycoprotein at the surface of the envelope retrovirion. The primary product is Env, which is processed by cleavage to form an N-terminal external protein called SU (for surface) and a C-terminal protein that spans the membrane called TM (for transmembrane). The order of genes in the provirus is the same as in the viral genome.

As described above, the genomes of complex Retroviruses contain a number of regulatory genes in addition to these four basic genes present in all Retroviruses genes will be described later on the next post.