Genomes of MLV, an oncogenic retrovirus, and HIV, a lentivirus.

The proviral (DNA) form of the viral nucleic acid (the form of the viral genome in infected cells) is shown. The 2 viruses share a similar organization for the gag, pol, and env genes.Gag and pol encode the viral core and replication enzymes, respectively, and are expressed directly from the viral promoter in the LTR. The env gene, expressed from a singly spliced mRNA, encodes the envelope glycoprotein. The additional accessory genes of HIV, expressed from multiply spliced mRNA, are shown. Although the roles of many are incompletely understood, they are important in the regulation of viral gene expression and mediation of infectivity. MLV, murine leukemia virus; HIV-1, human immunodeficiency virus type 1; LTR, long terminal repeat.