An infectious HIV particle consists of two identical strands of RNA packaged within a core of viral proteins and surrounded by a phospholipid bilayer envelope derived from the host cell membrane but including virally encoded membrane proteins (Fig. 20-3). The RNA genome of HIV is approximately 9.2 kb long and has the basic arrangement of nucleic acid sequences characteristic of all known retroviruses (Fig. 20-4). Long terminal repeats (LTRs) at each end of the genome regulate viral gene expression, viral integration into the host genome, and viral replication. The gag sequence encodes core structural proteins. The env sequence encodes the envelope glycoproteins gp120 and gp41, which noncovalently associate with each other and are required for infection of cells. The pol sequence encodes reverse transcriptase, integrase, and viral protease enzymes required for viral replication. In addition to these typical retrovirus genes, HIV-1 also includes six other regulatory genes, namely, the tat, rev, vif, nef, vpr, and vpu genes, whose products regulate viral reproduction and host immune evasion in various ways. The functions of these genes are summarized in Figure 20-4.
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