Retroviruses

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Chapter 26

Retroviruses

Retroviridae: General Features

• Midsized viruses with an enveloped capsid containing two copies of a single-stranded (+) RNA genome, transfer RNA (tRNA), reverse transcriptase, integrase, and protease.

• Two human pathogens—human immunodeficiency virus (HIV) and human T lymphotropic virus (HTLV)—are retroviruses.

Retroviral genes

1. All retroviral genomes contain three genes—gag, pol, and env—and are flanked by long-terminal repeats (Table 26-1).

TABLE 26-1

Retrovirus Genes and Their Function

Gene Virus Function
gag All Group-specific antigen: core and capsid proteins
int All Integrase
pol All Polymerase: reverse transcriptase, protease, integrase
pro All Protease
env All Envelope: glycoproteins
tax HTLV Transactivation of viral and cellular genes
tat HIV-1 Transactivation of viral and cellular genes
rex HTLV Regulation of RNA splicing and promotion of export to cytoplasm
rev HIV-1 Regulation of RNA splicing and promotion of export to cytoplasm
nef HIV-1 Alteration of cell activation signals; progression to AIDS (essential)
vif HIV-1 Virus infectivity, promotion of assembly, blocks a cellular antiviral protein
vpu HIV-1 Facilitates virion assembly and release, decrease of cell surface CD4
vpr (vpx*) HIV-1 Transport of complementary DNA to nucleus, arresting of cell growth
LTR All Promoter, enhancer elements

HIV, human immunodeficiency virus; HTLV, human T lymphotropic virus; LTR, long-terminal repeat (sequence).

*In HIV-2.

From Murray PR, Rosenthal KS, Pfaller MA: Medical Microbiology, 6th ed. Philadelphia, Mosby, 2009, Table 64-2.

2. Complex retroviruses, such as HIV, have several other genes encoding auxiliary and regulatory proteins (e.g., nef, tat, and rev)

Key HIV proteins (Fig. 26-1)

Replication

1. Attachment and fusion

2. Formation of HIV provirus

3. Viral messenger RNA (mRNA) and genome replication

4. Assembly

Genetic variation

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