Rna

I 8.34 The process of reproduction differs in positive-strand RNA viruses and negative-strand RNA viruses.

RNA viruses capable of integrating into the genome of their hosts, much as temperate phages insert themselves into bacterial chromosomes, are called retroviruses (FIGURE 8.35a). Because the retroviral genome is RNA, whereas that of the host is DNA, a retrovirus must produce reverse transcriptase, an enzyme that synthesizes complementary DNA (cDNA) from either an RNA or a DNA template. A retrovirus uses reverse transcriptase to make a double-stranded DNA copy from its single-stranded RNA genome. The DNA copy then integrates into the host chromosome to form a provirus, which is replicated by host enzymes when the host chromosome is duplicated ( FlGURE8.35b).

When conditions are appropriate, the provirus undergoes transcription to produce numerous copies of the original RNA genome. This RNA codes for viral proteins and serves as genomic RNA for new viral particles. As these viruses escape the cell, they collect patches of the cell membrane to use as their envelopes.

All known retroviral genomes have in common three genes: gag, pol, and env ( FIGURE 8.36), each encoding a precursor protein that is cleaved into two or more functional proteins. The gag gene encodes the three or four proteins that make up the viral capsid. The pol gene codes for reverse transcriptase and an enzyme, called integrase, that inserts the viral DNA into the host chromosome. The env gene codes for the glycoproteins, which appear on the viral envelope that surrounds the viral capsid.

Some retroviruses contain oncogenes (Chapter 20) that may stimulate cell division and cause the formation of tumors. The first retrovirus to be isolated, the Rous sarcoma virus, was originally recognized by its ability to produce connective-tissue tumors (sarcomas) in chickens.

The human immunodeficiency virus (HIV) is a retro-virus that causes acquired immune deficiency syndrome. AIDS was first recognized in 1982, when a number of homosexual males in the United States began to exhibit symptoms of a new immune-system-deficiency disease. In that year, Robert Gallo proposed that AIDS was caused by a retrovirus. Between 1983 and 1984, as the AIDS epidemic became widespread, the HIV retrovirus was isolated from AIDS patients.

HIV is thought to have appeared first in Africa in the 1950s or 1960s. It is closely related to several retroviruses found in monkeys and may have evolved when a monkey retrovirus mutated and infected humans. HIV is transmitted by sexual contact between humans and through any type of blood-to-blood contact, such as that caused by the sharing of dirty needles by drug addicts. Until screening tests could identify HIV-infected blood, transfusions and clotting factors used by hemophiliacs also were sources of infection.

HIV principally attacks a class of blood cells called helper T lymphocytes ( FIGURE 8.37). HIV enters a helper T cell, undergoes reverse transcription, and integrates into the chromosome. The virus reproduces rapidly, destroying the T cell as new virus particles escape from the cell. Because helper T cells are central to immune function and

Retrovirus k \ I

Core-shell proteins

Viral protein coat (capsid)

Reverse transcriptase t I %

-Viral-envelope glycoprotein

Core-shell proteins

Envelope

Reverse transcriptase t I %

Retrovirus

Viral protein coat (capsid)

Single-stranded RNA genome (two copies)

I 8.35 A retrovirus uses reverse transcription to incorporate its RNA into the host DNA. (a) Structure of a typical retrovirus. Two copies of the single-stranded RNA genome and reverse transcriptase enzyme are shown enclosed within a protein capsid. The capsid is surrounded by a viral envelope that is studded with viral glycoproteins. (b) The retrovirus life cycle.

are destroyed in the infection, AIDS patients have a diminished immune response — most AIDS patients die of secondary infections that develop because they have lost the ability to fight off pathogens.

The HIV genome is 9749 nucleotides long and carries gag, pol, env, and six other genes that regulate the life cycle of the virus. HIV's reverse transcriptase is very error prone, giving the virus a high mutation rate and allowing it to evolve rapidly, even within a single host. This rapid evolution makes the development of an effective vaccine against HIV particularly difficult.

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