The DNA version of a retrovirus integrates into the chromosomes of the host cell.

Although a retrovirus genome consists of single-stranded RNA, the virus particle actually contains two identical ssRNA molecules (Fig. 17.25).These are bound together by base pairing with two molecules of transfer RNA stolen from the previous host cell. In addition to binding the two virus RNA molecules together in the virion, the tRNA has another role. It is used as a primer by the reverse transcriptase when starting a new strand of DNA.

When a retrovirus enters a new cell the outer envelope merges with the host cell membrane and the core particle or nucleocapsid is released into the cytoplasm and disassembles, liberating the ssRNA. One of the two ssRNA molecules is then used by the reverse transcriptase to make the double stranded DNA form of the retrovirus. This dsDNA now enters the nucleus of the host cell. The retrovirus dsDNA has repeated sequences at each end, the long terminal repeats (LTRs). These are direct not inverted repeats and are required for integration of the retrovirus DNA into the host cell DNA (Fig. 17.26A). The site of integration is more or less random and once integrated, the retrovirus DNA is there to stay. It has become a permanent part of the host cell chromosome.

The integrated retrovirus DNA is transcribed to give messenger RNA molecules that are capped and tailed just like the mRNA of a typical eukaryotic cell (Fig. 17.26B; see Ch. 12 for processing of mRNA). The retrovirus RNA molecules exit the nucleus to the cytoplasm. Some are translated to produce viral proteins and others are packaged into virus particles. Thus the virus particle actually contains mRNA molecules. However, when infecting a new cell, this mRNA is used as a template to make DNA instead of being used as a message. Because the incoming viral RNA does not get trans-

long terminal repeats (LTRs) Direct repeats found at the ends of the retrovirus genome which are required for integration of the retrovirus DNA into the host cell DNA

FIGURE 17.25 Retrovirus Particle Contains Two ssRNA Molecules

FIGURE 17.25 Retrovirus Particle Contains Two ssRNA Molecules

The genome of retroviruses has a unique structure. Two molecules of ssRNA are held together by base pairing. In addition, two tRNA molecules from the previous host are also base-paired with the two ssRNA molecules. The tRNA acts as a primer for reverse transcriptase.

A Long terminal repeat (LTR)

LTR dsDNA of retrovirus LTR +

Host chromosome

LTR Retrovirus DNA LTR

FIGURE 17.26 Integration and Transcription of Retrovirus DNA

(A) After double-stranded retrovirus DNA is made by reverse transcriptase, the dsDNA integrates into the host chromosome. The DNA is flanked by two long terminal repeats (LTR) that facilitate the insertion. (B) Once integrated into the host chromosome, the retrovirus DNA is transcribed and expressed as any other gene in the host cell. The retrovirus RNA is processed by addition of the 7-methyl-G-cap and the poly(A) tail.

B Host chromosome

LTR Retrovirus DNA LTR


(by host cell enzymes)

Viral RNA

CAP AND TAIL (by host cell enzymes)

Viral mRNA

Cap PolyA tail

Subviral Infectious Agents 477

Use of Reverse Transcriptase to Make cDNA

Reverse transcriptase is now widely used in genetic engineering for making DNA copies of RNA (see Ch. 22 for details). It is especially useful in obtaining copies of eukaryotic genes that lack the non-coding intervening sequences. Many genes from higher animals have more non-coding DNA than coding sequence and for many purposes the coding sequence alone is easier to handle. Such complementary DNA (cDNA) copies of eukaryotic genes are made using the messenger RNA (which lacks introns due to processing) as template. Although the cDNA lacks the introns found in the original natural DNA gene it still encodes the same protein.

Retroviruses sometimes pick up host genes and carry them to another animal.

lated, several molecules of reverse transcriptase must be packaged along with the RNA in the retrovirus particle.

As with integrated bacterial viruses, retroviruses can transduce host cell genes. Chromosomal DNA close to the retrovirus integration site may be fused to virus sequences by deletion of intervening DNA. The fused DNA may then be transcribed as a unit. The RNA may be processed (removing any introns present in the original host genes) and packaged into virus particles. As with bacteriophage lambda, this first gives a defective virus, in which host genes have replaced virus genes. However, recombination with a wild-type retrovirus can generate a functional virus that carries host genes as well as a complete retrovirus genome. Occasional retroviruses carry onco-genes and may cause cancer. Oncogenes are genes that are involved in regulating cell division in animals and when mutated cause cancer. The oncogenes carried by viruses are originally of animal origin and were been picked up by the virus from some previous host animal. Although these cancer-causing viruses have attracted most notice, in principle any host gene close to the site of integration could be moved by retrovirus transduction.

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