Concept 25 Some viruses store genetic information in RNA.
David Baltimore and Howard Temin explain work on the Rous sarcoma virus.
Hi, I’m David Baltimore. And, I'm Howard Temin. In the '70s, we were working on the Rous sarcoma virus (RSV) and other viruses that can cause tumors. RSV is named after Peyton Rous. In 1911, he found that this virus causes tumors in chickens. Rous passed an extract from tumor cells over filters that excluded any material larger than a virus. This filtrate produced tumors when injected into healthy chickens. Since anything larger than a virus was filtered out, the implications were that viruses caused tumors. But no one knew how the viruses worked to cause cancer. In the early '70's, I proposed that, during infection, tumor viruses integrate their DNA into the host cell DNA. However, this would prove a problem for RSV, which stores its genetic information as RNA, not DNA. David and I both became interested in how RSV might use its RNA as a template to make DNA. To figure this all out, I stripped the virus of its outer coat and used the inner core in a cell-free system. I added all the nucleotides; thymine was the radioactive tracer. The viral components in this cell-free system produced radioactive DNA. I knew the radioactive product was DNA because it was not affected by RNase. But the product was destroyed by the enzyme DNase. I did a few more experiments. I incubated the viral cores with trypsin — an enzyme that digests protein. I then added nucleotides to the trypsin-digested viral cores. I did not get any DNA product. Similarly, if I added RNase at the beginning of the reaction ... ... and then added nucleotides ... ... there also was no DNA product. I concluded that RSV has a special enzyme that uses RNA as a template to make DNA. I initially named this enzyme "RNA-dependent DNA polymerase," but it later became known as reverse transcriptase. Howard Temin independently did a similar set of experiments. Crick's Central Dogma — where information flows from DNA to RNA to protein — does not apply to viruses like RSV. A reverse flow of information from RNA to DNA happens first, and these viruses thus became known as retroviruses. Our discovery of reverse transcriptase helped to clarify the life cycle of RSV and other retroviruses. Retroviruses consist of an outer envelope, an inner capsid, and a central core containing the RNA template and the reverse transcriptase enzyme. When the virus infects a cell, the viral envelope fuses with the host cell membrane and the capsid disassembles — releasing the RNA and reverse transcriptase into the cytoplasm of the host cell. The reverse transcriptase then synthesizes a complementary DNA strand using the viral RNA as a template. Then the enzyme degrades the RNA and synthesizes a second DNA strand. This DNA makes its way into the nucleus and integrates into a chromosome of the host cell. The typical retrovirus RNA looks like this: it has two ends called long terminal repeats (LTR), and only three genes — gag, pol, and env. The LTRs are sequences that help the virus integrate into the host genome. Once integrated,the virus uses the host replication machinery to produce RNA — containing the LTR's and genes between them — and the other viral proteins. pol codes for the reverse transcriptase. gag encodes the capsid and other core proteins. ...while env produces components of the outer viral envelope. The proteins are then assembled into a viral particle with the viral RNA inside. Once it leaves the cell, the virus is capable of infecting other cells. Tumor viruses, like RSV, have picked up an additional gene; called an oncogene, src contributes to malignancy.
Scientists are working on ways to use retroviruses as vectors in gene therapies. If done correctly, retroviruses can be engineered to carry genes into the body.
What are the advantages or disadvantages of using retroviruses as vectors in gene therapy?