Reverse transcriptase Rous sarcoma virus

Rous sarcoma virus reverse transcriptase second(s)... 

This enzyme is associated with the virions of RNA tumor viruses such as the Rous sarcoma virus (RSV). The enzyme has remarkable enzymatic activity in that it can catalyze several seemingly diverse steps in the synthesis of double-stranded DNA from the single-stranded RNA viral genome. The enzyme uses a tRNA for tryp-tophan as a primer to make a copy of DNA that is complementary to the viral RNA. The resulting RNA-DNA hybrid is converted to a double-stranded DNA molecule by ribon-uclease (RNase)H and DNA-dependent DNA polymerase activities that are intrinsic to reverse transcriptase. 

The first successful experience that used oligonucleotides to inhibit gene expression and virus replication was presented by Zamecnik and Stephenson in 1978 (24). They synthesized a 13-mer oligodeoxynucleotide complementary to the 5 and 3 reiterated terminal sequences of the Rous sarcoma virus 35S RNA and showed that exposure of infected fibroblasts to this oligomer led to a 99% decrease in reverse transcriptase activity in the medium, which also correlated with a decrease in cellular transformation, This study showed that such compounds may have a therapeutic advantage by specifically targeting genetic sequences that are critical to disease processes. 

It is now almost exactly 50 years since I went as a postdoctoral fellow learn about cell culture in Harry Rubin s group in the Virus Laboratory at the University of California at Berkeley. The use of cell cultures had created the breakthroughs in quantitative animal virology, which led, inter alia, to the production of polio vaccines (albeit at the cost of the lives of hundreds of thousands of rhesus monkeys, whose kidney cells were used to produce the viruses for the vaccines). We worked on Rous sarcoma virus (RSV) and chicken leucosis viruses in chick embryo fibroblast cell cultures. Rubin and Temin had developed an assay for RSV, based on the production of foci of virus-transformed fibroblasts, and Temin did the crucial experiments which showed that RSV, an RNA virus, made a DNA copy of itself, which was used to produce new virus particles. The enzyme involved was reverse transcriptase, and its discovery was one of the most important leaps forward in cell and molecular biology. 

Genome of the Rous sarcoma virus (RSV). The single-stranded RNA is about 10 kb. Four proteins are encoded in this retrovirus gag, a core protein pol, reverse transcriptase env, glycoprotein of the envelope and src, a nonessential viral gene whose product is responsible for cellular transformation. LTR, Long terminal repeat sequences present at the ends of each viral RNA molecule. 

In 1970, the enzyme reverse transcriptase (S17) was discovered in the particles of viruses such as Rous sarcoma virus. During the early hours of infection, the viral RNA genome is transcribed into DNA by reverse transcriptase. The viral DNA is then integrated into the cell s genome. 

Part or all of the viral genome becomes linearly inserted into chromosomes of transformed cells. For RNA viruses like Rous sarcoma virus, the viral genome must be converted to double-strand DNA before this insertion can occur. The viral enzyme that synthesizes DNA from a single-strand RNA template is called reverse transcriptase, and viruses containing reverse transcriptase are called retroviruses (see here). 

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