Reverse Transcriptase

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Two enzymes in one: A retroviral RT can perform several functions. As its name indicates, it can build DNA strands on an RNA template. This reaction is performed in the polymerase active site, formed by two sets of arms surrounding the RNA template and DNA (top of illustration). After building the DNA strand, the enzyme then removes the original RNA strand by cleaving it into pieces. This is performed by a nuclease active site at the opposite end of the enzyme. Finally, it builds a second DNA strand matched to the one that was just created to form the final DNA double helix. This reaction is also performed by the polymerase site.

Reverse transcriptase (/tran-SKRIP-tayz/), also RNA-dependent DNA polymerase (or simply RT for short): the enzyme used by retroviruses to create DNA copies of their RNA genomes, an essential step in their cycle of replication. It produces a single strand of DNA using an RNA template. In a natural setting, the template is the genome of a retrovirus. However, reverse transcriptase is also used in the laboratory to make cDNAs (complementary DNA sequences) from RNA sequences. Reverse transcription begins with the binding of a primer tRNA to the primer binding site.

The retroviral RT performs a remarkable feat, reversing the normal flow of genetic information, but does a sloppy job. The polymerases that normally make DNA and RNA in cells are very accurate and make very few mistakes. An RT enzyme, on the other hand, makes many. The RT of the AIDS virus HIV makes about one mistake in every 2,000 bases it copies (this rate is thousands of times higher than that of an ordinary polymerase).

One might suppose this would cause serious problems. But, in fact, this high error rate allows HIV to rapidly mutate and produce drug-resistant strains. Fortunately, modern treatments combining several drugs are often effective in combating this problem since the virus cannot mutate to evade all of them at the same time. ABOUT THE DISCOVERY OF REVERSE TRANSCRIPTASE | DIAGRAM OF REVERSE TRANSCRIPTION

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