Reverse transcriptase
In biochemistry, a reverse transcriptase, also known as RNA-dependent DNA polymerase, is a DNA polymerase enzyme that transcribes single-stranded RNA into single-stranded DNA. Normal transcription involves the synthesis of RNA from DNA; hence, reverse transcription is the reverse of this.
Reverse transcriptase was discovered by Howard Temin at the University of Wisconsin-Madison, and independently by David Baltimore in 1970. The two shared the 1975 Nobel Prize in Physiology or Medicine with Renato Dulbecco for their discovery.
Commonly used examples of reverse transcriptases include:
- HIV-1 reverse transcriptase from the human immunodeficiency virus type 1 (PDB: 1HMV)
- M-MLV reverse transcriptase from the Moloney murine leukemia virus
- AMV reverse transcriptase from the avian myeloblastosis virus
- Telomerase reverse transcriptase that maintains the telomeres of eukaryotic chromosomes

# Function

## Viruses
The enzyme is encoded and used by reverse-transcribing viruses, which use the enzyme during the process of replication. Reverse-transcribing RNA viruses, such as retroviruses, use the enzyme to reverse-transcribe their RNA genomes into DNA, which is then integrated into the host genome and replicated along with it. Reverse-transcribing DNA viruses, such as the hepadnaviruses, transcribe their genomes into an RNA intermediate and then, using reverse transcriptase, back into DNA.

## Eukaryotes
Self-replicating stretches of eukaryotic genomes known as retrotransposons utilise reverse transcriptase to move from one position in the genome to another via a RNA intermediate. They are found abundantly in the genomes of plants and animals. Telomerase is another reverse transcriptase found in many eukaryotes, including humans, which carries its own RNA template; this RNA is used as a template for DNA replication.

## Prokaryotes
Reverse transcriptases are also found in bacterial retrons, distinct sequences which code for reverse transcriptase, and are used in the synthesis of msDNA.

# Structure
Reverse transcriptase enzymes include an RNA-dependent DNA polymerase and a DNA-dependent DNA polymerase, which work together to perform transcription. In addition to the transcription function, retroviral reverse transcriptases have a domain belonging to the RNase H family which is vital to their replication.

# Replication fidelity
Reverse transcriptase has a high error rate when transcribing RNA into DNA since, unlike DNA polymerases, it has no proofreading ability. This high error rate allows mutations to accumulate at an accelerated rate relative to proofread forms of replication. The commercially available reverse transcriptases produced by Promega are quoted by their manuals as having error rates in the range of 1 in 17,000 bases for AMV and 1 in 30,000 bases for M-MLV

# Applications

## Antiviral drugs
As HIV uses reverse transcriptase to copy its genetic material and generate new viruses (part of a retrovirus proliferation circle), specific drugs have been designed to disrupt the process and thereby suppress its growth. Collectively, these drugs are known as reverse transcriptase inhibitors and include the nucleoside and nucleotide analogues zidovudine (Retrovir),  lamivudine (Epivir) and tenofovir (Viread), as well as non-nucleoside inhibitors, such as nevirapine (Viramune).

## Molecular biology
Reverse transcriptase is commonly used in research to apply the polymerase chain reaction technique to RNA in a technique called reverse transcription polymerase chain reaction (RT-PCR).  The classical PCR technique can only be applied to DNA strands, but with the help of reverse transcriptase, RNA can be transcribed into DNA, thus making PCR analysis of RNA molecules possible. Reverse transcriptase is also used to create cDNA libraries from mRNA. The commercial availability of reverse transcriptase greatly improved knowledge in the area of molecular biology as, along with other enzymes, it allowed scientists to clone, sequence and characterise DNA.