Reverse transcriptase

Some tiny germs use a special tool.

This tool turns one kind of code into another kind. It turns air into a new code. This helps the germs make copies of themselves.

Some germs use this to live in people. Other cells use it too. It helps cells keep their parts healthy. Scientists study this tool in labs. It helps them learn how germs work. This can help us stay well.

Some tiny germs use a special tool to make copies. This tool is an enzyme called reverse transcriptase.

Most living things use DNA to hold their code. Some viruses use RNA instead. Reverse transcriptase changes RNA into DNA. This way, the virus can hide its code inside a host cell.

This tool works in three steps. First, it builds a DNA strand using the RNA as a guide. Second, a part called RNase H breaks down the old RNA. Third, it builds a second DNA strand. This makes a complete double-stranded DNA copy.

This tool is not perfect. It makes mistakes often. These mistakes are called mutations. Mutations can even help some germs resist drugs. Scientists like Howard Temin and David Baltimore found these tools in 1970. They won a Nobel Prize for their work. Today, scientists use these enzymes in labs. They use them to study genes and find new ways to treat sickness.

One drug, called zidovudine, works by stopping this enzyme.

Reverse transcriptase is a special enzyme used to change RNA into DNA. This process is called reverse transcription. Most living things use DNA as their main code, but some viruses use RNA instead. For these viruses, the enzyme is a vital tool for survival. Without it, a virus like HIV could not put its code into a host cell. This would mean the virus could not make copies of itself.

This enzyme works through three main steps to build a double-stranded DNA copy. First, it uses a piece called a primer to start building a DNA strand from the RNA. Next, a part of the enzyme called RNase H breaks down the original RNA template. Finally, the enzyme uses the remaining pieces to build a second DNA strand. This creates a complete, double-stranded piece of DNA.

Scientists discovered these amazing tools in 1970. Howard Temin found them at the University of Wisconsin–Madison. At the same time, David Baltimore isolated them at MIT. They were studying different types of viruses, like the murine leukemia virus. Because of their big discovery, they shared the 1975 Nobel Prize in Physiology or Medicine. This prize recognized how important these enzymes are to science.

Different enzymes have different sizes and parts. The HIV-1 reverse transcriptase has two subunits weighing 66 and 51 kDa. The M-MLV enzyme from the Moloney murine leukemia virus is a single 75 kDa part. Another type, from the avian myeloblastosis virus, has parts weighing 63 and 95 kDa. These enzymes are not perfect and make many mistakes. These mistakes are called mutations, and they happen because the enzyme lacks a way to check its work.

We can see how this enzyme works in many places. In our own bodies, an enzyme called telomerase uses this method to help maintain our chromosomes. Scientists also use these enzymes in laboratories every day. They use them for tasks like RNA sequencing and PCR tests. Even medicines like zidovudine are designed to work by stopping this enzyme. It is a tiny tool that has a huge impact on biology.

Reverse transcriptase (RT) is a specialized enzyme that performs reverse transcription. This is the process of converting RNA into DNA. In the classical central dogma of biology, information typically flows from DNA to RNA. Reverse transcriptase expands this understanding by allowing information to move from RNA back to DNA. This enzyme is vital for many life forms. It helps certain viruses replicate their genomes within a host. It also helps retrotransposons move within a genome. In eukaryotic cells, a specific version helps maintain the ends of chromosomes.

The mechanism of reverse transcription is a complex, multi-step biochemical process. It begins when a lysyl tRNA acts as a primer. This primer attaches to a specific site on the viral RNA called the primer-binding site (PBS). The enzyme then uses its RNA-dependent DNA polymerase activity to add DNA nucleotides. These are added to the 3′ end of the primer to create a DNA strand complementary to the RNA. Following this, a domain called RNase H degrades parts of the original RNA template. This degradation is necessary to allow the synthesis of the second DNA strand.

To create a complete double-stranded cDNA, the enzyme performs several specific actions. First, the tRNA primer "jumps" to the 3′ end of the viral genome. This allows the newly synthesized DNA to hybridize with the complementary R region on the RNA. The enzyme then performs DNA-dependent DNA polymerase activity. This activity copies the first DNA strand into a second, antisense strand. This sequence of events results in a double-stranded DNA molecule. Once formed, this DNA can be integrated into the host genome by an enzyme called integrase. This allows the virus to use the host's own machinery to make new RNA copies.

There are several distinct types of reverse transcriptases studied by scientists. The HIV-1 reverse transcriptase, used by the human immunodeficiency virus, consists of two subunits. These subunits have molecular weights of 66 kDa and 51 kDa. In contrast, the M-MLV reverse transcriptase from the Moloney murine leukemia virus is a single 75 kDa monomer. Another example is the AMV reverse transcriptase from the avian myeloblastosis virus. It is composed of two subunits weighing 63 kDa and 95 kDa. Finally, telomerase is a reverse transcriptase found in eukaryotes that maintains telomeres.

The history of this discovery is tied to two major scientific achievements in 1970. Howard Temin discovered reverse transcriptases at the University of Wisconsin–Madison using Rous sarcoma virions. Independently, David Baltimore isolated the enzyme at MIT from murine leukemia virus and Rous sarcoma virus. Their work changed how we understand genetic flow. Because of these discoveries, Temin and Baltimore shared the 1975 Nobel Prize in Physiology or Medicine. This prize was shared with Renato Dulbecco. Before the term "reverse transcriptase" was used, the enzyme was called RNA-dependent DNA polymerase.

Reverse transcription is known for being an extremely error-prone process. This is because the enzyme lacks proofreading ability, which most DNA polymerases possess. These errors lead to mutations, which can result in things like drug resistance in viruses. For example, the error rate for AMV is approximately 1 in 17,000 bases. For M-MLV, the error rate is about 1 in 30,000 bases. These mutations accumulate quickly due to the lack of a correction mechanism. Interestingly, this error-prone nature might even contribute to the diversity of life through template switching.

Today, reverse transcriptase has many important connections to modern science and medicine. In laboratories, it is used for molecular cloning, RNA sequencing, and polymerase chain reaction (PCR). It is also essential for genome analysis. In medicine, drugs like zidovudine (AZT) are used to inhibit this enzyme. By stopping the enzyme, the drug can help prevent the virus from replicating. Scientists are also working on new tools like the reverse transcribing xenotranscriptase (RTX). This engineered enzyme can both copy and proofread RNA, which could improve the accuracy of future sequencing tests.