Reverse transcriptase (RT) is an enzyme that plays a unique role in molecular biology. Unlike most biological processes where DNA is transcribed into RNA, RT catalyzes the synthesis of DNA using an RNA template, a process known as reverse transcription. Its discovery in 1970 revolutionized the field, expanding the central dogma of molecular biology, which previously described a one-way flow from DNA to RNA to protein. The presence of RT in various organisms highlights its diverse and fundamental roles.
In Retroviruses
Reverse transcriptase is most famously associated with retroviruses, a family of RNA viruses including the human immunodeficiency virus (HIV). These viruses carry their genetic information as single-stranded RNA and use RT to convert it into a double-stranded DNA copy upon infecting a host cell. This viral DNA then integrates into the host cell’s genome. The integration of this viral DNA, known as a provirus, allows the host cell’s machinery to transcribe viral genes, leading to the production of new viral RNA and proteins necessary for replication.
HIV relies on reverse transcriptase for its life cycle, making the enzyme a target for antiviral therapies. Medications like nucleoside reverse transcriptase inhibitors (NRTIs) and non-nucleoside reverse transcriptase inhibitors (NNRTIs) interfere with RT’s activity. NRTIs act as faulty building blocks, terminating the DNA chain during synthesis, while NNRTIs bind to the enzyme, altering its function. These inhibitors have transformed HIV infection into a manageable chronic condition.
In Eukaryotic Genomes
Reverse transcriptase activity is also an integral part of eukaryotic genomes, contributing to chromosome maintenance and genomic evolution. One example is telomerase, an enzyme that adds repetitive DNA sequences to the ends of chromosomes, called telomeres. Telomeres protect chromosome ends from degradation and fusion, and their shortening with each cell division is linked to cellular aging. Telomerase contains a reverse transcriptase component (TERT) that uses an RNA template within the enzyme to synthesize telomeric DNA repeats.
Telomerase activity is low or absent in most somatic cells, leading to telomere shortening over time. However, in stem cells, germ cells, and most cancer cells, telomerase is highly active, enabling these cells to maintain telomere length and divide indefinitely. This sustained telomere maintenance allows cancer cells to bypass aging limits and proliferate uncontrollably. Targeting telomerase activity is an area of ongoing research for potential cancer therapies.
Another presence of reverse transcriptase in eukaryotes is within retrotransposons, mobile genetic elements that “jump” or copy themselves to different locations in the genome. These elements, such as LINE-1 (L1) elements, replicate via an RNA intermediate, requiring RT to convert their RNA back into DNA for insertion into new genomic sites. LINE-1 elements encode their own reverse transcriptase, which is crucial for their retrotransposition process. Retrotransposons make up a substantial portion of eukaryotic genomes, and their activity can influence genomic stability and evolution.
In Prokaryotic Systems
Reverse transcriptase activity extends beyond eukaryotes and viruses, also appearing in prokaryotic systems. Group II introns are self-splicing RNA molecules found in bacteria. These introns are mobile genetic elements that possess an intron-encoded protein (IEP) with reverse transcriptase activity. This RT enables the introns to move within the genome through a process called retrotransposition.
During retrotransposition, the excised intron RNA can reverse splice into a DNA target site, and the intron-encoded reverse transcriptase then synthesizes a DNA copy of the intron. This allows the intron to insert itself into new locations. The widespread occurrence of group II introns and their associated reverse transcriptase in bacteria suggests an ancient role for reverse transcription in genetic mobility across different domains of life. These bacterial RTs are phylogenetically related to those in eukaryotic retrotransposons and telomerase, indicating a shared evolutionary history.