RNA-dependent DNA Polymerase

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  • RNA-dependent DNA polymerase (RdDp), commonly known as reverse transcriptase, is an enzyme that synthesizes complementary DNA (cDNA) from an RNA template. 
  • This enzymatic activity is most famously associated with retroviruses, such as HIV (Human Immunodeficiency Virus), but it is also found in retrotransposons, hepatitis B virus, and certain mobile genetic elements in both prokaryotic and eukaryotic genomes. The discovery of reverse transcriptase in the early 1970s by Howard Temin and David Baltimore fundamentally reshaped molecular biology, challenging the traditional view of the “central dogma” that information flows strictly from DNA to RNA to protein.
  • Reverse transcriptase is a hallmark of the Retroviridae family. Upon infection of a host cell, the viral RNA genome is released and used as a template by the reverse transcriptase to produce a double-stranded DNA copy. This cDNA is then integrated into the host genome by another viral enzyme, integrase, forming a provirus. Once integrated, the viral genome is transcribed and translated using host machinery, allowing for the production of new viral particles.
  • Structurally and functionally, reverse transcriptase is a multifunctional enzyme with three main activities:
    • RNA-dependent DNA polymerase activity, which synthesizes DNA from the RNA genome.
    • RNase H activity, which degrades the RNA strand of the RNA-DNA hybrid, enabling second-strand DNA synthesis.
    • DNA-dependent DNA polymerase activity, which synthesizes the second DNA strand to complete the double-stranded viral DNA.
  • Reverse transcriptase lacks the high-fidelity proofreading mechanisms of cellular DNA polymerases, resulting in a high mutation rate. This contributes to viral diversity and rapid evolution, particularly in retroviruses like HIV, where mutation facilitates immune evasion and the emergence of drug resistance. The error-prone nature of reverse transcription is both a challenge for antiviral treatment and a driving force for viral adaptability.
  • In addition to its role in retroviral replication, RdDp plays a central part in the life cycle of hepatitis B virus (HBV), a DNA virus that replicates through an RNA intermediate. HBV reverse transcriptase synthesizes DNA within viral particles from an RNA pre-genome via reverse transcription, making it unique among DNA viruses.
  • Beyond virology, RNA-dependent DNA polymerase is a crucial tool in biotechnology and molecular biology. The ability of reverse transcriptase to generate DNA from RNA is the basis for reverse transcription polymerase chain reaction (RT-PCR), a widely used method for detecting RNA expression, diagnosing infections (such as SARS-CoV-2), and studying gene regulation. Enzymes like Moloney murine leukemia virus (M-MLV) reverse transcriptase and avian myeloblastosis virus (AMV) reverse transcriptase are commonly used in laboratory applications.
  • From a therapeutic perspective, reverse transcriptase is a key antiviral drug target, particularly in the treatment of HIV/AIDS. Reverse transcriptase inhibitors (RTIs) are classified into two main groups:
    • Nucleoside/nucleotide RT inhibitors (NRTIs), such as zidovudine (AZT) and tenofovir, mimic natural nucleotides and cause premature chain termination during DNA synthesis.
    • Non-nucleoside RT inhibitors (NNRTIs), such as efavirenz and nevirapine, bind to a non-active site on the enzyme, inducing conformational changes that inhibit function.
  • These inhibitors form the backbone of combination antiretroviral therapy (ART), which has significantly improved the prognosis for people living with HIV. However, the emergence of drug-resistant mutations remains a major obstacle, necessitating continuous development of new drugs and treatment strategies.
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