Reverse transcriptase

Reverse transcriptase
Name	Reverse transcriptase
EC number	2.7.7.49
CAS number	9068-38-6

Reverse transcriptase. This enzyme, an RNA-dependent DNA polymerase, catalyzes the synthesis of DNA from an RNA template, reversing the typical flow of genetic information. Its discovery in 1970 by Howard Temin and David Baltimore, working independently on the Rous sarcoma virus, fundamentally altered molecular biology and earned them the 1975 Nobel Prize in Physiology or Medicine. The enzyme is a defining component of retroviruses like HIV and is also encoded by retrotransposons and certain DNA viruses.

Introduction

The existence of reverse transcriptase was postulated by Howard Temin as part of his provirus hypothesis, which challenged the central dogma of molecular biology. Simultaneous confirmation by David Baltimore in the Rous sarcoma virus provided the first evidence that genetic information could flow from RNA to DNA. This groundbreaking work, recognized by the Nobel Assembly at Karolinska Institutet, revealed a critical mechanism for viral replication and genome evolution. The enzyme's activity is central to the life cycle of retroviruses and has been exploited in numerous biotechnological applications.

Function

The primary function of reverse transcriptase is to transcribe a single-stranded RNA genome into a complementary DNA (cDNA) strand. This process initiates at a primer, often a tRNA molecule packaged within the virion, and utilizes the enzyme's RNA-dependent DNA polymerase activity. Following synthesis, the enzyme's RNase H domain degrades the original RNA template, allowing a second DNA strand to be synthesized by its DNA-dependent DNA polymerase activity, forming a double-stranded DNA provirus. This DNA is then integrated into the host genome by the viral integrase enzyme, a key step in the infection cycle of HIV.

Structure

Reverse transcriptase is a multifunctional protein, typically existing as a heterodimer in viruses like HIV. The enzyme comprises several distinct domains, including the polymerase domain which contains sub-sites for nucleotide binding and catalysis. A structurally separate RNase H domain is responsible for degrading the RNA strand in the RNA-DNA hybrid. The structure, elucidated through techniques like X-ray crystallography, reveals a right-handed conformation resembling other polymerases, such as those from the bacteriophage family. Key structural motifs, including the "fingers," "palm," and "thumb" subdomains, facilitate template binding and processivity.

Types

Reverse transcriptases are categorized based on their origin and specific properties. Retroviral reverse transcriptases, such as those from Moloney Murine Leukemia Virus (M-MLV) and Avian Myeloblastosis Virus (AMV), are widely used in molecular biology. Cellular versions are encoded by retrotransposons like the Ty elements in Saccharomyces cerevisiae and LINE-1 elements in the human genome. Additionally, some DNA viruses, including Hepatitis B virus (HBV), which utilizes a reverse transcriptase during its replication within the hepatocyte nucleus, encode related enzymes. Telomerase, which maintains telomere ends, is a specialized reverse transcriptase containing an RNA template component.

Applications

The enzyme is indispensable in modern biotechnology and molecular diagnostics. Its ability to generate complementary DNA (cDNA) from messenger RNA (mRNA) is the foundational step in techniques like reverse transcription polymerase chain reaction (RT-PCR), crucial for detecting SARS-CoV-2 RNA. It is used in constructing cDNA libraries for genomic studies and in sequencing methodologies developed by companies like Illumina. Furthermore, reverse transcriptase is employed in gene cloning experiments and in the study of gene expression patterns in organisms from E. coli to humans.

Inhibitors

Inhibitors of reverse transcriptase are a major class of antiviral drugs, particularly in the treatment of HIV/AIDS and Hepatitis B virus infection. These are divided into nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs), such as zidovudine (AZT) and tenofovir, which are incorporated into the nascent DNA chain, and non-nucleoside reverse transcriptase inhibitors (NNRTIs), like efavirenz and nevirapine, which bind allosterically to the enzyme. The development of these inhibitors, pioneered by researchers like Anthony Fauci at the National Institutes of Health (NIH), has been central to Highly Active Antiretroviral Therapy (HAART), transforming HIV from a fatal diagnosis to a manageable chronic condition.

Category:Enzymes Category:Molecular biology Category:Virology