Reverse transcriptase is a specialized enzyme that synthesizes DNA using an RNA template. This process, reverse transcription, runs opposite the usual flow of genetic information in cells, where transcription normally copies DNA into RNA. Reverse transcriptases are functionally related to DNA polymerase-type proteins and carry out the synthesis of complementary DNA (cDNA) from single-stranded RNA and often the synthesis of the second complementary DNA strand.

Mechanism and domains

Most reverse transcriptases have at least two enzymatic activities: an RNA-dependent DNA polymerase that copies the RNA template into single-stranded cDNA, and an RNase H activity that degrades the RNA strand of an RNA–DNA hybrid to allow synthesis of the complementary DNA strand. Some enzymes also possess DNA-dependent DNA polymerase activity for second-strand synthesis. Biochemically they vary in fidelity, processivity and temperature tolerance, properties that influence both viral replication and laboratory performance.

Biological roles

The best-known biological users of reverse transcriptase are retroviruses, which package the enzyme to convert their RNA genomes into DNA that can integrate into a host genome. Cellular activities also employ reverse transcription: telomerase extends chromosome ends by adding DNA repeats using an internal RNA template, and many eukaryotic genomes contain mobile genetic elements (retrotransposons) that replicate via an RNA intermediate and reverse transcription.

Laboratory uses

Purified reverse transcriptases are essential tools in molecular biology. They are used to produce cDNA from messenger RNA for cloning, sequencing and expression analysis, and they are the first step in reverse transcription polymerase chain reaction (RT-PCR) and quantitative RT-PCR (qRT-PCR) diagnostics. Commonly used enzymes in research derive from viral sources such as Moloney murine leukemia virus or avian myeloblastosis virus; engineered variants improve thermostability and reduce error rates for demanding applications.

Practical considerations

  • Priming methods: cDNA synthesis can be initiated with oligo(dT) primers that anneal to poly(A) tails, random primers that sample all RNA, or gene-specific primers for targeted studies.
  • Fidelity and inhibitors: different reverse transcriptases make errors at different rates; in medicine, inhibitors of viral reverse transcriptases form a main class of antiretroviral drugs and are selected to reduce viral replication.
  • Downstream steps: after cDNA synthesis, DNA templates are compatible with cloning, PCR amplification and sequencing workflows.

History and impact

The discovery that an enzyme could copy RNA into DNA reshaped understanding of genetic information flow and genome dynamics. Reverse transcription underlies key aspects of virus biology, genome evolution and modern molecular diagnostics. For foundational context see entries on enzymes, the chemistry of RNA and the structure of DNA, and consult resources on transcription and the family of DNA polymerase-related proteins for mechanism and classification.