David Baltimore's Discovery of Reverse Transcriptase
The story of a discovery that shattered a fundamental dogma of biology.
In the late 1960s, a core principle of biology seemed unshakable: the "Central Dogma." This concept, famously articulated by Francis Crick, stated that genetic information flows in one direction only—from DNA to RNA to protein. It was a neat, one-way street. The idea that information could flow backwards from RNA to DNA was considered biological heresy 3 .
Yet, David Baltimore, then a young virologist at MIT, was not one to be constrained by dogma. His career, sparked by a transformative high school summer at the Jackson Laboratory in Bar Harbor, Maine, had been defined by a relentless pursuit of how viruses, particularly RNA viruses, operate 2 5 .
His work on poliovirus and vesicular stomatitis virus had already shown him that RNA could be copied into more RNA. When he turned his attention to RNA tumor viruses, a class of viruses known to cause cancer, he was prepared to follow the data wherever it led, even if it meant challenging one of biology's most fundamental beliefs 2 3 . His independent and curious nature, fostered by mentors who believed in giving students "as much freedom as possible," had primed him for a breakthrough 5 .
Specialized in RNA viruses and their replication mechanisms
Conducted groundbreaking work at Massachusetts Institute of Technology
Applied rigorous biochemical techniques to viral enzymes
By 1970, a puzzling contradiction had emerged. Howard Temin, another brilliant virologist, had long argued based on his work with RNA tumor viruses that these viruses must have a DNA intermediate in their life cycle. This "provirus hypothesis" suggested that the viral RNA genome was being copied into DNA, which then integrated into the host cell's own DNA, sometimes causing cancer 3 5 .
Without direct proof of an enzyme that could perform this reverse flow of information, Temin's idea was met with widespread skepticism, even from influential figures like Max Delbrück 5 .
Baltimore, working independently and from a different angle, was also circling the same problem. He was an expert in the enzymology of viruses—studying the specific enzymes viruses use to replicate. He had successfully identified the RNA-dependent RNA polymerase in poliovirus and decided to apply the same rigorous biochemical techniques to the RNA tumor viruses 2 5 . His approach was simple in concept: if Temin was right, these virus particles should contain an enzyme that can make DNA using an RNA template.
The established belief that genetic information flows only in one direction:
Temin's revolutionary proposal that RNA viruses use a DNA intermediate:
Baltimore's genius lay in designing a clear, direct experiment to test for the heretical enzyme. He later explained that he followed a paradigm established by Arthur Kornberg, who had pioneered the study of DNA-synthesizing enzymes 5 .
Baltimore obtained samples of two RNA tumor viruses, the Rauscher murine leukemia virus and the Rous sarcoma virus, from a virus repository at the NIH 5 .
He disrupted the virus particles to release their internal contents, creating a crude extract. To this extract, he added the essential building blocks for DNA synthesis 3 7 .
The key to the experiment was that the virus's own genomic RNA served as the template. If a reverse transcriptase enzyme was present, it would use this RNA to build a complementary DNA strand.
One of the DNA building blocks was radioactively labeled. This allowed Baltimore to track whether DNA was being synthesized 7 .
The products of the reaction were analyzed to confirm that the synthesized material was, in fact, DNA complementary to the viral RNA 3 .
| Reagent/Material | Function in the Experiment |
|---|---|
| RNA Tumor Virions (Rauscher MuLV, Rous Sarcoma Virus) | Source of the proposed reverse transcriptase enzyme and its native RNA template. |
| Deoxynucleotide Triphosphates (dNTPs) | The essential building blocks (A, T, G, C) for synthesizing a new DNA strand. |
| Radioactively Labeled dNTP (e.g., ^3H-dTTP) | Allows for sensitive detection and measurement of newly synthesized DNA. |
| Divalent Cation (Mg²⁺) | A critical cofactor required for the catalytic activity of the polymerase enzyme. |
| Detergent | Breaks open the viral lipid envelope, releasing the internal enzymes and genetic material. |
The experiment was a resounding success. Baltimore's test tubes showed clear evidence of DNA synthesis dependent on the presence of the disrupted virus 3 . He had discovered the enzyme we now know as reverse transcriptase.
In a stunning coincidence, Howard Temin and his colleague Satoshi Mizutani performed a very similar experiment at the exact same time and reached the identical conclusion.
The acceptance of this revolutionary concept was, surprisingly, instantaneous 5 . The independent confirmation from two respected labs made the finding hard to dismiss.
Famously, scientist Sol Spiegelman attended a talk on the discovery at Cold Spring Harbor, returned to his lab, and had reproduced the results by the following Monday 5 . The Central Dogma had been successfully amended.
The discovery of reverse transcriptase fundamentally reshaped biology and medicine. It earned Baltimore, Temin, and their mentor Renato Dulbecco the 1975 Nobel Prize in Physiology or Medicine 3 .
It defined retroviruses as a unique family of viruses that use reverse transcriptase in their life cycle. This later proved critical in understanding HIV, the AIDS virus 3 .
Reverse transcriptase became a cornerstone tool in molecular biology. It allows scientists to convert mRNA into complementary DNA (cDNA), enabling gene cloning, cDNA libraries, and RT-PCR 7 .
Beyond the lab bench, Baltimore became a leading voice in science policy, advocating for a robust response to the AIDS epidemic and later serving as president of Caltech 3 4 . His career stands as a testament to the power of curiosity, rigorous experimentation, and the courage to challenge established truths.
The story of reverse transcriptase reminds us that in science, even the most deeply held dogmas are open to revision when faced with clear, reproducible evidence.