Anniversary Of RNA Double-Helix Discovery

A few years back, the 50th anniversary of the discovery of the structure of the DNA double helix by biologists James D. Watson and Francis H. C. Crick was grandly celebrated.

Last month marked a less celebrated anniversary: 50 years since the discovery of the RNA double helix. It was in the July 20, 1956, Journal of the American Chemical Society (78, 3548) that Alexander Rich and David R. Davies of the National Institute of Mental Health, in Bethesda, Md., reported for the first time that RNA could form a double helix. Rich is now professor of biophysics at Massachusetts Institute of Technology, and Davies is chief of the molecular structure section in the laboratory of molecular biology at the National Institute of Diabetes & Digestive & Kidney Diseases, in Bethesda.

In fact, it's a double anniversary. It's also 50 years since the discovery of the nucleic acid hybridization reaction−the reaction of two unstructured polynucleotides to form a double helix based on the specificity of hydrogen bonding between the monomer units. Where was that reported? It's the same work, reported in the same short paper.

At the time the paper was published, Rich had been working for three years to find out whether RNA could form a double helix. In the classic paper on the structure of double-helical DNA, Watson and Crick had pointed out that RNA could not form a double helix like the one they had described for DNA because the 2′ hydroxyl on each RNA nucleotide would interfere with such a structure.

"So if RNA couldn't form that type of double helix, could it form any double helix?" Rich says he speculated at the time. In 1954, Rich and Watson, then postdoctoral fellows at California Institute of Technology, tried to address this question by studying X-ray diffraction patterns of RNA fibers. "We published a couple of papers about this, but they were inconclusive," Rich says.

A hint of an RNA double helix was seen soon thereafter. In 1955, an enzyme that catalyzes the synthesis of polyribonucleotides was discovered by enzymologist Severo Ochoa of New York University, enabling researchers to make RNA polynucleotides such as polyriboadenylic acid (polyA) and polyribouridylic acid (polyU). In March 1956, NYU biochemistry professor Robert C. Warner obtained polyA and polyU made that way, mixed them together, and studied the mixture spectroscopically. The study suggested that polyA and polyU were interacting with each other in the mixture, but the interpretation wasn't clear.

At about the same time, Rich, then at the National Institutes of Health, was also studying RNA polynucleotides. "We discovered that polyA and polyU alone didn't have any particular structure," Rich says. "But when we mixed the two together, to our astonishment, I could draw out a fiber, and there was a diffraction pattern of a double helix, quite different from the DNA one."

That was the discovery of the RNA double helix. It was also the discovery of nucleic acid hybridization. But the reaction wasn't called "hybridization" until later, because in 1956 that term had not yet been coined.

"There had never been another reaction between polymers in which two different monomer units combined with great specificity," Rich says. Therefore, chemists were very skeptical about his findings at the time, he says. "PolyA and polyU were each very large macromolecules, and they were highly negatively charged. Therefore, why would they combine? Because they were so long, polymer chemists felt they would probably be entangled and would never be able to sort themselves out."

Rich himself was surprised about the RNA double-helix discovery. Within a couple of weeks of having sent his paper to JACS for review, Rich wrote about it to his postdoctoral mentor, Caltech chemistry professor Linus Pauling. In the letter, Rich wrote that the hybridization reaction was a most remarkable thing and that it was completely reproducible. The letter "is filled with astonishment," he says.

The discoveries of the RNA double helix and nucleic acid hybridization paved the way for the elucidation of many other phenomena, including the polymerase chain reaction, antisense nucleic acids, and RNA interference. The discoveries represented "a paradigm shift in the way people thought about nucleic acids," Rich says. "They began to think of them as being more mobile and more reactive than they had thought up to then."

Prior to the 1956 discoveries, Rich says, he and colleagues "talked a lot about RNA, but nobody, including myself, suggested, 'Why don't you mix together polyA and polyU?' I think that speaks to the fact that it wasn't at all obvious that that could work. When I asked Francis Crick about this a few years ago, he said, 'Maybe it's because we all felt it would require an enzyme.' Prior to the discovery, people had no idea that hybridization could occur by itself."

So what induced Rich to mix the two together? "Well, I tell you, I've asked my colleagues and searched through my memory, and I don't actually know," Rich says.