The story of DNA is one of the most fascinating of modern science. Contrary to popular belief, the discovery of the chemical structure and biological function of deoxyribonucleic acid (DNA) did not occur within several years in the twentieth century and was not accomplished by a small, select group of scientists. Solving the problems of DNA was similar to the painstaking work in assembling the many isolated pieces of a large jigsaw puzzle. A great number of scientists working in a variety of fields contributed to the final outcome, but few ever received anything more significant than the personal satisfaction of having been a participant.
In 1928, Frederick Griffith, a British geneticist, discovered what he called a transforming principle in which a nonvirulent bacteria was turned into a virulent one. It was not until sixteen years later that Griffith’s “transforming principle” was identified as DNA by Avery, MacLeod, and McCarty.
The first in a new series “Bridging Science and Medicine”, this exhibit features Oswald Avery’s research that led to the development of the first vaccine for pneumococcal pneumonia, but it also led him and colleagues Colin M. MacLeod and Maclyn McCarty to make an unexpected discovery in 1944: that DNA is the substance that transmits hereditary information, a finding that would set the course for biological research for the rest of the century.
Transformation continues to provide the most compelling evidence that DNA is genetic material. A purified molecule of DNA can be taken up by an organism, propagated, and passed down from generation to generation, forever changing the heredity of the recipient and its descendants. When the introduced DNA changes a trait of the recipient cell, the cell is said to be “transformed.” Remarkably, several recent experiments with the same formal structure as these DNA experiments lead to the conclusion that protein can also transform cells (, ).
The protracted saga of transformation began in 1928 when F. Griffith, a British army surgeon, reported that an avirulent strain of the pneumococcus bacterium could be converted to a virulent strain using an extract of heat-killed virulent pneumococcus (). As bacterial genetics had not yet been discovered, the transformation of an avirulent to a virulent bacterium by some inanimate material was burdened with the stigma of alchemy—in the vernacular of that time, the metamorphosis of one species into another.
Despite this mystique, work by a group at the Rockefeller demonstrated that Griffith's finding was reproducible. The Rockefeller group discovered the chemical composition of the “transforming principle” by purifying and characterizing the active component from heat-killed virulent cells. In 1944, Avery, McCarty, and MacLeod reported that the active principle was “a nucleic acid of the deoxyribose type…”(). They ruled out proteins, the contemporary favorite, because the activity of the transforming principle could be enzymatically destroyed by DNase but not proteinase.
At the time, the import of this discovery was lost on most (because of confusion about the chemical composition of DNA), but to others like the young Joshua Lederberg, it represented an unprecedented view of the future (“…terrific and unlimited in its implications…”) (J. Lederberg, personal communication). And Lederberg had seen the future. Transformation now forms the basis for all genetic engineering—bacterial genes into mice, mouse and human genes into microbes. DNA is now synonymous with genetic information.
Today, a completely synthetic DNA molecule can be transformed into a recipient cell and confer a desired, novel trait. Moreover, changes engineered into the DNA sequence of the molecule have predictable effects on the phenotype of the transformed cell. That modified DNA can subsequently be reisolated from the transformed organism or its descendants and the transformation process repeated, is a monotonous but satisfying affirmation that DNA is the hereditary material.
Recent experiments in yeast show that proteins can transform cells in the same formal sense that is so convincing for DNA (, ). Yeast cells have a prion-like factor called [PSI+] (). [PSI+] is hereditary by classical genetic criteria—[PSI+] is transmitted from parents to their progeny both in somatic divisions (mitosis) and in sexual divisions (meiosis). A cross of [PSI+] by [psi−] results in all [PSI+]progeny. Although these breeding experiments demonstrated that the [PSI+] factor is not chromosomal DNA, the actual molecule responsible for [PSI+]-based inheritance remained obscure. What was missing was vertical transmission, isolation of a molecule that could transform a [psi−] organism to [PSI+].
The identification of a protein as the key molecule in [PSI+] inheritance set the stage for this transformation experiment. The yeast prion [PSI+] results from self-propagating aggregation of Sup35p (), a protein required for efficient termination of translation. In the [psi−] state, the Sup35p is unaggregated and active, whereas in the [PSI+] state it is aggregated and inactive. The [PSI+] aggregates of Sup35p enhance the ability of ribosomes to read through nonsense mutations. This ability of [PSI+] to suppress nonsense mutations provides a clear-cut assay that distinguishes [PSI+] from [psi−] cells. The [PSI+] prion propagates when a misfolded version of the Sup35 protein templates the aggregation of the properly folded Sup35 protein, thereby converting a [psi−] cell to a [PSI+] cell. This seeding of the Sup35p aggregation can be recapitulated in vitro.
With the essential ingredients in hand, several laboratories devised procedures that enabled purified Sup35p aggregates to transform a [psi−] cell to [PSI+]. What is so striking is the high efficiency with which these Sup35p aggregates infect cells and convert them to [PSI+] (the levels are comparable to those of a control where the transformation frequency of a DNA-based trait was measured). The purity of the Sup35 protein is not in doubt because it was first produced in recombinant bacteria in the [psi−] form, purified from the bacteria, and then converted to the aggregated [PSI+] form in vitro prior to its addition to [psi−] yeast. The [PSI+] trait newly acquired by transformation is then transmitted from generation to generation (Figure 1A). Infectivity with these aggregates is sensitive to proteinase but not to DNase. Moreover, just as DNA transformation with modified molecules confers a novel trait, so protein transformation with [PSI+] aggregates of altered conformation confers heritable [PSI+] prions with the distinctive properties of the aggregate used for transformation (Figure 1B). This experiment rules out de novo indirect induction of the resident soluble Sup35p ().
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