Part VIII: Joshua Lederberg, 1958 Prize in Physiology or Medicine
By Joseph Luna
“You say [it was] a wonderful scientific achievement?” said Paul Ehrlich. “My dear colleague, for seven years of misfortune I had one moment of good luck!”
Joshua Lederberg, then only 13 or so, read these final lines of The Microbe Hunters and closed his copy, exhilarated. Paul de Kruif’s semi-non-fictional account of twelve great microbiologists had inspired the young Lederberg and cemented his desire to be one of them. It was an odd life choice to make in 1941, but Lederberg was no ordinary teenager. After graduating high school at age 15, Lederberg headed straight to Columbia University. He graduated three years later with a degree in zoology just shy of his nineteenth birthday and continued on at Columbia for medical school as part of a wartime Navy program.
His precociousness had not gone unnoticed, for Lederberg also sought a scientific mentor as an undergrad, and found one in a young assistant professor named Francis Ryan. Having trained with George Wells Beadle and Edward Tatum for his postdoc, Ryan established his laboratory to study the bread mold Neurospora as a new model for microbial genetics. Within a year, Lederberg all but abandoned his medical studies to work in Ryan’s lab, partly due to a single paper that both stunned and spurred the young men to action.
Across town at Rockefeller in 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty established that DNA was the molecule of heredity in Pneumococcus bacteria. Suddenly the race was on to characterize the role that DNA played in other micro-organisms; Lederberg and Ryan leaped at the chance to try this out in their favorite fungus. Whereas the Rockefeller group established DNA as the key ingredient for transforming non-virulent bacteria to more deadly forms, Lederberg and Ryan aimed to uncover whether DNA could also be responsible for correcting nutritional mutants in Neurospora. In other words, they sought to confirm that manipulating genes as Beadle and Tatum had done was the same as manipulating DNA.
They started with Neurospora mutants that could not make the amino acid leucine. These bugs could only grow when leucine was present in the media, and would die otherwise. Next, they attempted to transform these mutants using DNA from normal Neurospora to restore leucine production. As they suspected, they were able to recover bugs that could grow in the absence of leucine. Yet there was a catch, they figured out that this was not due to the DNA they were introducing into cells, but instead because the mutant microbes had reverted to their parental, or prototroph, condition. But where they failed to show transformation, they succeeded in showing something else: Lederberg and Ryan had invented a prototrophic recovery method to isolate rare natural revertants (termed “back mutations”) to show that induced mutations could sometimes spontaneously switch back to their ancestral condition. Microbes, they discovered, were ceaselessly tinkering.
Their original hypothesis, to correct a mutation at will with DNA transformation in Nuerospora was a spectacular failure, but it got Lederberg to thinking that maybe transformation wasn’t all there was. Maybe there was a way for microbes to transform each other naturally and exchange genetic information. And maybe this might’ve gone unnoticed because it was such a rare event, just like back-mutations were a rare event.
Hence, one failure became an opportunity: Lederberg decided to use his prototrophic recovery method to try to find instances of genetic exchange between bacteria. It was a bold idea, and about as far from the fungus Neurospora as could be imagined. Unlike Neurospora, bacteria by and large reproduce asexually, that is, they make copies of themselves not by shuffling their genetic information but by dividing and making identical daughter cells. Thus any idea of crossing two mutants, as the key method for any genetics experiment, seemed out of the question. There was a strong argument to be made that Lederberg was wasting his time.
Undaunted, he started by isolating nutritional mutants of a benign intestinal bacterium called Escherichia coli, and started testing whether E. coli with different mutations could correct one another, no transformation required. He failed, numerous times. Perhaps sensitive to the frustrating impasse reached by his student, or reticent to sink any further time or money in the project, Ryan suggested collaborating with his former mentor, Edward Tatum, who had just moved to Yale University and who had made a wide array of E. coli mutants. Maybe one of them could work? In short order, Lederberg hopped on the train and found himself in Tatum’s lab in New Haven. It was a move simultaneously bold and foolish, as he effectively dropped out of medical school to pursue these studies. Just imagine for a moment, what his mother must’ve thought of her 21 year old former prodigy medical student turned homeless graduate student son.
But none of that would matter, because within the first 6 weeks, Lederberg hit paydirt. Using Tatum’s recently isolated K12 strain of E. coli, Lederberg appropriated two double mutants (strain A could not make the vitamin biotin and the amino acid methionine; strain B could not make the amino acids threonine and proline), mixed them together and tested whether any bacteria could grow in minimal media that by definition lacked the above components. Strain A by itself was dead, same for strain B, but the mixed cultures would yield colonies at a frequency of about one in ten million. The use of double mutants ensured that this wasn’t a reversion, but instead mutant A had acquired the genetic components from mutant B to correct the mutations. In other words, bacteria could exchange genetic information. They could be used to test the nature of the gene.
Lederberg was 22 when he made this discovery, received a PhD from Yale a year later after mapping the E. coli chromosome, and shared the Nobel prize at 33 with Beadle and Tatum, for launching bacterial genetics. His addendum to The Microbe Hunters could’ve read, “My dear colleague, I had a single moment of good luck in over 10 million tries!”