Birds Do It, Bees Do It
Lederberg and Tatum Discover Bacterial Mating and Phage Recombination (1946)
As Cole Porter reminds us, "Birds do it, bees do it," and, in fact, even bacteria and bacteriophages do it! Well - maybe not fall in love - but at least exchange genes. In most bacteria, some of the DNA is contained in plasmids - circular structures similar to chromosomes, but smaller and with far less information. Plasmids contain genetic information that is not usually essential, but can come in handy - like genes for antibiotic resistance. Some plasmids, called conjunctive plasmids, also code for the ability to transfer themselves to another bacterial cell. This process is called conjugation.
In bacteria, as in people, successful mating requires that the two individuals be compatible. Among bacteria, opposites seem to attract - for successful mating one cell must contain a conjunctive plasmid the other lacks. A frequently studied conjunctive plasmid is the F plasmid of E. coli. Cells that carry the F plasmid are called F+, those that lack it are F-. An F+ bacterium can grow a special appendage, which it uses to reach out to a potential mate and draw it close. Once the two cells are in direct contact, the plasmid DNA of the F+ bacterium is replicated, and a copy is passed into the F- partner, so that both bacteria are now F+ and have all the genes associated with that plasmid.
The existence of this mating system was discovered in 1946 by Joshua Lederberg and Edward Tatum. Tatum was a well-established bacteriologist and biochemist, whose studies of nutritional mutants in bread mold had led to the one gene-one enzyme hypothesis. He had also discovered "double mutant" bacteria, a wild-type bacteria which required two supplemental substances to survive. Lederberg was a medical student, who had taken a leave of absence from his studies to work in Tatum's lab.
Lederberg and Tatum decided to test whether two types of double mutant bacteria could mate, raising the strains both together and separately to see if there was any exchange of genetic material between them. To understand how their experiment worked, consider two hypothetical double mutant strains: we'll call one A- B- C+ D+ and the other A+ B+ C- D-. The A- B- C+ D+ strain cannot live unless you add substances A and B to its diet, but like normal bacteria, it can make its own C and D. Conversely, the A+ B+ C- D- strain needs an external source of substances C and D, but makes its own A and B just fine.
Lederberg and Tatum found that when such strains were grown separately, each remained true to form: A- B- C+ D+ bacteria never developed the ability to live without substances A and B and A+ B+ C- D- bacteria never developed the ability to live without C and D. However, when the two strains were grown together, some bacteria appeared that were able to live without any nutritional supplements; they had become A+B+C+D+. The likelihood of this occurring by mutation is infinitesimal. They concluded that the bacteria had mated and exchanged genetic information.
It is interesting to note that while this was a well-designed experiment, Lederberg and Tatum were also lucky. Although they had no way of knowing it, the two mutant strains that they used were of compatible mating types. The strain that Tatum had originally used to create the mutants contained the F plasmid (which was F+), but one of the mutants had lost it (becoming F-). If both double mutants had contained the F plasmid (if both were F+), the two strains would have been unable to mate and no recombination between the double mutant bacteria would have been possible, postponing the discovery of bacterial mating to a later day.
Double mutants were also used during the same year by Alfred Hershey and Max Delbruck to demonstrate that bacteriophages can also exchange genetic material. Like Lederberg and Tatum, they used two double mutant strains of phage, with two different and complementary genetic markers. They grew each strain of phage alone and also grew the two in combination. Their results were analogous to Lederberg's and Tatum's. New combination phenotypes arose only in the mixed cultures - cultures in which the bacteria were infected with both types of phage - proving that even phage, simple as they are, can exchange genes.
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