Conjugation

In 1946, Edward Tatum and Joshua Lederberg (the latter aged only 21 and still a student!) demonstrated a second form of genetic transfer between bacteria. These experiments involved the use of auxotrophic mutants (Box 11.8), which have lost the ability to make a particular enzyme involved in the biosynthesis of an essential nutrient. These can be recognised experimentally by their inability to grow on a medium lacking the nutrient in question (Figure 11.26). The results of Tatum and Lederberg's experiments suggested that a process was taking place in bacteria akin to sex in eucary-otes, in which there was a recombination of the cells' genetic material (Figure 11.27). Transformation, as demonstrated by Griffith, could not explain their results, since the addition of a DNA-containing extract of one strain to whole cells of the other did not result in prototroph formation. This was confirmed in an ingenious experiment in which Bernard Davis showed that Tatum and Lederberg's results were only obtained if direct cell-to cell contact was allowed (Figure 11.28).

Auxotroph e.g. leu-

Needs to have leucine provided

No growth on minimal medium

Prototroph e.g. leu+

Synthesizes own leucine

Growth on minimal medium

Growth on minimal medium

Figure 11.26 Auxotrophic mutants provide useful genetic markers. They are unable to synthesise a particular nutrient, and can be detected by their inability to grow on a minimal medium (one containing only inorganic salts and a carbon source such as glucose)

Figure 11.27 Tatum and Lederberg: sexual recombination in bacteria. Two strains with complementary genotypes (auxotrophic for certain genes, prototrophic for others) are each unable to grow on a minimal medium. When the two are mixed, however, colonies are formed, indicating that recombinant bacteria prototrophic for all the genes have been formed. The bacteria are washed before plating out to avoid the carry-over of nutrients from the initial broth to the minimal medium. From Black, JG: Microbiology: Principles and Explorations, 4th edn, John Wiley & Sons Inc., 1999. Reproduced by permission of the publishers

Figure 11.27 Tatum and Lederberg: sexual recombination in bacteria. Two strains with complementary genotypes (auxotrophic for certain genes, prototrophic for others) are each unable to grow on a minimal medium. When the two are mixed, however, colonies are formed, indicating that recombinant bacteria prototrophic for all the genes have been formed. The bacteria are washed before plating out to avoid the carry-over of nutrients from the initial broth to the minimal medium. From Black, JG: Microbiology: Principles and Explorations, 4th edn, John Wiley & Sons Inc., 1999. Reproduced by permission of the publishers

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