Molecular Genetics of Prokaryotes Problem Set

Problem 4: Exchange of genetic information between bacteria

Tutorial to help answer the question

Cells of an E. coli strain that are trp lac Z met bio were mixed with cells of an E. coli strain that are trp lac Z met bio and cultured for several hours. Then cells were removed, washed, and transferred to minimal media containing lactose as the only sugar source. A few cells were able to grow on minimal media with lactose, and formed colonies. How did these few cells become trp lac Z met bio?

Tutorial

Gene recombination

In this problem, the experimental results point to gene recombination between the two different E. coli strains. The met

bio

genes of one strain have been recombined with the trp

lac Z

genes of the second strain to create a new strain of genotype trp

lac Z

met

bio

. We normally associate genetic recombination with a sexual process, whereby the two DNAs are present in the same cell, and undergo homologous recombination. Did this occur by a sexual process involving conjugation and mixing of DNAs of two living cells?

Sex in bacteria

The occurrence of sex in bacteria was first described by Joshua Lederberg and Edward Tatum in 1946 (Nature, volume 158, page 558), who were studying mixed cultures of E. coli strains with various nutritional mutations. The mutant strains differed from the wild type strains in lacking the ability to synthesize growth factors such as amino acids and vitamins, similar to the strains in the present problem.

In one experiment, two triple mutants of E. coli, one requiring threonine, leucine, and thiamine, and the second requiring biotin, phenylalanine, and cystine were grown in mixed cultures. At very low frequency, recombinant strains with no growth-factor requirement were obtained. They ruled out spontaneous mutations and transformation by the culture medium as the source of recombinant strains. In the words of Lederberg and Tatum, "These experiments imply the occurrence of a sexual process in the bacterium Escherichia coli."

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Updated: July 15, 1999
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