An Hfr strain is used to map three genes in an interrupted mating experiment. The cross is Hfr/a⁺b⁺c⁺ rif x F⁻/a⁻b⁻c⁻ rif^T (No map order is implied in the listing of the alleles; rif^T is resistance to the antibiotic rifampicin.) The a⁺ gene is required for the biosynthesis of nutrient A, the b⁺ gene for nutrient B, and c⁺ for nutrient C. The minus alleles are auxotrophs for these nutrients. The cross is initiated at time = 0 and at various times, the mating mixture is plated on three types of medium. Each plate contains minimal medium (MM) plus rifampicin plus specific supplements that are indicated in the following table. (The results for each time interval are shown as the number of colonies growing on each plate.) Time of Interruption _ 5 min 10 min 15 min 20 min Nutrients A and B 0 0 4 21 Nutrients B and C 0 5 23 40 Nutrients A and C 4 25 60 82 What is the purpose of rifampicin in the experiment?

Key Concepts

Hfr Strains and Gene Mapping

Hfr (high frequency of recombination) strains are bacterial strains that facilitate the transfer of chromosomal genes during conjugation. In gene mapping experiments, Hfr strains are used to determine the order and distance between genes based on the timing of their transfer. By interrupting the mating process at various time intervals, researchers can infer the relative positions of genes based on which genes are transferred and expressed.

Auxotrophy and Nutritional Requirements

Auxotrophy refers to the inability of an organism to synthesize a particular compound required for its growth, necessitating the addition of that compound to the growth medium. In this experiment, the a⁻, b⁻, and c⁻ alleles represent auxotrophic mutants that require specific nutrients A, B, and C, respectively. The presence of these nutrients in the medium allows for the selection of successful gene transfer events from the Hfr strain to the F⁻ strain.

Role of Rifampicin

Rifampicin is an antibiotic that inhibits bacterial RNA synthesis by targeting RNA polymerase. In this experiment, rifampicin is used to prevent the growth of the Hfr donor strain after conjugation, allowing only the recipient F⁻ strain to grow if it has successfully received and expressed the transferred genes. This selective pressure is crucial for accurately assessing the success of gene transfer and mapping the genes based on the observed colony growth.