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 Can the location of the rif gene be determined in this experiment? If not, design an experiment to determine the location of rif relative to the F factor and to gene b.

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, the order and distance between genes can be determined by measuring the time it takes for genes to be transferred from the Hfr strain to the F- recipient. The interrupted mating technique allows researchers to identify which genes are transferred at specific time intervals, providing insights into their relative positions on the chromosome.

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. By analyzing the growth of colonies on media supplemented with these nutrients, researchers can infer which genes have been successfully transferred and their order based on the time of appearance of colonies.

Rifampicin Resistance and Gene Location

Rifampicin is an antibiotic that inhibits bacterial RNA synthesis, and the rif^T gene confers resistance to this antibiotic. In the context of the experiment, determining the location of the rif gene relative to other genes involves analyzing the growth patterns of colonies on selective media. If rif is located close to the F factor or gene b, it will be transferred along with these genes during conjugation, affecting the number of colonies that can grow in the presence of rifampicin. Understanding this relationship is crucial for mapping the rif gene's position.