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 the c⁺ gene 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.)

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.

Verified step by step guidance

1. Step 1: Understand the experimental setup. An Hfr strain (donor) with genotype a⁺b⁺c⁺ rif is crossed with an F⁻ strain (recipient) a⁻b⁻c⁻ rif^T. The rif^T allele confers resistance to rifampicin, and the experiment uses interrupted mating to map gene order based on the time of entry of genes into the recipient during conjugation.
2. Step 2: Analyze the data table showing colony counts on minimal medium plus rifampicin and specific nutrient supplements (A&B, B&C, A&C) at different interruption times (5, 10, 15, 20 minutes). The appearance of colonies indicates successful transfer of the corresponding genes allowing growth on selective media.
3. Step 3: Determine the order of gene transfer for a, b, and c by comparing the earliest time points at which colonies appear on plates supplemented with different nutrient pairs. For example, if colonies appear earlier on A&C than on A&B, gene c likely enters before gene b.
4. Step 4: Assess whether the rif gene location can be inferred from this data. Since rif^T is used to select for recipient cells and no direct timing data for rif transfer is given, the position of rif relative to a, b, and c cannot be determined from this experiment alone.
5. Step 5: Design an experiment to map rif relative to the F factor and gene b. For example, perform interrupted mating using an Hfr strain rif^T and an F⁻ strain rif^S (sensitive), then plate on minimal medium plus antibiotic rifampicin and supplements for gene b. By recording the time at which rif resistance and b gene transfer occur, the relative order and distance of rif to b and the F factor can be established.

Key Concepts

Hfr Strain and Interrupted Mating

An Hfr (high frequency recombination) strain contains the F factor integrated into its chromosome, allowing transfer of chromosomal genes to an F- recipient during conjugation. Interrupted mating involves stopping conjugation at various times to determine the order and relative distances of genes based on when they are transferred.

Gene Mapping Using Time of Entry

Gene mapping in bacteria uses the time it takes for genes to transfer during conjugation to infer their order and relative distances. Genes transferred earlier appear in recombinants sooner, and the number of colonies growing on selective media at different times reflects gene transfer timing.

Use of Selective Media and Auxotrophs in Genetic Analysis

Selective media containing specific nutrients and antibiotics allow identification of recombinant bacteria that have acquired certain genes. Auxotrophic mutants require supplements to grow, so plating on minimal media plus supplements reveals which genes have been transferred based on colony growth.