The phage P1 is used as a generalized transducing phage in an experiment combining a donor strain of E. coli of genotype leu⁺ phe⁺ ala⁺ and a recipient strain that is leu⁻ phe⁻ ala⁻. In separate experiments, transductants are selected for leu⁺ (Experiment A), for ala⁺ (Experiment B), and for phe⁺ (Experiment C). Following selection, transductant genotypes for the unselected markers are identified. The selection experiment results below show the frequency of each genotype.

What compound or compounds are added to the minimal medium to select for transductants in Experiments A, B, and C?

Verified step by step guidance

1. Step 1: Understand the concept of transduction and selection. Transduction is a process where bacterial DNA is transferred from one bacterium to another by a phage. In this experiment, the phage P1 is used to transfer genetic material between donor and recipient strains. Selection involves growing bacteria on a minimal medium supplemented with specific compounds to identify transductants that have acquired the desired genotype.
2. Step 2: Analyze the genotype being selected in each experiment. In Experiment A, transductants are selected for leu⁺, meaning the medium must contain leucine to allow only leu⁺ bacteria to grow. In Experiment B, transductants are selected for ala⁺, so the medium must contain alanine. In Experiment C, transductants are selected for phe⁺, requiring phenylalanine in the medium.
3. Step 3: Examine the genotype frequencies for unselected markers in each experiment. For Experiment A, the frequencies of phe⁻ ala⁻, phe⁺ ala⁻, phe⁻ ala⁺, and phe⁺ ala⁺ indicate the likelihood of co-transduction of phe and ala with leu. Similarly, for Experiment B and C, the frequencies of unselected markers provide insights into the genetic linkage between leu, phe, and ala.
4. Step 4: Determine the compounds added to the minimal medium for selection. To select for leu⁺ transductants in Experiment A, leucine is added to the medium. For ala⁺ transductants in Experiment B, alanine is added. For phe⁺ transductants in Experiment C, phenylalanine is added. These compounds ensure that only bacteria with the corresponding genotype can grow.
5. Step 5: Reflect on the results and genetic linkage. The frequency data suggests varying degrees of linkage between the genes leu, phe, and ala. Genes that are closer together on the bacterial chromosome are more likely to be co-transduced, as seen in the higher frequencies of certain genotypes. This information can be used to map the relative positions of these genes.

Key Concepts

Generalized Transduction

Generalized transduction is a process by which a bacteriophage (phage) transfers genetic material from one bacterium to another. In this process, the phage can incorporate any segment of the host's DNA into its own genome during the lytic cycle. When the phage infects a new bacterial cell, it can introduce this DNA, leading to genetic recombination and the potential for new phenotypes in the recipient strain.

Selection of Transductants

In genetic experiments, selection of transductants involves using specific growth conditions to isolate bacteria that have acquired desired traits through transduction. For example, in the given experiments, minimal media supplemented with specific amino acids (like leucine, phenylalanine, or alanine) allows only those bacteria that have successfully taken up the corresponding genes to grow, while others are unable to survive due to their auxotrophic nature.

Auxotrophy and Prototrophy

Auxotrophy refers to the inability of an organism to synthesize a particular compound required for its growth, often due to mutations in genes involved in biosynthetic pathways. Prototrophic organisms, on the other hand, can synthesize all necessary compounds from basic nutrients. In the context of the experiments, the donor strain is prototrophic for certain amino acids, while the recipient strain is auxotrophic, necessitating the addition of specific compounds to the growth medium to select for successful transductants.