A triple-auxotrophic strain of E. coli having the genotype phe⁻ met⁻ ara⁻ is used as a recipient strain in a transduction experiment. The strain is unable to synthesize its own phenylalanine or methionine, and it carries a mutation that leaves it unable to utilize the sugar arabinose for growth. The recipient is crossed to a prototrophic strain with the genotype phe⁺ met⁺ ara⁺. The table below shows the selected marker and gives cotransduction frequencies for the unselected markers.

Identify the compounds present in each of the selective media.

Verified step by step guidance

1. Step 1: Understand the problem. The goal is to identify the compounds present in each selective media based on the genotype of the recipient strain (phe⁻ met⁻ ara⁻) and the donor strain (phe⁺ met⁺ ara⁺), as well as the cotransduction frequencies provided in the table.
2. Step 2: Recall the concept of selective media. Selective media are designed to allow the growth of organisms with specific genotypes while inhibiting others. For example, if a media lacks phenylalanine, only strains that can synthesize phenylalanine (phe⁺) will grow.
3. Step 3: Analyze the table. The table provides cotransduction frequencies for unselected markers when a specific marker is selected. For example, when phe⁺ is selected, 2% of colonies contain met⁺ and 51% contain ara⁺. This information helps determine which markers are linked and which compounds are required for growth.
4. Step 4: Determine the compounds in the selective media for each marker. For phe⁺ selection, the media likely lacks phenylalanine, forcing only phe⁺ strains to grow. For met⁺ selection, the media likely lacks methionine, allowing only met⁺ strains to grow. For ara⁺ selection, the media likely contains arabinose as the sole sugar source, allowing only ara⁺ strains to utilize it for growth.
5. Step 5: Use the cotransduction frequencies to infer genetic linkage. Higher cotransduction frequencies indicate closer genetic linkage between markers. For example, the high cotransduction frequency (79%) between met⁺ and phe⁺ suggests these genes are closely linked, which can help interpret the results of the experiment and the compounds required in the media.

Key Concepts

Auxotrophy and Prototrophy

Auxotrophy refers to the inability of an organism to synthesize a particular compound required for its growth, necessitating supplementation from the environment. In contrast, prototrophic organisms can synthesize all necessary compounds from basic nutrients. In this experiment, the E. coli strain is auxotrophic for phenylalanine (phe⁻) and methionine (met⁻), while the prototrophic strain can grow without additional supplements.

Transduction

Transduction is a process of horizontal gene transfer in bacteria, where bacteriophages (viruses that infect bacteria) transfer genetic material from one bacterium to another. This method can introduce new traits, such as the ability to synthesize certain compounds, into the recipient strain. Understanding transduction is crucial for interpreting the genetic changes observed in the E. coli strains during the experiment.

Cotransduction Frequencies

Cotransduction frequencies indicate the likelihood that two genetic markers are transferred together during transduction. High cotransduction frequencies suggest that the markers are located close to each other on the bacterial chromosome, while low frequencies imply they are farther apart. Analyzing these frequencies helps determine the genetic linkage and the effectiveness of the transduction process in the experiment.