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. Selected Selected Colonies Containing the Marker Unselected Marker (%) _ phe⁺ met⁺ ara⁺ met⁺ 4 - 7 phe⁺ - 2 51 met⁺, phe⁺. - - 79 ara⁺ 68 5 - Identify the compounds present in each of the selective media.

Key Concepts

Auxotrophy and Prototrophy

Auxotrophy refers to the inability of an organism to synthesize a particular compound required for its growth, necessitating the addition of that compound in the growth medium. In contrast, prototrophy indicates the ability to 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 synthesize these compounds.

Transduction

Transduction is a process of genetic transfer in bacteria mediated by bacteriophages (viruses that infect bacteria). During this process, a bacteriophage can transfer genetic material from one bacterial cell to another, allowing for the introduction of new traits, such as prototrophy for specific amino acids. Understanding transduction is crucial for interpreting the genetic changes observed in the recipient strain after crossing with the prototrophic strain.

Cotransduction Frequencies

Cotransduction frequencies refer to the likelihood that two genetic markers are transferred together during transduction. This frequency can provide insights into the physical proximity of genes on the bacterial chromosome; closely linked genes are more likely to be cotransduced. Analyzing the cotransduction frequencies in the experiment helps determine which unselected markers are associated with the selected marker, aiding in the identification of compounds in the selective media.