The general transducing phage P1 is grown on a wild type bacterial strain (cys +
ID: 13485 • Letter: T
Question
The general transducing phage P1 is grown on a wild type bacterial strain (cys+ pyr+ trp+). The resulting phage are mixed with an auxotrophic bacteria that require cysteine, pyrimidine, and tryptophan (cys- pyr- trp-). The auxotroph cannot synthesize these molecules due to the mutation; therefore the only way that they can survive is for you to supplement their diet with them. Recombinants are selected on medium containing pyrimidine and tryptophan. Results are shown below:
Spread on Plate Colonies arising on plate
Phage only (108) 0
Recipient cells only (108) 4
Mixture of phage and cells (108 of each) 200
What is the genotype of these four colonies?
Explanation / Answer
The genotype of the four colonies is ( cys+ , pyr- , trp- ). These four colonies are revertants. This means these bacteria acquired a new mutation that corrects the parent cys defect. They don't need to correct the pry or trp synthesis mutations because they are provided in the medium.