The general transducing phage P1 is grown on a wild type bacterial strain (cys +

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.

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