You have an F- strain of E. coli that is resistant to streptomycin (Str(r)) but

Question

You have an F- strain of E. coli that is resistant to streptomycin (Str(r)) but requires the following amino acids for growth on minimal medium: arginine, cysteine, methionine, phenylalanine, and proline. You do a series of interrupted mating experiments with two different Hfr strains, Hfr1 and Hfr2, both of which are wildtype for all amino acid markers, but are Lac(-) and Str(s) (sensitive to streptomycin).

The following data are the times in minutes at which you first observed recombinant progeny containing the indicated amino-acid marker (prototrophy).

Results of F- x Hfr1 cross

Arg+ = 5

Cys+ = 65

Met+ = 75

Phe+ = 35

Pro+ = 10

Results of F- x Hfr2 cross

Arg+ = 40

Cys+ = 70

Met+ = 60

Phe+ = 10

Pro+ = 35

You do a new mating experiment with the same strains. You interrupt the mating after 60 minutes and plate on a medium that selects for progeny that are recombinant for the phenylalanine gene (Phe+). What supplements/antibiotics must be added to the minimal medium in order to select for all Phe+ recombinants without regard for the other auxotrophic phenotypes? Pick from the following additions: A=arginine, C=cysteine, M=methione, F=phenylalanine, P=proline, S=streptomycin. Give your answer in any order without spaces (e.g. CMFS).

Explanation / Answer

Answer

Since both the Hfr starins are lac (-) and Str (S), in order to obtain recombinants with Phe+ after 60 mins of uninterrupted mating the minimal media plate should have streptomycin, proline, phenylalanine, and arginine. Thus, the ordre is SPFA.

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