Below is a schematic map of a hybrid gene constructed from elements of the lac a

Question

Below is a schematic map of a hybrid gene constructed from elements of the lac and trp operons. In words, the CRP binding site and promoter from the lac operon have been fused to the operator and transcribed sequence from the trp operon. This is one of those hybrid systems in which components from different genes will each act autonomously.

            This hybrid gene was transformed into a series of mutant E. coli hosts, and the expression of the trpA gene product was measured. All of the hosts were derived from a strain that had its normal trp operon deleted so the only possible source of trpA enzyme is the hybrid gene. In the table below, the genotype of the host is to the left, and additions to the medium are across the top. Use pluses or minuses to indicate whether the trpA enzyme will be produced by each genotype under each of the conditions. The basic incubation conditions were minimal medium with glycerol as the carbon source. CRP is the gene for the catabolite repressor protein, trpRSis a mutant of the trp repressor that binds to the operator regardless of tryptophan concentrations, and CRPS is a mutant of the CRP protein that binds to the CAP site in the regardless of glucose concentrations.

Genotype of host

+tryptophan

+tryptophan

+glucose

Genotype of host

Minimal Medium

+tryptophan

                        +glucose

+tryptophan

+glucose

trpR+,CRP+ trpR-,CRP+ trpR+,CRP- trpR-,CRP- trpRS,CRP+ trpRS,CRP- trpR+,CRPS trpR-,CRPS Below is a schematic map of a hybrid gene constructed from elements of the lac and trp operons. In words, the CRP binding site and promoter from the lac operon have been fused to the operator and transcribed sequence from the trp operon. This is one of those hybrid systems in which components from different genes will each act autonomously. This hybrid gene was transformed into a series of mutant E. coli hosts, and the expression of the trpA gene product was measured. All of the hosts were derived from a strain that had its normal trp operon deleted so the only possible source of trpA enzyme is the hybrid gene. In the table below, the genotype of the host is to the left, and additions to the medium are across the top. Use pluses or minuses to indicate whether the trpA enzyme will be produced by each genotype under each of the conditions. The basic incubation conditions were minimal medium with glycerol as the carbon source. CRP is the gene for the catabolite repressor protein, trpRSis a mutant of the trp repressor that binds to the operator regardless of tryptophan concentrations, and CRPS is a mutant of the CRP protein that binds to the CAP site in the regardless of glucose concentrations.

Explanation / Answer

Genotype of host

+tryptophan

+tryptophan

+glucose

Genotype of host

Minimal Medium

+tryptophan

                        +glucose

+tryptophan

+glucose

trpR+,CRP+             +         +                      +         + trpR-,CRP+              +           -                     +         + trpR+,CRP-              -            -                 -           - trpR-,CRP-              +           +                     -          + trpRS,CRP+             +           +                    -           + trpRS,CRP-             +           +                    +           + trpR+,CRPS              +            -                    +            + trpR-,CRPS                               +                       +                         +                           +

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