The trp operon in E. coli is similar to the lac operon, in that it is a negative
ID: 166863 • Letter: T
Question
The trp operon in E. coli is similar to the lac operon, in that it is a negative operon. trpR encodes a repressor, trpO is the operator sequence, and trpA encodes the first enzyme in tryptophan biosynthesis. The regulation is different in that it is repressible, however: in the trp system, the repressor must bind to the amino acid tryptophan in order to bind to the operator and repress transcription. A trpR-mutant is not able to bind tryptophan. A trpO- mutant cannot be bound by the repressor, and a trpA- mutant produces an inactive enzyme. Based on these data complete the following table.
Use a (+) to indicate activity and a (-) to indicate inactivity (-) of the trpA gene.
Genotype
No tryptophan
Tryptophan
R+O+A+
R-O+A-/R+O-A+
R+O-A+/R+O+A-
R+O-A-/R+O+A+
Genotype
No tryptophan
Tryptophan
R+O+A+
R-O+A-/R+O-A+
R+O-A+/R+O+A-
R+O-A-/R+O+A+
Explanation / Answer
R+O+A+............is wild, shows gene activity only in the presence of tryptophan. R-O+A-/R+O-A+............shows no gene activity both in the absence and in the presence of tryptophan. The reason is tryA- produces an inactive enzyme. R-O-A+/R+O+A-............shows gene activity even in the absence of tryptophan. The reason is trpR-mutant is not able to bind tryptophan. A trpO- mutant cannot be bound by the repressor. Thus continuous enzyme activity could be seen. R+O-A-/R+O+A+............shows gene activity both in the absence and in the presence of tryptophan.
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