The trp operon in E. coli is similar to the lac operon. trpR encodes a repressor
ID: 51762 • Letter: T
Question
The trp operon in E. coli is similar to the lac operon. trpR encodes a repressor, trpO is the operator sequence, and trpA encodes the first enzyme in tryptophan biosynthesis. The regulation is different in this way, however: in the trp system, the repressor must bind to tryptophan before it can 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-
Genotype
No tryptophan
Tryptophan
R+O+A+
R-O+A-/R+O-A+
R+O-A+/R+O+A-
Explanation / Answer
Genotype No Tryptophan Tryptophan
R+O+A+ + -
Here when theres no Tryptophan, repressor cannot bind operator without it and cannot repress the expression of trpA gene. So we have trpA activity.
When Truptophan is present, repressor can bind to operator and inhibit trpA gene. So no activity.
R-O+A-/R+O-A+ No Trytophan Tryptophan
+ +
2 alleles of the genes are inherited. 1 from male parent and the other from female parent. Here since in R-O+A- there is no trpA gene, the trpA gene can only be expressed from the R+O-A+. Here since its O-, the repressor cannot bind the opeartor site no matter whether Tryptophan is present or absent. trpA gene will always be expressed.
R+O-A+/R+O+A- No Tryptophan Tryptophan
+ +
Similarly to the previous case,R+O-A+ the repressor cannot bind the mutant operator neither in the presence of Tryptophan nor in the absence of it. So it cannot repress the transcription of trpA.
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