Question

The trp operon: Bacterial cells can take up the amino acid tryptophan from their surroundings, or, if the external supply is insufficient, they can synthesize tryptophan from small molecules in the cell.  

When external supplies of tryptophan are plentiful, the cells suppress transcription of the trp operon, which encodes the tryptophan biosynthetic enzymes. When external supplies of tryptophan are not plentiful, the cells express the trp operon.

The trp operon repressor protein inhibits transcription of the genes in the trp operon. Upon binding tryptophan, the tryptophan repressor binds to a site in the promoter of the operon and represses transcription.

A. (2 pts) Why is tryptophan-dependent binding to the operon a useful property for the tryptophan repressor?

B. (4 pts) How would regulation of transcription of tryptophan biosynthetic enzymes be affected in cells that express a mutant form of the tryptophan repressor that (i) cannot bind to the DNA or (ii) bind to DNA even when no tryptophan is bound to it? Also address how tryptophan synthesis would be affected and how that would affect the cell. Please bullet point your response (i) and (ii) for each mutant for ease of grading.

Answer 1

Ans (A):- Tryptophan dependent binding of Repressor to the operon is important because these operon are responsible for production of enzymes responsible for making Tryptophan inside the cells. Suppose, if Repressor will binds to operon without assistance to Tryptophan, then cell won't be able to produce Tryptophan even if the Tryptophan is needed to the cells. So, the Repressor will only bind to operon when Tryptophan is present means here cell known that it doesn't have to synthesize Tryptophan as it is present. It is the better way to regulate Tryptophan operon. Thus, bacterial cells have evolved such a mechanism that it won't waste energy in making Tryptophan when it present and if Tryptophan is missing these Repressor can't hault the synthesis of tryptophan. Thus, these useful property of tryptophan operon.

Ans (B) (i) When Tryptophan Repressor mutant does not bind to the DNA.

1. Tryptophan synthesizing gene will be expressed constitively.

2. Tryptophan operon will loose its complete regulation as Repressor is mutated. It can no longer repress the gene.

3. This mutant will affect the cell growth as most of the cell enery will be channelize towards Tryptophan enzymes synthesis.

(ii) Repressor binds to DNA even in the absence of tryptophan.

1. The Tryptophan operon will be completely shut down.

2. The Tryptophan synthesizing enzymes won't be produced even if it is needed to the cell.

3. The cell will die in the absence of Tryptophan in media because cells can't synthesize it's own Tryptophan.

4. Cells can't even synthesize it's protein because Tryptophan absence leads to haulting of ribosome if it encounter Tryptophan codon.

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