To understand how the trp operon functions, you do a series of experiments that

Question

To understand how the trp operon functions, you do a series of experiments that
introduce a copy of the trp operon into an E. coli cell that already contains a trp operon in its
own DNA. In each of the following scenarios, explain how the normal control of the trp operon
will be affected (i.e. will genes be turned on and off in the same way? will they be on all the
time? off all the time? etc.), and then explain what effect the addition of the extrachromosomal
version of the gene will have.
A. The cellular operon consists of a mutant repressor gene that cannot bind trp and normal
structural genes (those that make the products from the trp operon). The extra operon consists of
a normal set of genes, with a normal repressor.
B. There is a mutation in the operator region such that it cannot bind trp repressor. The extra
operon contains a normal repressor and a normal set of structural genes.
C. There is a mutation in the trpC gene (the third of 5 structural genes in the operon) that
introduces a stop codon for translation. The extra operon consists of a normal set of genes, with a
normal repressor.

Explanation / Answer

A. The cellular operon consists of a mutant repressor gene. This mutant repressor protein cannot bind to tryptophan. Hence the trp operon will always be on.
In the presence of the extra operon which consists of a normal repressor, the cell would use this repressor to regulate trp synthesis in the cell. ------------------------------------------------ B. The cellular operator region is mutated such that the repressor cannot bind. In this case, the trp operon will always be on. The presence of the extra operon will not make any difference. Even though the extra operon has a normal operator, it cannot replace the mutated operon in the cellular operon. -------------------------------------------------- C. A mutation is present in the trpC gene of the cell which introduces a stop codon. In this cell, due to this mutation, transcription of trpC does not occur. This leads to the failure in formation of repressor-trp complex needed for regulation. Hence the trp operon will always be on. The presence of a normal trpC gene in the extra operon makes up for this mutation. It synthesizes a normal protein which allows the cell to regulate the synthesis of trp.

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