A theoretical operon (amo) in E. coli contains several structural genes encoding

Question

A theoretical operon (amo) in E. coli contains several structural genes encoding enzymes involved in the biosynthesis of a particular amino acid. Unlike the lac operon, in which the repressor gene is separate from the operon itself, for the amo operon, the repressor gene is found within the operon itself. When the end product amino acid is present, it binds to the repressor, and this complex binds to the operator, repressing the operon. In the absence of the amino acid, the repressor fails to bind to the operator, and transcription proceeds. Characterize this operon, comparing it to the regulation of the lac and trp operons. Be sure to discuss how regulation is affected by having the repressor gene as part of the operon. Then, consider the following mutations: a) Mutation in the operator region; b) Mutation in the promoter region; c) Mutation in the repressor gene. In each case, state whether the operon will be transcribed or not, and describe how the mutation affects the regulation of the operon. Compare each response to the equivalent situation in the lac and trp operons.

Explanation / Answer

The type of operon is a repressor operon (or repressible operon) as, it is activated or repressed on the quantity or level of the end product expressed. It is also regulated on the basis of regulated of negative feedback mechanism.

This operon is strikingly similar to trp in terms of regulation as trp operon is also a repressor operon.

In trp operonThis operon contains five structural genes ( trp E, trp D, trp C, trp B, and trp A) which codes for the enzyme tryptophan synthetase and  a repressor  regulator gene called trp R, which has a promoter where RNA polymerase binds and synthesizes mRNA for a regulatory protein. The protein synthesized by trp R binds to the operator during high levels of trp in the environment which then causes the transcription to be blocked.

While the regulation for lac operon is different. The lac operon has three structural genes ( lacZ, lacY and lacA) and three regulator genes ( lacI, lacP/promoter, lacO/operator). The lacI is responsible for the synthesis of repressor protein. When lactose is unavailable in the environment, the repressor protein in its active form binds to the operator region and in presence of an inducer (lactose or allolactose) it binds itself to the inducer to release the repressor complex from the operator to commence translation (negative control of lac operon).

Now in the following senarios, for the hypothetical operon.:

1. Mutation in operator region.

This may have positive effect on translation of the operon as the degree of binding gets reduced, so that in case of high concentations of the expressed amino acids, binding of the repressor protein occurs to a lesser extent or binding doesn't occur at all (depending on the mutation).

2. Mutation in promoter region.

This would have a negative impact as mutation may lead to binding of promoter to the rna polymerase nonfunctional thus preventing translation of the operon.

3. Mutation in repressor gene.

This would have a positive impact on the translation of the operon as the production of structurally modified repressor will not be able to bind to the operator. So the translation can proceed.

The results are similar for trp operon

The result for lactose operon is as follows :

1. Mutation in operator gene will prevent binding of repressor protein so will have a positive impact.

2. Mutation in promoter will prevent the rna polymerase binding, thus a negative inpact

3. Mutation in repressor gene will produced altered repressor protein with low or nil capability of binding to operator thus would have a positive impact.

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