BIO 301 Study Guide - Quiz Guide: Operon, Repressor, Corepressor

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Bio 301 assignment 4: in bacteria, the his operon is a repressible operon that encodes enzymes responsible for the production of the amino acid histidine. The co-repressor for the regulation of this operon is charged his-trnahis. For the following mutants, determine whether the operon genes will be transcribed, and briefly explain your reasoning. (6 points: mutation of the promoter. If mutations at promoter region occurred, rna polymerase will not be able to recognize the promoter region and bind to it in order to begin the transcription. So, even if the operon is still active, transcription will not begin: mutation of the operator. If there is a mutation at the operator, repressor protein will not be able to bind to it in order to prevent transcription. So, there will be continuous transcription: mutation of the repressor, such that it cannot bind to charged his-trnahis. Mutation of the repressor will also result in continuous transcription.

Related Questions

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.

A transcription factor G can act as either a repressor or an activator of operon M, depending on its conformational state.

In one conformational state, transcription factor G binds site 1 in the promoter of operon M. When bound to site 1, it is an activator of operon M transcription. Factor G cannot bind to site 1 if ligand X is bound to it.

When factor G binds to ligand X, factor G undergoes a conformational change such that it now binds site 2 in the promoter and represses operon M. Because of the way in which it acts, ligand X would be called a(n) ____.

Imagine that there is a mutation in the gene that encodes transcription factor G such that operon M is always transcribed.

Which of these mutations in the gene that encodes transcription factor G is consistent with that outcome?

More than one answer may be correct.

'The mutation changes transcription factor G such that it ___'

cannot bind site 1

cannot bind site 2

cannot bind to ligand X

cannot dissociate from ligand X once it has bound

Continuing with the system in Questions 4 and 5, imagine that you are now analyzing mutations in the promoter of operon M. If site 1 is mutated such that factor G cannot bind, operon M will never be ____

repressed

activated

If site 2 is mutated such that factor G cannot bind, operon M will never be ____

repressed

activated

Still on system described in Q4,5,6,7. If there is a mutation in the DNA encoding biosynthesis genes such that ligand X cannot be made, operon M will never be ___

repressed

activated

The following test deals with the regulation of the trp operon of Escherichia coli. Two mutants of this operon are described: in mutant A, the operator region of the operon carries a point mutation so that it is unable to carry out its function; mutant B expresses a trp repressor protein unable to bind tryptophan. Review the principle of the regulation of this operon and then solve the following questions related to the behavior of the mutant cells using the options below.

A: Mutant A B: Mutant B C: Both of them D: Neither of them

a) _____ In the presence of tryptophan these cells behave like wild-type cells. b) _____ In the absence of tryptophan these cells behave like wild-type cells. c) _____ These cells are able to synthesize tryptophan.
d) _____ These cells are able to break down tryptophan.

e) _____ Tryptophan is able to act as an inducer in these cells.
f) _____ Tryptophan is able to act as a corepressor in these cells.
g) _____ The structural genes of the trp operon can be transcribed in these cells.
h) _____ In the presence of tryptophan RNA polymerase binds to the promoter of the trp operon in these cells.
i) _____ In the absence of tryptophan RNA polymerase binds to the promoter of the trp operon in these cells.
j) _____ These cells die if tryptophan is omitted from their medium.
k) _____ These mutants can be ââcuredââ by expressing wild-type trp repressor in them.
l) _____ These mutants can be ââcuredââ by transforming them with a wild-type trp operator.

BIO 301 Study Guide - Quiz Guide: Operon, Repressor, Corepressor

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