Biochemistry 2288A Lecture Notes - Lecture 3: Trp Operon, Enzyme, Tryptophan

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29 Dec 2017

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The 5 genes required for typtophan (amino acid) biosynthesis are transcribed from a common promoter. An operon is a group of genes that is being regulated and group under one promoter00. When is the trp operon expressed: when there is a lot of tytophan in the cellular environment, when there is little typtophan in the cellular environment, when the glucose is low, all the time. The trp operon is expressed when there is no or little tryptophan in the growth medium. For example: the cells need to make their own tryptophan. Trp promoter: between -10 and 035 sequences there is a dna sequence (binding site) called the trp operator that binds a protein called the trp repressor; precisely between - Important point: trp operator = dna element, trp repressor = proteins that binds dna. Genetics has been critical in understanding the regulation of the trp operon.

Related Questions

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.

When tryptophan is present at high levels, synthesis of tryptophan in bacteria is regulated allosterically when

A. two molecules of tryptophan bind regulatory sites on the first enzyme in the pathway for tryptophan synthesis, causing a change in conformation that inhibits the enzyme activity, and thus decreases tryptophan biosynthesis

B. two molecules of tryptophan bind the Trp repressor, causing a change in conformation so that the repressor is inhibited from binding to the operator

C. two molecules of tryptophan bind the Trp repressor, causing a change in conformation so that the repressor binds the operator and blocks expression of the genes encoding the enzymes that synthesize tryptophan

D. two molecules of tryptophan bind the Trp repressor, causing a change in conformation so that the repressor binds the operator and increases the level of expression of the genes encoding the enzymes that synthesize tryptophan

E. two molecules of tryptophan bind regulatory sites on the first enzyme in the pathway for tryptophan synthesis, causing a change in conformation that activates the enzyme activity, and thus increases tryptophan biosynthesis

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.

Biochemistry 2288A Lecture Notes - Lecture 3: Trp Operon, Enzyme, Tryptophan

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