Biology 1002B Chapter Notes - Chapter 14.1: Lac Operon, Lac Repressor, Trp Operon

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amethystarmadillo903

3 May 2018

School

Western University

Department

Biology

Course

Biology 1002B

Professor

Tom Haffie Denis Maxwell

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14.1a The Operon is a Unit of Transcription

• Operon: a cluster of prokaryotic genes and the DNA sequences involved in their regulation

• Operator: DNA sequence that controls transcription of an operon; it is a binding sequence

for a regulatory protein

• Regulatory protein: DNA-binding protein that binds to a operator and affects the

expression of an associated gene or genes

o Gene that codes for regulatory protein is separate from operon

o Repressor: prevents operon genes from being expressed when bound to operator

o Activator: increases likelihood of transcription when bound to operator

o A number of repressors or activators control more than one operon

• Transcription unit: a region of DNA that transcribes a single primary transcript

o Each operon can contain many genes and is transcribed as a unit from promoter

into a single mRNA

o Operon is promoter + operator + transcription unit

o Usually proteins encoded by genes in same operon is used in catalytic steps of same

pathway

14.1b The lac Operon Transcribed when Inducer Inactivates Repressor

• Lac operon contains promoter, operator, and genes lacZ, lacY, lacA (for lactose

metabolism)

• A separate regulatory gene lacI which is upstream and nearby of lac operon encodes for

lac repressor

o Lac repressor synthesized in ACTIVE form

o When lactose absent, lac repressor binds to operator and blocks RNA polymerase

from binding to promoter

o Repressor occasionally comes off, RNA can transcribe - there is always a low

concentration of lac operon gene products in cell

o When lactose present, it gets converted to allolactose which inactivates lac

repressor, and now polymerase can bind to promoter

• Inducible operon: operon whose expression is increased by an inducer molecule

o Allolactose is the inducer

• As lactose is used up, lac operon turns off because no more allolactose to inactivate

repressor

14.1c Positive Regulatory System in Transcription of lac Operon

• Net yield of energy from lactose is less than that for glucose since lactose must be

converted into glucose and this requires energy

• Lac operon can respond to availability of glucose

• CAP (catabolite activator protein) bind to DNA upstream of lac promotor and bends DNA

to make promoter more accessible to RNA polymerase - transcription INCREASES

o CAP is synthesized in INACTIVE form that can only bind to DNA after it is activated

by binding with cyclic AMP (cAMP, which is a nucleotide)

o cAMP levels are inversely related to uptake of glucose; when glucose is abundant,

cAMP levels are low and CAP is mostly inactive

• Positive regulation of lac operon

o Lactose present and glucose absent

• Lactose is converted to inducer allolactose and repressor is inactivated

• CAP is activated by high cAMP levels since no glucose

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Document Summary

14. 1b the lac operon transcribed when inducer inactivates repressor. Inducible operon: operon whose expression is increased by an inducer molecule: allolactose is the inducer, as lactose is used up, lac operon turns off because no more allolactose to inactivate repressor. 14. 1c positive regulatory system in transcription of lac operon: net yield of energy from lactose is less than that for glucose since lactose must be converted into glucose and this requires energy. Lactose is converted to inducer allolactose and repressor is inactivated: cap is activated by high camp levels since no glucose, rna polymerase binds efficiently to promoter and transcription rate is high, lactose present and glucose present. 14. 1d transcription of the trp operon gene for tryptophan biosynthesis: tryptophan is an essential aa. Inducible and repressible operons both illustrate negative gene regulation because both are regulated by a repression that turn of gene expression.

Related Questions

Complete the following to describe the role that an operon plays in the synthesis of enzymes in prokaryotes.

An operon consists of an operator gene, a promoter gene, and the structural genes. When a repressor is bound to the operator, RNA polymerase cannot bind to the _________operatorpromoterstructural genes to start transcription of the _________operatorpromoterstructural genes. When a(n) _________helicaseinducerrepressorribosome is present, it binds to and inactivates the _________helicaseinducerrepressorribosomeRNA polymerase. This inactive _________helicaseinducerrepressorribosomeRNA polymerase can no longer bind to the operator. As a result, _________helicaseinducerrepressorribosomeRNA polymerase can bind to the promoter, leading to eventual transcription of the _________operatorpromoterstructural genes that encode the enzymes to be synthesized.

-operator

-promoter

-structural genes

carminechinchilla384

1.) What type of control best describes the lac operon when it is regulated by allolactose? Explain the features of this type of regulation using the lac operon as an example.

2.) What is positive gene regulation?

a) when an activator protein binds to the operator of a gene to turn expression off

b) when a repressor protein binds to the operator of a gene to turn expression off

c) when an activator protein binds to the operator of a gene to turn expression on

d) when a repressor protein binds to the operator of a gene to turn expression on

e) when a protein binds to DNA to change gene expression

3.) In the lactose operon of E. coli, what is the function of each of the following genes or sites: a) regulator, b) operator, c) promoter, d) structural gene Z and e) structural gene Y?

a) regulator gene

b) operator

c) promoter

d) structural gene Z

e) structural gene Y

4.) In a negative, repressible regulatory mechanism, transcription of the structural gene(s) occurs in

a) the presence of the co-repressor, but not in its absence.

b) the absence of the co-repressor, but not in its presence.

c) the presence of the inducer, but not in its absence.

d) the absence of the inducer but not in its presence.

e) the absence or presence of the co-repressor.

What are answers and can you explain them to me?

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.

Biology 1002B Chapter Notes - Chapter 14.1: Lac Operon, Lac Repressor, Trp Operon

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