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Chapter 9: Microbial Genetics The Operon: Regulation of bacterial gene expressio


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Controlling enzymes
Bacterial cells carry out a huge number of chemical reactions catalyzed by enzymes
Bacterial cells must respond rapidly to changing environments e.g., presence of a new carbon source
Enzyme activity can be regulated by feedback inhibition of synthesized enzyme
- BUT making enzyme requires energy (1200 ATPs/ave. size protein)
- SO it is better to stop synthesizing enzymes that are not needed
Regulation of gene expression (Figure 8.7)
Genes through transcription and translation direct the synthesis of proteins
60-80% of genes are not regulated
- Constitutive: fixed rate of expression
- Enzymes that cells need constantly e.g., glycolysis
Other genes such as those for enzymes involved in lactose metabolism or amino acid production are regulated and only expressed when required
The process of transcription plays an important role in the ability of bacteria to respond to changing environments
- mRNA compared to DNA is inherently unstable, so the expression levels of certain proteins can be controlled at the level of transcription
Transcriptional control
Two genetic control mechanisms which regulate transcription of mRNA and, therefore the synthesis of corresponding protein
Induction-turning on gene expression
Repression-turning off gene expression
Inhibition of gene expression
Often in response to an excess of endproduct, shuts down synthesis of the enzyme
Mediated by regulatory proteins called repressors
- Block RNA polymerase from initiating transcription
Repressible genes are transcribed until they are repressed
Turns on transcription of a gene
A substance which acts to induce transcription of a gene is an inducer, often the substrate
Enzymes synthesized in the presence of inducers are called inducible enzymes
Example: lactose utilization genes
Inducible genes are not transcribed until they are induced
The operon
Multiple genes are arranged in the same orientation and are closely linked on the DNA
Genes in an operon are transcribed on a single RNA transcript, but are translated individually to form multiple proteins
A mechanism for coordinate control of genes involved in a single process e.g., a metabolic pathway
Genes are coordinately regulated by regulating transcription of the operon mRNA
The term "operon" not only includes the structural genes in the operon but also the regulatory sequences controlling transcription
- Promoter-site of RNA polymerase binding
- Operator-site of binding of a regulatory protein
Operons and regulation
~27% of E. coli genes are in operons
Many of the genes arranged in operons are regulated
For example,
- genes for the lactose utilization are controlled by the presence of lactose
- genes for flagella are controlled by temperature
Genes for enzymes required all the time are not arranged in operons
Two regulated operons
Two E. coli operons reflect how operons in general are regulated
The lac operon
- Encodes inducible catalytic enzymes involved in the lactose utilization and uptake
The arg operon
- Encodes repressible anabolic enzymes involved in the production of the amino acid arginine
The lac operon
Three structural genes
- lacZ: beta-galactosidase which splits lactose to glucose and galactose
- lacY: a permease involved in transport of lactose into the cell
- lacA: a transacetylase, function unknown
A regulatory gene lacI
-The LacI protein is a repressor of the lac operon
A promoter where RNA polymerase binds to transcribe the operon
An operator site where the LacI repressor binds to block transcription
In the absence of lactose (Figure 9.19a)
In the absence of lactose, the repressor binds to operator site preventing binding of RNA polymerase and transcription
In the presence of lactose (Figure 9.19b)
The lac operon is an inducible operon
Lactose acts as the inducer
Lactose induces enzyme expression by binding to the LacI repressor preventing its binding to the operator site
Catabolite repression
The "glucose effect"
Glucose is more rapidly metabolized than lactose, and is a preferred carbon source
Therefore, the presence of glucose represses the lac operon allowing glucose to be used first
How does glucose control the lac operon?
Apart from an operator site, the lac operon contains a site for binding of CRP (cyclic AMP receptor protein).
In order for the lac operon to be transcribed CRP must be bound.
Cyclic AMP (cAMP) is a molecule in the cell that serves as cellular alarm signal (glucose sensor).
When available glucose is high, levels of cAMP are low
When available glucose is low, levels of cAMP are high
CRP only binds to the lac operon when cAMP levels are high (glucose low)
Therefore, even when lactose is present, if glucose is also present, the lactose operon will not be transcribed
The arg operon
A repressible operon: genes are transcribed until turned off or repressed
Three structural genes argCBH encoding enzymes in the arginine biosynthetic pathway
In the absence of arginine (Figure 20a)
The arg operon is transcribed and enzymes for the synthesis of arginine are produced
In the presence of arginine (Figure 20b)
Arginine acts as a corepressor and represses its own synthesis
Like feedback inhibition but acts on enzyme synthesis rather than activity
Summary of gene regulation
The production of many bacterial proteins are controlled by regulating transcription
Functionally related genes are arranged in operons to allow coordinate regulation
Operons can be
- Induced in the presence of substrate
Repressed in the presence of endproduct
Inducible and repressible operons are the major ways in which genes are regulated
Genes can also be regulated by
- Activators - regulatory proteins that are required for gene transcription
- Attenuators - specific sequences in the DNA that prevent either transcription or translation of the gene except in response to certain conditions

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