The Lac Operon
The Lac Operon
Lac Operon merupakan salah satu sistem pengendalian enzim. Pengendalian pada ekspresi DNA dilakukan
untuk memastikan translasi dilakukan pada kodon yang tepat. Pengendalian dilakukan dengan 3 kemungkinan
cara yaitu, pengendalian pada tahap Transkripsi, pengendalian pada tahap Translasi dan pengendalian secara
simultan (transkripsi dan translasi).
Lac-Operon. Lac berasal dari kata Lactose dan Operon merupakan sederetan/sekelompok functional gene (gen
yang fungsional/aktif). Bagan Lac Operon dapat digambarkan sederhana seperti di bawah ini.
Dalam operon, terdapat 2 macam gen. Gen pengendali yang selalu ada pada DNA polimerase dan Gen struktural
yang mengkode protein target. Pada pembahasan ini Gen strukturalnya merupakan Lactose operon karena
mengkode laktosa. Gen ‘i’ mengekspresikan protein ‘i’, dapat berlaku sebagai represor maupun aktivator. Disebut
represor jika keterlibatannyamenghalangi aktivasi enzim struktural. Sebaliknya, disebut aktivator jika
keterlibatannya mengaktifkan enzim struktural sehingga transkripsi dapat berjalan. Gen ‘p’ adalah Promotor dan
‘o’ adalh Operator.
Ada 2 macam kelompok pengendalian sintesis enzim. Pengendalian Positif (+) dan Pengendalian Negatif (-).
Berikut bagannya.
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The Lac Operon
Keterangan :
Molekul Signal adalah molekul asing bukan protein (mirip dengan protein) yang terikat pada represor/aktifator.
(a) Adanya Molekul Signal mengakibatkan terjadinya dissosiasi pada represor sehingga menyebabkan
transkripsi berjalan.
(b) Adanya Molekul Signal mengakibatkan represor terikat pada template DNA sehingga transkripsi tidak
berjalan,
(c) Adanya Molekul Signal mengakibatkan activator ter dissosiasi sehingga transkripsi tidak berjalan
(d) Adanya Molekul Signal mengakibatkan activator dapat menempel pada template DNA sehingga
transkripsi berjalan.
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The Lac Operon
Salah satu contoh Lac-Operon ada pada Bakteri E.Coli.
The lac Operon in E. coli
Introduction
This BioCoach activity will help you understand how gene expression is regulated in bacteria, causing genes to
be turned on and off in response to environmental signals. You will find information on how genes in bacteria
are grouped together in functional groups called operons, the functions of various segments of DNA in operons,
how specific molecules start and stop transcription within operons, and how certain molecules affect the rate of
this transcription. Animations in the activity show these processes. You can check your understanding of the lac
operon by using the interactive Practice exercises and the Self-Quiz at the end of the module.
Concept 1: Gene Regulation in Bacteria
Bacteria adapt to changes in their surroundings by using regulatory proteins to turn groups of
genes on and off in response to various environmental signals.
The DNA of Escherichia coli is sufficient to encode about 4000 proteins, but only a fraction of these are made at
any one time. E. coli regulates the expression of many of its genes according to the food sources that are
available to it.
Concept 2: The Lactose Operon
An operon is a cluster of bacterial genes along with an adjacent promoter that controls the
transcription of those genes.
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The Lac Operon
When the genes in an operon are transcribed, a single mRNA is produced for all the genes in that operon. This
mRNA is said to be polycistronic because it carries the information for more than one type of protein.
Concept 3: The lac Operator
The operator is a short region of DNA that lies partially within the promoter and that interacts with
a regulatory protein that controls the transcription of the operon.
Here's an analogy. A promo
Concept 4: The lac Regulatory Gene
The regulatory gene lacI produces an mRNA that produces a Lac repressor protein, which can bind
to the operator of the lac operon.
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The Lac Operon
In some texts, the lacI regulatory gene is called the lacI regulator gene. Regulatory genes are not necessarily
close to the operons they affect.
The general term for the product of a regulatory gene is a regulatory protein. The Lac regulatory protein is
called a repressor because it keeps RNA polymerase from transcribing the structural genes. Thus the Lac
repressor inhibits transcription of the lac operon.
Concept 5: The Lac Repressor Protein
In the absence of lactose, the Lac repressor binds to the operator and keeps RNA polymerase from
transcribing the lac genes.
Animate
It would be energetically wasteful for E. coli if the lac genes were expressed when lactose was not present.
The effect of the Lac repressor on the lac genes is referred to as negative regulation.
Concept 6: The Effect of Lactose on the lac Operon
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The Lac Operon
When lactose is present, the lac genes are expressed because allolactose binds to the Lac
repressor protein and keeps it from binding to the lac operator.
Allolactose is an isomer of lactose. Small amounts of allolactose are formed when lactose enters E. coli.
Allolactose binds to an allosteric site on the repressor protein causing a conformational change. As a result of
this change, the repressor can no longer bind to the operator region and falls off. RNA polymerase can then bind
to the promoter and transcribe the lac genes.
Concept 7: The lac Inducer: Allolactose
Allolactose is called an inducer because it turns on, or induces the expression of, the lac genes.
The presence of lactose (and thus allolactose) determines whether or not the Lac repressor is bound to the
operator.
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The Lac Operon
Allolactose binds to an allosteric site on the repressor protein causing a conformational change. As a result of
this change, the repressor can no longer bind to the operator region and falls off. RNA polymerase can then bind
to the promoter and transcribe the lac genes.
Concept 8: Feedback Control of the lac Operon
When the enzymes encoded by the lac operon are produced, they break down lactose and
allolactose, eventually releasing the repressor to stop additional synthesis of lac mRNA.
Animate Reset
Messenger RNA breaks down after a relatively short amount of time.
Concept 9: Energy Source Preferences of E. coli
Whenever glucose is present, E. coli metabolizes it before using alternative energy sources such as
lactose, arabinose, galactose, and maltose.
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The Lac Operon
Animate
Glucose is the preferred and most frequently available energy source for E. coli. The enzymes to metabolize
glucose are made constantly by E. coli.
When both glucose and lactose are available, the genes for lactose metabolism are transcribed at low levels.
Only when the supply of glucose has been exhausted does does RNA polymerase start to transcribe the lac
genes efficiently, which allows E. coli to metabolize lactose.
Concept 10: The Effect of Glucose and Lactose on the lac Operon
When both glucose and lactose are present, the genes for lactose metabolism are transcribed to a
small extent.
Maximal transcription of the lac operon occurs only when glucose is absent and lactose is present. The action of
cyclic AMP and a catabolite activator protein produce this effect.
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The Lac Operon
Concept 11: The Effect of Glucose and Cyclic AMP on the lac Operon
The presence or absence of glucose affects the lac operon by affecting the concentration of cyclic
AMP.
The concentration of cyclic AMP in E. coli is inversely proportional to the concentration of glucose: as the
concentration of glucose decreases, the concentration of cyclic AMP increases.
Animate
Cyclic AMP is derived from ATP.
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The Lac Operon
Concept 12: The Effect of Lactose in the Absence of Glucose on the lac Operon
In the presence of lactose and absence of glucose, cyclic AMP (cAMP) joins with a catabolite
activator protein that binds to the lac promoter and facilitates the transcription of the lac operon.
In some texts, the catabolite activator protein (CAP) is called the cAMP-receptor protein.
Animate
When the concentration of glucose is low, cAMP accumulates in the cell. The binding of cAMP and the catabolite
activator protein to the lac promoter increases transcription by enhancing the binding of RNA polymerase to the
lac promoter.
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