Lac+Operon


 * Refer this website for animations related to Lac operon**

http://www.phschool.com/science/biology_place/biocoach/lacoperon/quiz.html?x=62&y=11 **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. 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.

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 promoter is like a doorknob, in that the promoters of many operons are similar. An operator is like the keyhole in a doorknob, in that each door is locked by only a specific key, which in this analogy is a specific regulatory protein. **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. Animate In some texts, the lacI regulatory gene is called the lacI regulator gene. Regulatory genes are not necessarily close to the operons they affect.
 * Concept 3: The lac Operator**

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.

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.
 * Concept 5: The Lac Repressor Protein**

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** 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. Animate 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. 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.

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.
 * Concept 8: Feedback Control of the lac Operon**

Animate Reset Messenger RNA breaks down after a relatively short amount of time.

Whenever glucose is present, E. coli metabolizes it before using alternative energy sources such as lactose, arabinose, galactose, and maltose. 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.
 * Concept 9: Energy Source Preferences of 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.

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.
 * Concept 10: The Effect of Glucose and Lactose 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.
 * Concept 11: The Effect of Glucose and Cyclic AMP 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.
 * Concept 12: The Effect of Lactose in the Absence of Glucose on the lac Operon**

http://www.phschool.com/science/biology_place/biocoach/lacoperon/quiz.html?x=62&y=11
 * Self-Quiz**

You may refer to this illustration while answering any of the questions in the Self-Quiz in above mentioned site.

Mutations typically disable a gene. Predict the phenotype of the following mutants. If you answer correctly, you can watch an animation demonstrating the phenotype of that mutant.

1. Predict the phenotype of a lacI mutant.

a.

The lac genes would be expressed efficiently only in the absence of lactose.

b.

The lac genes would be expressed efficiently only in the presence of lactose.

c.

The lac genes would be expressed continuously.

d.

The lac genes would never be expressed efficiently.

2. If a second wild type or normal copy of the lacI gene (just lacI and not lacZ, lacY, or lacA) is introduced into the lacI mutant cell, what would be the phenotype of this partial diploid (also referred to as a merodiploid)?

a.

The lac genes would be expressed efficiently only in the absence of lactose.

b.

The lac genes would be expressed efficiently only in the presence of lactose.

c.

The lac genes would be expressed continuously.

d.

The lac genes would never be expressed efficiently.

3. Predict the phenotype of a lacI S or "super-repressor" mutant. A lacI S mutant synthesizes a repressor that cannot bind to the inducer.

a.

The lac genes would be expressed efficiently only in the absence of lactose.

b.

The lac genes would be expressed efficiently only in the presence of lactose.

c.

The lac genes would be expressed continuously.

d.

The lac genes would never be expressed efficiently.

4. Predict how a lacI S mutant would be affected by the construction of a merodiploid that has a second normal copy of the lacI gene.

a.

The lac genes would be expressed efficiently only in the absence of lactose.

b.

The lac genes would be expressed efficiently only in the presence of lactose.

c.

The lac genes would be expressed continuously.

d.

The lac genes would never be expressed efficiently.

5. Predict the phenotype of an operator mutant (O c) which would prevent the binding of the repressor.

a.

The lac genes would be expressed efficiently only in the absence of lactose.

b.

The lac genes would be expressed efficiently only in the presence of lactose.

c.

The lac genes would be expressed continuously.

d.

The lac genes would never be expressed efficiently.

6. Predict the phenotype of a promoter mutant (lacP) which has a mutation in the promoter for the lac operon.

a.

The lac genes would be expressed efficiently only in the absence of lactose.

b.

The lac genes would be expressed efficiently only in the presence of lactose.

c.

The lac genes would be expressed continuously.

d.

The lac genes would never be expressed efficiently.

7. Predict the phenotype of a lacZ mutant, which has a mutation in the gene for β-galactosidase.

a.

The production of all protein products would be affected.

b.

The production of β-galactosidase would be affected, but other protein products would be unaffected.

c.

The production of β-galactosidase would be affected, and the production of some other protein products might also be affected.

d.

The production of β-galactosidase would be unaffected, but other protein products would be affected.

8. Predict the phenotype of a lacY mutant, which has a mutation in the gene for lactose permease.

a.

The lac genes would be expressed efficiently only in the absence of lactose.

b.

The lac genes would be expressed efficiently until the lactose supply in the cell is exhausted.

c.

The lac genes would be expressed continuously.

d.

Expression of the lac genes would cease immediately.