Introduction to lactose operon, what is the role of lactose operon

Lactose operons are a group of genes involved in lactose breakdown, which is composed of repressors and manipulation sequences of the lactose system, so that a group of genes related to lactose metabolism are synchronously regulated. In 1961, F Jacob and J Mood proposed the famous operon theory based on the study of the system. In the lactose system operon of Escherichia coli, the structural genes of -galactosidase, galactoside osmose, and galactoside transacylase are arranged on chromosomes in the order of lacz(z), lac y(y), and lac a(a), and there is a manipulation sequence lac o(o) upstream of z, and a promoter lac p(p) in front of it, which is the structural pattern of the operon (lactose operon).

The regulatory gene lac I(I), which encodes a repressor in the lactose manipulation system, is located in close proximity to P.

Composition of lactose operon: Escherichia coli lactose operon contains three structural genes, Z, Y and A, which encode galactosidase, permealine and galactoside acetyltransferase, respectively, in addition to a manipulation sequence O, a promoter p and a regulatory gene I.

Negative regulation of repressors:

In the absence of lactose, the repressor protein encoded by the I gene binds to the manipulation sequence O, and the lactose operon is in a repressive state and cannot synthesize the three enzymes that decompose lactose.

In the presence of lactose, lactose acts as an inducer to induce the allosterism of the repressor protein, which cannot bind to the manipulation sequence, and the lactose operon is induced to open up the synthesis of three enzymes that decompose lactose. Therefore, this regulatory mechanism of lactose operon is inducible negative regulation.

Positive regulation of CAP:

There is a CAP binding site upstream of the promoter, when Escherichia coli changes from an environment with glucose as a carbon source to an environment with lactose as a carbon source, the concentration of CAMP increases, and it binds to CAP to make CAP allosteric, CAP binds to the CAP binding site near the initiation sequence of the lactose operon, activates RNA polymerase activity, promotes the transcription of structural genes, promotes the transcription of structural genes after the regulatory protein binds to the operon, positively regulates the lactose operon, and accelerates the synthesis of three enzymes that decompose lactose.

Regulation of glucose metabolite repression effect: synthesis of glucose metabolites --- straight lacmRNA.

Coordinated regulation: The negative regulation of the repressor encoded by the I gene in the lactose operon and the positive regulation of CAP are coordinated and mutually restricted.

Intracellular lactose levels are sensed and the expression of galactosidase is regulated according to the level of intracellular lactose.

Lactose operon has the following parts: (1) Regulatory genes: regulate secretion of repressors.

2) Promoter: Initiates the transcription and translation of genes. (3) Manipulative genes:

Binding site with lactose. (4) Galactosidase genes z, y, a.

In the absence of lactose, the regulatory gene expresses a repressor protein, which binds to the oppressor located behind the promoter, preventing transcription from proceeding, and the galactosidase gene is not transcribed. When lactose is present, lactose binds to the repressor protein so that the repressor cannot bind to the operant gene, and the galactosidase gene is transcribed.

Applications: (1) It is used to identify whether the target gene is inserted into the vector.

The carrier contains a modified lactose operon. The lactose operon retains only the n-terminus of the galactosidase gene and inserts multiple cloning sites into the galactosidase gene. The vector is hosted by Escherichia coli that can only express galactosidase C-terminal polypeptides.

When galactosidase C-terminal polypeptide or N-terminal polypeptide is alone, it is inactive; When the two coexist, they can exert galactosidase activity, which can decompose X-gal into a blue product.

Transfer the vector into E. coli cultured in medium containing X-gal and induce with IPTG (lactose analogue), if the colonies turn blue, the gene of interest is not inserted; If the colony is white, it indicates that the gene of interest is inserted.

2) It is used to specifically express the target gene. If the lactose operon is added before the gene of interest, the gene is not expressed when there is no lactose in the cell. Gene expression occurs when lactose or IPTG is added to the medium.

Positive and negative regulatory mechanisms of lactose operons:

1. The lactose operon (lac) is composed of a regulatory gene (lac I), a promoter (lac p), a manipulation gene (lac O) and a structural gene (lac z, lac y, lac a). LAC I encodes a repressor, and LAC Z, LAC Y, and LAC A encode -galactosidase, -galactoside permeability enzyme, and -galactoside transacetylase, respectively.

2. Negative regulation of repressor proteins: when there is no lactose in the medium, the repressor protein binds to the manipulative genes in the operon, preventing the expression of structural genes.

When there is lactose in the culture medium, the lactose (really isolactose) molecule binds to the repressor protein, causing a conformational change of the repressor protein, which cannot bind to the manipulative gene, so that the RNA polymerase can normally catalyze the structural gene on the transcription operon, that is, the operon is induced to be expressed.

3. CAMP-CAP is an important positive regulator, which can bind to the promoter region on the manipulation and initiate gene transcription. The glucose content in the medium decreased, the CAMP synthesis increased, and the CAMP and CAP formed a complex and bound to the promoter to promote the expression of lactose operon.

4. Coordinated regulation: the two mechanisms of negative regulation of repressor protein encoded by lactose operon regulation gene and positive regulation of CAP are coordinated and mutually restricted.

Structural genes related to bacterial functions are often linked together to form a gene cluster. They encode different enzymes in the same metabolic pathway. A gene cluster is regulated by the same way, one open and one closed.

That is, they form a regulated unit, and other genes related to functions are also included in this regulatory unit, such as the gene encoding an enzyme, and although its products are not directly involved in catalytic metabolism, it can transport small molecule substrates into cells.

Lactose operonThese include regulatory genes, initiating genes, manipulative genes, and structural genes. The lactose operon Escherichia coli's lac operon is regulated in two ways: one is rightRNA polymeraseCombine toPromoterup regulation (positive regulation); The second is the regulation of manipulative genes (negative regulation).

Operon in glucose-bearing.

The regulatory mechanism of Escherichia coli can not use lactose in the culture medium, and lactose can only be used when it is changed to lactose: when there is only lactose in the medium, because the metabolite of lactose isolactose is an inducer of the lac operon, it can bind to the allosteric site of the repressor protein and change the conformation.

Disruption of the affinity of repressor proteins to manipulate genes.

Unable to bind to the manipulation gene, RNA polymerase binds to the promoter, and smoothly manipulates the gene to transfect the structural gene to produce a large number of enzymes that break down lactose, which is why lactose is used when there is only lactose in the culture medium of E. coli.

When glucose is added to lactose-containing media, the reason for the inability to take advantage of lactose is that in the regulation of the LAC operon, there is a degrader gene-activating protein (CAP), which, when specifically bound to the promoter, promotes the binding of RNA polymerase to the promoter and promotes transcription (due to the binding energy of CAP.

Promotes transcription, known as a positive regulatory modality).

However, the free CAP cannot bind to the promoter, and when there is enough CAMP in the cell, the CAP first forms a complex with the CAMP before this complex can bind to the promoter.

The degradation products of glucose can reduce the amount of camp in cells, and when glucose is added to the lactose medium, the cAMP concentration decreases and CAP cannot bind to the promoter. At this time, even if lactose exists, RNA polymerase cannot bind to the promoter, and although the inhibition of the manipulation gene has been lifted, it cannot be transcribed, so the milk cavity can still not be used to block sugar.

Not lactose, not lactose.

It is the cleolactose produced by lactose metabolism that binds to the repressor proteins produced by the regulatory genes, not the lactose itself.

Lactose operon mechanism:

Inhibition: Regulates the transcription of genes into mRNA to synthesize repressor proteins, which are able to recognize and bind to manipulator genes due to their conformation, due to the lack of lactose, RNA polymerase.

It cannot bind to the initiating gene, and the structural gene is also suppressed, and as a result, the structural gene cannot transcribe mRNA and cannot translate enzyme proteins.

Induction: In the presence of lactose, lactose metabolism produces allolactose, which can bind to the repressor protein produced by the regulatory gene, so that the repressor protein changes its conformation, and cannot be combined with the manipulative gene, losing the repressive effect, and as a result, RNA polymerase binds to the initiating gene, activates the structural gene, transcribes mRNA, and translates the enzyme protein.

Negative feedback: cytoplasm.

In the possession — galactose shouts scum silver.

After the enzyme, it catalyzed Liang Kai to decompose lactose into galactose and glucose.

When lactose is broken down, it causes repressor proteins to bind to manipulative genes, causing structural genes to shut down.

Composition of lactose operon: The E. coli lactose operon contains three structural genes Z, Y, and A, which encode galactosidase, osmose, and galactosidase acetyltransferase, respectively. In addition, there is an operand sequence O, a promoter p, and a regulatory gene I.

Negative regulation of inhibitory proteins: In the absence of lactose, the inhibitory protein encoded by gene I binds to the operational sequence O, and the lactose operon is in a repressive state, unable to synthesize the three enzymes that break down lactose.

In the presence of lactose, lactose is used as an inducer to induce the allosteric inhibitor protein that cannot bind to the control sequence. The lactose operon is induced to turn on the synthesis of three enzymes that break down lactose. Therefore, the regulatory mechanism of lactose operon is negative regulation.

Structural genes for functions associated with bacteria are often linked together to form a group of genes. They encode different enzymes in the same metabolic pathway. A gene cluster is regulated in the same way, one open and one closed. They thus form a regulated unit.

Other relevant functional genes are also included in this regulatory unit, such as genes encoding enzymes, whose products are not directly involved in catalytic metabolism but enable the transport of small molecule substrates to cells.

The lactose operon is composed of the regulatory gene Laci, the initiator gene P, the regulatory gene O, and three structural genes, Lacz, Lacy, and Laca.

The order on chromosomes is: p--laci--- p--o--lacz--- lacy---laca

LACI has its own promoter and terminator and can express genes on its own. It encodes a repressor protein that binds both to the manipulative gene LACO and to inducers (isolactose, iPTG, etc.). When the repressor protein binds to LACO, it affects the binding of RNA polymerase to LACP and hinders the passage of RNA polymerase through LACO, so that structural genes cannot be transcribed; When the repressor protein binds to the inducer and its conception changes and cannot bind to LACO, then the open LACO can be transcribed and translated to form an expression product that utilizes lactose.

Inhibition: Regulate the transcription of mRNA by genes and synthesize repressor proteins, due to the lack of lactose, repressor proteins can recognize manipulative genes and bind to manipulative genes because of their conception, because RNA polymerase cannot bind to initiating genes, structural genes are also inhibited, and as a result, structural genes cannot transcribe mRNA and cannot translate enzyme proteins. Induction:

When lactose exists, lactose metabolism produces isolactose, which can bind to the repressor protein produced by the regulatory gene, so that the repressor protein changes its conformation, can no longer bind to the manipulative gene, and loses the repressive effect, as a result, RNA polymerase binds to the initiator gene, and activates the structural gene, transcribes mRNA, and translates the enzyme protein. Negative feedback: With galactosidase in the cytoplasm, it catalyzes the decomposition of lactose and glucose.

After lactose is decomposed, it causes repressor proteins to bind to manipulative genes, causing structural genes to be turned off.