The operon is what?

была впервые предложена Ж. Моно и Ф. Жакобом. The model of the operon was first proposed by J. Mono and F. Jacob. In 1961, they examined the processes associated with the development of Escherichia coli cells. In particular, the mechanisms of control of DNA sites encoding milk sugar (lactose) were studied .

Description

Mono and Jacob proposed a scheme for coordinated control over the activity of structural DNA sections. Scientists gave it the name " operon." схемы следующий. The principle of the scheme is as follows. The transcription of a group of DNA regions encoding polypeptides that are closely related to each other functionally is controlled by two elements. The first is the regulator, and the second is the operator. The latter is represented by a sequence of nucleotides adjacent to the controlled structural regions. If the product of the regulator is a protein-compressor, its attachment to the operator will block the transcription of the DNA links. This, in turn, is a prerequisite for the formation of steric hindrances for the coupling of RNA polymerase to a specific promoter site. The latter is necessary for the initiation of transcription. Other developments will occur if the apoinductor acts as a regulator. In this situation, his connection with the operator will create conditions for transcription.

Specificity

последовательность ДНК, которая состоит из тесно сцепленных кодирующих участков, промотора и оператора. The operon is a DNA sequence that consists of closely linked coding regions, a promoter and an operator. The control element can be located next to it or at a certain distance from it. The operator is often located between the structural sections and the promoter. There are low-molecular substances with which the operon can be controlled . в частности, эффекторы, которые являются индукторами или крепрессорами структурных участков, включенных в схему. This is, in particular, effectors, which are inducers or cushions of the structural sections included in the scheme.

Classification

There is an inducible or repressed operon. зависит от характера влияния на их деятельность молекул-эффекторов. This depends on the nature of the effect on their activity of the effector molecules. In induced structures, it is attached to the repressor, blocking binding to the operator. Accordingly, there are obstacles for transcription of structural sites. называется негативной. Such regulation of operons is called negative. At the same time, induced structures can be under positive control. In this case, the effector binds to the regulatory protein molecule, activating its apoinducer. Joining the operator, he provides the possibility of transcription. These types of control also apply to repressed structures. If the regulation is negative, the effector acting as the corepressor is connected to the inactive repressor and activates it. As a result, the latter gets the ability to dock with the operator, thereby blocking transcription. If the control is positive, the binding occurs with the active apoinducer. This complex can not connect to the operator. Accordingly, the structural areas are not transcribed.

conclusions

With negative control, thus, the effector is connected to the repressor, causing inactivation or activation. Accordingly, the transcription of the operon is induced or repressed. In case of positive control, the connection is made to the apoinductor. This process blocks or permits transcription. The result depends on the shape that the apoinductor acquires when attached to the effector.

The structure of the operon of E. coli

The E. coli has a structure that is capable of fermenting milk sugar. It includes a promoter, an operator and three structural regions of DNA. Enzymes of gengalactosidase, galactosidepermease, thiogalactoside transacetylase are encoded. For each of them there is a gene. The operon includes sections lac Z, lac A, lac Y. The first encodes gengalactosidase, which catalyzes milk sugar to glucose and galactose. Lac Y interacts with galactoside permease. This enzyme provides transportation of different sugars. Lac A encodes thiogalactoside transacetylase. Its role, however, in the process of utilization of milk sugar is not clear. As a rule, all proteins are in the cells of Escherichia coli in trace amounts. But when growing it in an environment where lactose acts as the sole source of energy and carbon, the number of enzymes grows 1000 times.

Constitutional synthesis

включает в себя структурный участок lac 1. Им кодируется белок-репрессор. The lactose operon includes the lac 1 structural site. It encodes a repressor protein. In the active state, it is a tetramer, which is formed from four copies of the lac 1 region - polypeptides, including 360 amino acids. Cells with changes in this gene are constitutive for the synthesis of proteins encoded by lac Z, Y and A. This situation is possible with mutations not only in the repressor, but also in the operator. Such changes are always cis-dominant. This is due to the fact that the operator, unlike the repressor, can influence the possibility of transcription only if it is located in the immediate vicinity of the promoter. If there are induced compounds in the cell, they enter into competition with the operator for the repressor molecules.

Inductors

They can be different connections. Milk sugar acts simultaneously as an inductor, and as a substrate. In normal cells and in the absence of induced compounds, the residual activity of the enzymes allows the penetration of lactose in minimal amounts. As a result of the catalytic reaction, the milk sugar is transformed into allolactose. He, in turn, communicates with the repressor and provokes his detachment from the operator. This allows the RNA polymerase to dock with the promoter. As a result, lac Z, A and Y transcription is triggered. Compounds acting only as inducers are IPTG and TMG used to study the control of the lactose operon.

Positive elements

Their detection is due to diaxia. The essence of this phenomenon is that the utilization of milk sugar begins only after using all the glucose present in the medium. Diaxia is one of the manifestations of catabolic repression. This glucose effect is known from the 1940's. last century. It is expressed in the inability of the E. coli to catabolize different carbohydrates in the presence of glucose. It, in turn, acts as a more efficient source of energy.

Mechanism of Effect

It was able to decipher Pastan and Perlman. They found two elements by which the lac operon is transcribed . небольшая молекула-эффектор – цАМФ (циклический аденозинмонофосфат) и белок-активатор катаболизма САР. This is a small molecule-effector - cAMP (cyclic adenosine monophosphate) and a protein-activator of CAB catabolism. In eukaryotes, the first acts as a mediator of the action of hormones. It was found that when cAMP is added to E. coli cells growing in a medium with the presence of glucose, their speed slows down, but at the same time catabolic repression is removed. This, in turn, causes the possibility of expression of the lac operon in the simultaneous presence of lactose and glucose. After a while, an inverse relationship was revealed. The activity of the enzyme that synthesizes cAMP is suppressed by glucose.

Conclusion

Transcription of the operon is under double control - negative and positive. The CAP-cAMP complex allows the RNA polymerase to bind to the template DNA prior to the start of the process. At present, scientists have deciphered the complete nucleotide chain of the regulatory region of the lac operon, in which an operator and a promoter are present. Moreover, in 1969, his pure DNA containing the fragment lac 1, the fully promoter and operator sequences, lac Z and the lac Y fragment was isolated in 1969. It was established that a significant role in the interaction of multimeric proteins with DNA belongs to symmetrical structures - palindromes . In the lac operon operator, there are 26 of them. At the same time, 14 of them differ in specificity. In different chains they are read alike, but in the opposite direction. The palindrome was also found in the promoter region interacting with the CAP-cAMP complex.