The operon model was first proposed by J. Mono and F. Jacob. In 1961, they investigated processes associated with the development of E. coli cells. In particular, the mechanisms of control of DNA sections encoding milk sugar (lactose) were studied .
Description
Mono and Jacob proposed a scheme for coordinated control of the activity of structural sections of DNA. Scientists gave it the name " operon". The principle of operation of the circuit is as follows. The transcription of a group of DNA regions encoding polypeptides that are closely linked functionally to each other 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 a protein compressor acts as a regulator product, then its attachment to the operator will block the transcription of DNA units. This, in turn, is a prerequisite for the formation of steric barriers to the connection of RNA polymerase with a specific promoter site. The latter is necessary to initiate transcription. A different development of events will be if the apoinductor acts as a regulator. In this situation, its connection with the operator will create conditions for transcription.
Specificity
An operon is a DNA sequence that consists of closely linked coding regions, a promoter, and an operator. The control element may be located next to it or at a certain distance from it. The operator is often located between structural regions and the promoter. There are low molecular weight substances that can be controlled by an operon. These are, in particular, effectors, which are inductors or crepressors of structural sections included in the circuit.
Classification
There is an inducible or repressed operon. It depends on the nature of the influence of effector molecules on their activities. In inducible structures, it joins the repressor, blocking binding to the operator. Accordingly, obstacles arise for the transcription of structural sites. Such regulation of operons is called negative. However, inducible structures can be under positive control. In this case, the effector binds to a regulatory protein molecule, activating its apoinducer. Joining the operator, it provides the possibility of transcription. These types of controls also apply to repressed structures. If the regulation is negative, the effector acting as a corepressor connects to the inactive repressor and activates it. As a result, the latter gains the ability to dock with the operator, thereby blocking transcription. If the control is positive, binding occurs with the active apoinducer. This complex cannot connect to the operator. Accordingly, structural regions are not transcribed.
conclusions
With negative control, in this way, the effector connects to the repressor, causing inactivation or activation. Accordingly, transcription of the operon is induced or repressed. In the case of positive control, accession is carried out to the apoinductor. This process blocks or allows transcription. The result depends on the shape that the apoinducer takes when attaching to the effector.
The structure of the operon E. coli
E. coli has a structure capable of fermenting milk sugar. It includes a promoter, an operator, and three structural regions of DNA. Enzymes gengalactosidase, galactosidepermease, thiogalactosidotransacetylase are encoded. Each of them has its own gene. The operon includes sections of lac Z, lac A, lac Y. The former encodes a gene galgactosidase that catalyzes milk sugar to glucose and galactose. Lac Y interacts with galactoside permease. This enzyme provides transport of various sugars. Lac A encodes a thiogalactoside transacetylase. Its role, however, in the process of utilization of milk sugar is not clear. As a rule, all proteins are found in the cells of E. 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.
Constitutive synthesis
The lactose operon includes the structural region of lac 1. It encodes a repressor protein. In the active state, it is a tetramer, which is formed from four copies of the lac 1 site - polypeptides that include 360 amino acids. Cells with changes in this gene are constitutive for the synthesis of proteins encoded by lac Z, Y and A. Moreover, this situation is possible in the presence of mutations not only in the repressor, but also in the operator. Such changes are always cis-dominant. This is due to the fact that an operator, unlike a repressor, can influence the possibility of transcription only if it is located in the immediate vicinity of the promoter. If induced compounds are present in the cell, they compete with the operator for repressor molecules.
Inductors
They may be different compounds. Milk sugar acts both as an inducer 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 quantities. As a result of the catalytic reaction, milk sugar is transformed into allolactose. He, in turn, communicates with the repressor and provokes its disconnection from the operator. This allows RNA polymerase to bind to the promoter. As a result, transcription of lac Z, A and Y is started. Compounds acting only as inducers are IPTG and TMG, which are used to study the control of the lactose operon.
Positive Acting Elements
Their identification is caused by diauxia. The essence of this phenomenon is that the utilization of milk sugar begins only after the use of all available glucose in the medium. Diauxia is one of the manifestations of catabolite repression. This glucose effect has been known since the 40s. last century. It is expressed in the inability of E. coli to catabolize various carbohydrates in the presence of glucose. She, in turn, acts as a more efficient source of energy.
Effect mechanism
He was able to decipher Pastan and Perlman. They found two elements by which the lac operon is transcribed . This is a small effector molecule - cAMP (cyclic adenosine monophosphate) and an activator of catabolism ATS. In eukaryotes, the first acts as a mediator of the action of hormones. It turned out that when cAMP was added to the cells of Escherichia coli growing in an environment with the presence of glucose, their speed slows down, but catabolic repression is removed. This, in turn, makes it possible to express the lac operon with the simultaneous presence of lactose and glucose. After some time, an inverse relationship was revealed. The activity of the enzyme synthesizing cAMP is suppressed by glucose.
Conclusion
The transcription of the operon is under double control - negative and positive. The CAP-cAMP complex allows RNA polymerase to bind to template DNA before the start of the process. Currently, scientists have deciphered the complete nucleotide chain of the regulatory region of the lac operon, in which there is an operator and a promoter. Moreover, in 1969, its pure DNA was isolated containing the lac 1 fragment, the completely promoter and operator sequences, lac Z and the lac Y fragment. The study found that a significant role in the interaction of multimeric proteins with DNA belongs to symmetrical structures - palindromes . There are 26 of them in the lac-operon operator. Moreover, 14 of them are specific. In different chains they are read the same, but in the opposite direction. A palindrome was also detected in the promoter region interacting with the CAP-cAMP complex.