Let us discuss about the two types of gene expression regulation.
The two types of gene expression regulation are: (1) Negative Regulation and (2) Positive Regulation. And also discuss about some important terms used in connection with the regulation of gene expression.
Most of the genes of an organism produce specific proteins (enzymes), which, in turn produce specific phenotypes. The genes whose mRNA transcripts are translated into protein are known as structural genes. Every cell of an organism possesses all the structural genes normally present in the species, but only a small fraction of them are functional in any cell at a given time.
In prokaryotes, cells generally synthesize only those enzymes which they need in a given environment. For example, E. coli cells grown in the presence of lactose produce abundant (up to 3000 molecules/cell) β-galactosidase, the enzyme that hydrolyses lactose. However, very little of this enzyme (less than 3 molecules/cell) is produced in the absence of lactose.
In eukaryotes, the cells of different organs produce different proteins needed for their function. Red blood cells contain a high concentration of hemoglobin, while leucocytes (white blood cells) have no hemoglobin at all.
Apparently, there is a precise control on the kinds of proteins or enzymes product in a given tissue or cell at a given time. Such a control on gene activity, i.e., protein production, that permits the function of only those genes whose products are required in a given cell at a given time is termed as gene regulation.
Synthesis of enzyme depends mainly on two factors in a degradative process, the synthesis of enzyme depends on the availability of the molecule to be degraded. If the molecule is in more quantity, the enzyme synthesis will be more and vice versa. In a biosynthetic pathway, the synthesis of an enzyme is controlled by the end product. If the end product is more, the enzyme synthesis will be less and vice versa.
There are two types of gene regulation, viz:
(1) Negative regulation, and
(2) Positive regulation.
(1) In negative regulation:
An inhibitor is present in the cell/system, that prevents transcription by inactivating the promoter. This inhibitor is known as repressor. For initiation of transcription, an inducer is required. Inducer acts as antagonist of the repressor. In the negative regulation, absence of product increases the enzyme synthesis and presence of the product decreases the synthesis.
(2) In positive regulation:
An effector molecule (which may be a protein or a molecular complex) activates the promoter for transcription. In a degradative system, either negative or positive mechanism may operate, while in a biosynthetic pathway negative mechanism operates (e.g., lac operon).
The phenomenon of gene expression can be elaborated further such as given below:
1. Gene expression is the mechanism at the molecular level by which a gene is able to express itself in the phenotype of an organism.
2. The mechanism of gene expression involves biochemical genetics. It consists of synthesis of specific RNAs, polypeptides, structural proteins, proteinaceous bio-chemicals or enzymes which control the structure or functioning of specific traits.
3. Gene regulation is the mechanism of switching off and switching on of the genes depending upon the requirement of the cells and the state of development.
4. It is because of this regulation that certain proteins are synthesized in as few as 5-10 molecules while others are formed in more than 100,000 molecules per cell.
5. There are two types of gene regulations positive and negative.
6. In negative gene regulation the genes continue expressing their effect till their activity is suppressed.
7. This type of gene regulation is also called repressible regulation.
8. The repression is due to a product of regulatory genes.
9. Positive gene regulation is the one in which the genes remain non-expressed unless and until they are induced to do it.
10. It is, therefore called inducible regulation.
11. Here a product removes d biochemical that keeps the genes in non-expressed state.
12. As the genes express their effect through enzymes, their enzymes are also called inducible enzymes and repressible enzymes.
Gene regulation is exerted at four levels:
1. Transcriptional level when primary transcript is formed.
2. Processing level when splicing and terminal additions are made.
3. Transport of mRNA out of nucleus into cytoplasm.
4. Translational level.
Important Terms used in Connection with the Regulation of Gene Expression:
In operon, protein molecules which prevent transcription. The process of inhibition of transcription is called repression.
The substance that allows initiation of transcription (e.g., lactose in lac operon). Such process is known as induction.
A combination of repressor and a metabolite which prevents protein synthesis. Such process is known as co-repression.
4. Inducible Enzyme:
An enzyme whose production is enhanced by adding the substrate in the culture medium. Such system is called inducible system.
5. Repressible Enzyme:
An enzyme whose production can be inhibited by adding an end product. Such system is known as repressible system.
6. Constitutive Enzyme:
An enzyme whose production is constant irrespective of metabolic state of the cell.
7. Negative Control:
Inhibition of transcription by repressor through inactivation of promoter, e.g., in lac operon.
8. Positive Control:
Enhancement of transcription by an effector molecule through activation of pro-motor.
The molecule that acts as an inducer or co-repressor in the operon model of E.coli.