REGULATION OF GENE EXPRESSION ( Lac Operon & Trp Operon)

REGULATION OF GENE EXPRESSION

Constitutive genes (House-keeping genes) - These genes are expressed constantly because their products are constant needed for cellular activity, eg. genes for glycolysis, gene of ATPase enzyme.

 Non-constitutive genes (Smart gene or Luxury gene) - These genes remain silent and are expressed only when the gene product is needed. They are switched 'on' or 'off' according to the requirement of cellular activities. Non-constitutive genes are of two types;  inducible and repressible.  The Inducible genes are switched on in presence of a chemical substance called inducer, required for the functioning of gene activity. The repressible genes continue to express themselves till a chemical,, often an end product of the metabolism inhibits or represses their activity. Such type of inhibition is called feed back inhibition or feed back repression.

The mechanism which stimulates the expression of certain genes and inhibits that of others is called regulation of gene expression. 

It is possible only if the organism has a mechanism of regulating gene activity by allowing some to function and others to restrain their activity through switching on and switching off system. This means, the genes are turned 'on' or 'off' as per requirement.

A set of genes is 'switched on'  when enzymes are required to metabolise a new substrate. The enzymes produced by these genes metabolise the substrate.

The molecules of metabolite that come to switch on of the genes are termed as inducers and the phenomenon  is called induction.

Similarly, certain genes which are in their 'switch on' state, continue to synthesise a metabolite till the later is produced in amount more than required or else, It is supplied to the cell from outside. In other words, certain genes continue to express themselves till the end product of inhibits or repress their  expression. Inhibition by an end product is known as 'feed back repression'.

Regulation of gene expression refers to a very broad term that may occur at various levels. Considering that gene expression results in the formation of a polypeptide, it can be regulated at several levels. In eukaryotes the regulation could be exerted at:-

1.  transcriptional level (formation of primary transcript). 

2. processing less (regulation of splicing).

3. transport of mRNA from nucleus to the cytoplasm.

4. translational level.

OPERON CONCEPT

In 1961, two French microbiologist Francis Jocob and Jacques Monad at the Pasteur Institute in Paris, proposed a mechanism called operon model for the regulation of gene action in E coli.

An operon is a part of genetic material or DNA, which acts as a single regulated unit having one or more structural genes-an operator gene, a promoter gene, a regulator gene.

Operons are of two types:-

 (i) inducible

 (ii) repressible

1. Inducible System (Lac operon of E. coli)

An Inducible operon system normally remains in switched off condition and begins to work only when the substance to be metabolised by it is present in the cell. Inducible operon system generally occurs in catabolic pathways. e.g. Lac operon of E. coli.

Active repressor + inducer = inactive repressor

An Inducible operon system consists of four types of genes.

Structural genes - These genes synthesise mRNAs, which in turn synthesise polypeptide or enzyme over the ribosomes. An operon may have one or more structural genes. Each structural gene of an operon is called cistron. The lac operon (lactose operon) of Escherichia coli contains three structural genes (Z, Y and A). These genes occur adjacent to each other and thus are linked. They transcribe a polycistronic mRNA molecule (a single stretch of mRNA covering all the three genes), that helps in the synthesis of three enzymes-β galactosidase (breaks lactose into glucose and galactose) lactose permease (helps in entry of lactose in cell from outside and transacetylase (transfers an acetyl group from acetyl Co A to β galactosidase).

Operator gene - It lies adjacent to the structural genes and directly controls the synthesis of mRNA over the structural genes. It is switched off by the presence of a repressor. An inducer can take away repressor and switch on the gene that directs the structural genes to transcribe.

Promoter gene - This gene is the site for initial binding of RNA polymerase. When the operator gene is turned on, the enzyme RNA polymerase moves over it and reaches the structural genes to perform transcription.

Regulator gene- It produces a repressor that binds to operator gene and stops the working of the  opertor gene.

Repressor - It Is a protein, producd by the regulator gene. It binds to the operator gene so that the transcription of structural gene stops. Repressor has two binding site (1) operator gene (2) effective molecule (inducer/corepressor. 

 Inducer- It is a chemical (substrate, hormone or some  other metabolite) which after coming in contact with the repressor, forms an inducer repressor complex. This complex cannot bind with the operator gene, which is thus switched on. The free operator gene allows the structural gene to transcrip mRNA to synthesise the enzymes. 

The inducer for lac operon of Escherichia coli is lactose (in fact allolactose as isomer of lactose). When the sugar lactose is added to the culture of E.coli, a few molecules of lactose gets into the bacterial cells by the action of the enzyme permease, a small amount of this enzyme is present in the cell even when the operon is not working. These few lactose molecules are then converted into an active from which acts as an inducer and binds to the repressor protein. The inducer repressor complex fails to join with the operator, which is turned on. The three genes are expressed as three enzymes to metabolise lactose. Allolactose is real inducer of lac operon. 

Repressible system (Tryptophan operon of E.coli)

A repressible operon system is normally in It's switch on state and continue to synthesise a metabolise till the latter is produced in amount more than required, or else it becomes available to the cell from outside. Repressible operon system is commonly found in anabolic pathway, eg. Tryptophan operon of E.coli.

[Inactive repressor + co-repressor =acitve repressor] 

Tryptophan operon of Escherichia coli is an example of repressible system. It consists of the following:

Structural genes- These genes are meant for transcription of mRNA, which in turn synthesise enzyme.

Tryptophan operon has five structural genes E, D, C,B and A. They lie in continuation and synthesise enzymes for five steps of tryptophan synthesis.

Operator gene (trp O)- It lies adjacent to the structural genes and controls the functioning of the structural genes. Normally, it is kept switched on, because the apo-repressor produced by the regulator gene does not bind to it. The operator gene is switched off when a co-repressor is available alongwith apo repressor.

Promoter gene (trp P)- It marks the site at which the RNA polymerase enzyme binds. When the operator gene is switched on, it moves from promotor gene to structural genes for transcription.

Regulator gene (trp R)- It produces a regulatory protein called apo-repressor for (Inactive repressor) possible blocking the activity of operator gene.

Apo-repressor- It is a regulatory protein synthesised by regulator gene. When a co-repressor substrate is available in the cell, the apo-repressor combines with the co-repressor to form a apo-repressor co-repressor complex. This complex binds with the operator gene and switches it off. Presence of apo-repressor alone, the operator gene is kept switched on because, by itself the apo-repressor is unable to block the working of operator gene.

Co-repressor- It is an end product of reactions catalysed by enzymes produced by the structural genes.

In the presence of tryptophan some molecules of tryptophan act as co-repressor, co-repressor-bind with inactive repressor. Co-repressor repressor complex bind with operator region and prevent the binding of RNA polymerase to the promoter, the trp-operon is off.

The repressor molecule has key role in regulation of lac-operon. Repressor molecule active or inactive. Active Repressor may be rendered inactive by addition of an inducer while the inactive repressor can be made active by addition of a co-repressor. Because the product of regulator gene the repressor act by shutling off the transcription of structural gene the operon model, as originally proposed by Jacob & Monod is referred as -negative control system.