If lactose is not supplied to E.coli cells, the presence of the enzyme (J galactosidase in hardly detectable, but as soon as the substrate (lactose) is added the production of the enzyme galactosidase increases by as much as 10,000 times.
The production of enzyme again gets reduced if lactose is removed from the system. Such enzymes whose synthesis can be increased many times by the presence of the substrate are called inducible enzymes.
The genetic system responsible for synthesis of such enzymes is called the inducible system. The inducible enzyme is an example of adaptive enzyme in the sense its (enzyme) synthesis gets increased due to the presence of the substrate.
In some other systems the reverse of the inducible system occurs for instance in a culture of E.coli if no amino acids are supplied from outside the cells can produced all the enzymes required for the production of amino acids.
On the other hand if a particular amino acid like histidine is added to the system the production of histamine synthesizing enzymes will fall down in these systems the addition of a product of a synthetic pathway will check the production of enzymes responsible for the synthetic pathway.
In other words increased presence of a substrate would reduce the synthesis of an enzyme which has the enhancing effect of the production of that substate.
Systems such as these are called repressible systems and the enzyme whose synthesis gets checked is known then as the repressible enzyme.
2. Inducer and Co-repressor:
It has been mentioned above that an enzyme gets induced or repressed by the presence of a particular substrate.
The substance which induces the synthesis of an enzyme is called the inducer and the substance which suppresses the synthesis of the enzyme is called the repressor. In the synthesis of p galactosidase, lactose acts as the inducer and in the suppression of histidine synthesizing enzyme histidine acts as the repressor.
Enzyme induction or repression is generally brought about by the substrate on which an enzyme can act. But in some instances there will be molecules which resemble the substrate which also can act as the inducers.
A typical example of this kind of induction is IPTG (isopropyl thiogalactoside). IPTG resembles lactose and can induce the production of the enzyme (3 galactosidase.
Such inducers which are not substrates themselves but can bring about induction because of similarities with the substrate are known as gratuitious inducers.
In the above paragraphs we have discussed as to how in the absence of lactose there is no synthesis of (3 galacotosidase. This means then in the absence of the inducer the genes responsible for the production of the enzyme protein do not function.
It will be interesting to find out as to why the enzymes do not function in the absence of lactose and how in the presence of lactose the genes regulating the enzyme synthesis get activated.
It has been found out that a group of molecules called repressors are present in the cell and they check the activity of the enzymes. In the presence of the inducers the repressors get inactivated. In some instances ordinarily the repressors are inactive but they become active in the presence of another group of molecules called co- repressors. The induction and repression is supposed to work in the following manner.
1. Inducible system: active repressor+ inducer= inactive repressor
2. Repressible system: Inactive repressor + corepressor = active repressor
3. The Operon Concept:
F.Jacob and J.Monod (1961) the Nobel Laureates have studied in detail the inducible system in the synthesis of the enzyme P galactosidase in E.coli.
In order to explain the induction and repression of enzyme synthesis they have proposed the involvement of a group of genes along with the presence of cytoplasmic inducers and repressors. Their model of gene action is popularly called the operon model.
This scheme of gene action proposed by Jacob and Monod as originally conceived by them consisted of an operator gene and a number of structural genes.
As per the current understanding however the operon system consists of a structural gene, operator gene, promoter gene, regulator gene and repressor, inducer and co-repressor. The following is an account of the components of the operon model.
4. Structural Genes:
There are many structural genes associated in an operon system. These direct the synthesis of the mRNA and govern the sequence of amino acids in a protein molecule.
Each structural gene might produce a particular kind of protein or all structural genes might regulate the production of a single protein.
The activities of the structural gene(s) are controlled by the promoter and operator site of the operon system. The most well studied structural genes (z,y and ac) are those of the lactose operon system in E.coli.
5. Operator Gene:
The operator gene is situated adjacent to the first structural gene. It switches on or switches off the functioning of the structural gene (protein synthesis).
In case a structural gene has to be suppressed, a repressor attaches itself to the operator to form an Operator-repressor complex. In the case of protein synthesis, the operator-repressor complex prevents the transcription by blocking the movement of RNA polymerase.
6. Promoter Gene:
The promoter gene is continuous with the operator gene and is believed to lie left to it. It is suggested that RNA polymerase binds to the promoter site during transcription. Three regions have been recognized in the promoter site. These are (a) recognition site, initial binding site and the mRNA initiation site (operator site).
(i) Recognition site:
Also called the cga site (catabolic gene activator site), it consists of certain palindrome sequence of DNA these symmetrical sections of DNA are recognized by proteins having symmetrically placed sub units.
This site, also called the CRP site (cyclic AMP receptor protein site) binds a Carotenoid protein to the promoter gene and thus facilities the binding of the enzyme RNA polymerase.
It has been found that in E.coli, CRP combines with cAMP (cyclic Adenosine monophosphate) forming a CRP + cAMP complex which binds to the promoter enhancing the binding of RNA polymerase and activates transcription. This regulation is called positive control.
(ii) Initial binding site:
This consists of seven bases (DNA) to which the RNA polymerase binds.
(iii) RNA initiation site:
The site where transcription begins is called initiation site. This is the region overlapping with the operator region. Regulator gene
The regulator gene directs the activity of the operator gene by producing inhibitor proteins called repressors. This repressor protein binds to the operator gene and blocks the path of RNA polymerase, thus preventing transcription.
If an inducer is present in the system, it binds to the repressor which undergoes conformational change and becomes inactive. As the inactive repressor cannot bind to the operator, the structural genes get activated and protein synthesis continues.
In a repressible system, if only a repressor is present, it is inactive by itself and cannot attach to the operator gene and hence transcription continues.
However when another protein called co repressor is present it attaches to the repressor forming a repressor-co repressor complex, which blocks the operator gene thus preventing transcription.