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2. INTRODUCTON Although Gregor John Mendel for the first time use the term Factor for hereditary units. This mystery of hereditary unit reveled till 1900s. In 1909 W. Johanson coined the term Gene. The earlier workers proposed various hypothesis to explain exact nature of gene. At present we all know that, ‘Genes are made of DNA’. Now we can define gene as, “Gene is a nucleic acid sequence that carries the information representing a particular polypeptide”.

3. Gene expressed itself through a series of steps involved in a sequential synthesis of products. This can be summarized as follows- DNA………… RNA…………PROTEIN During the course of evolution cell have evolved control mechanism to insure that proteins are synthesized in the required amount at a specific time. Thus we can say that organisms have evolved the ability to regulate the expression of a specific gene in response to environmental signal.

4. GENE EXPRESSION Gene expression means synthesis of a particular product due to gene action. Some of the gene product required by the cell under all growth conditions are called House keeping gene. These include constitutive gene & their expression is called constitutive gene expression. Some gene are expressed only when their product is needed for growth & development under a given environment their expression is turn off when product is not needed such genes are called Regulatory gene. Regulatory Genes includes- Inducible Gene Repressible Gene

5. REGULATION OF GENE EXPRESSION Regulation of gene expression may be defined as, set of mechanism controlling the activation or inactivation of a particular gene. The exhaustive mechanism/investigation have been established on the basis of study of gene expression in bacteria and viruses, that regulation of gene expression in prokaryotes occur mostly at two levels- Transcriptional level Translational level Some time RNA degradation & protein modification also play major role in regulating but most of the prokaryotic genes that are regulated are controlled at transcriptional level.

6. THE REGULATION OF GENE EXPRESSION IS OF TWO TYPES: Positive Regulation:- When the expression of genetic information is quantitatively increased by the presence of a specific regulatory element it is called positive regulation. It is regulated by inducible gene. Negative Regulation:- When the expression of genetic information diminished by the presence of specific regulatory element it is called Negative regulation.It is done by repressible gene. There are some mechanism have been given to explain the regulation of gene expression in prokaryotes.

7. OPERON MODEL This is the model for transcriptional regulation of gene expression firstly F. Jacobs and J. monad in [1961] on the basis of there study on inducible system for the synthesis of the β- galactosidase enzymes in E. Coli proposed a model in order explain the induction & repression of gene expression. They got Nobel prized for this in [1965]. According to Jacob & Monad an operon (Unit of Gene Expression & Regulation) includes- regulator gene Promoter gene Operator gene Structural genes..

8. LAC OPERON The lac operon is proposed by-Jacob & Monod (1961). Based on their study on regulation of lactose, metabolism of E. Coli. This operon is consists of 3-contigeous structural genes (Lac Z, Lac Y & Lac A), a promoter & a regulator & an operator gene. Lac Z Codes for β-galactosidase; an intracellular enzyme that cleaves the disaccharide lactose into glucose & galactose. Lac Y codes for the - β galctosidase preameese; a membrane bound transport protein that pumps lactose into the cell. Lac A code for β- galactosidase transacetylase; that transfers an acetyl group from Co-A to - β galactosidase.

10. NEGATIVE REGULATION OF LAC OPERON:- The lac genes are controlled by negative regulation. In the absence of lactose (inducer) the repression occur on the operator binding site & very low level of transcription of loc Z, Y & A occurs. When the lactose is available to the cell the low level of permease allow the up take of lactose & galactosidase catalyses the conversion of some lactose to allotactose. Allolactose acts as an inducer & binds to the lac repressor. This causes a change in conformation of repressor reducing its affinity for the lac operon. The removal of lac-repressor from operator site allow the RNAP to bind at the promoter site (P lac) to begin the transcription of lac Z, Y, A gene. The lac operon is an example of negative control of gene expression because bound repressor prevents the transcription of structural gene.

12. POSITIVE REGULATION OF LAC OPERON: The P-lac promoter is not a strong promoter. It requires the presence of specific proteins called cAMP receptor protein (CRP) or some time called as catabolic activator protein (CAP). When glucose is present in environment E-coli does not need any alternate carbon source such as lactose. There for lac operon is not normally activated. Glucose reduces the level of c-AMP in the cell. CRP (catabolic) receptor protein) which exist as a dimmer, can not bind to DNA in the absence of cAMP.

14. When glucose is absent level of cAMP in the cell is increases & CRP binds to cAMP. The CRP-cAMP complex then binds to the lactose operon promoter just upstream from site of RNAP. It is believed that CRP-cAMP complex increases the binding of RNAP to the promoter & thus increasing the transcription rate 5o times. This type of gene regulation is an example of positive control of gene regulation. As a fact we can say that the lac operon is subject to both positive & regulation.

15. TRYPTOPHAN OPERON The trp operon is an example of biosynthetic operon also called as repressible operon. The organization of five structural gene (trp E, trp D, trp C, trp B & trp A) & adjacent regulatory sequence of trp operon has been studied in details by Charles Yanofsky et;al in 1976. The five structural genes coding for the enzymes needed for tryptophan biosynthesis from chorismate. These structural gene codes for 3 enzymes required for the biosynthesis of tryptophan from chorismic acid. Anthranilate synthetase – Encoded by trp E & trp D Tryptophan synthetase- Encoded by trp B & trp A. N-(5’phosphoribosyl)–anthranilate isomerase Indole -3 glycerol phosphate synthetase- Encoded by the trp C. These structural gene are contiguous with promoter operator.

17.  The regulator gene code for tryptophan repressor lies away from trp operon.  When the tryptophan is absent RNAP Binds to the promoter region and transcription of structural gene occurs.  When the tryptophan is present in the environment it binds with the receptor molecule forming a receptor-co repressor complex the conformation of repressor changes due to formation of the complex & it aquires the active conformation. It then bind to the operator region of operon & prevent the binding of RNAP to the promoter. Thus transcription of structural gene is inhibits. Since we can say that, here the tryptophan act as a co-repressor.

19. BIOLOGICAL SIGNIFICANCE The ability of switch gene on and off of fundamental impotence because it enables cells to respond to a changing environment and is the basis of cell differentiation. Organisms adapted to the environmental changes by altering the gene expression. Regulation of gene expression is also required for the development, differentiation and adaptation. Many of mechanism that control gene expressions are used to respond hormones and therapeutic agents. An understanding of these mechanisms may lead to the development of agents that inhibit the function or arrest the growth of pathogen. The principle established in the studies of prokaryotes, coupled with a variety of molecular biology techniques, have lead to remarkable progress in analysis of gene regulation in eukaryotes.

20. CONCLUSION Regulation of gene expression is a biochemical process that affects the rates at which gene products are synthesized and degraded. Regulations of genes in organism provide the way to control the enzyme production in the cell at required time. Hence checks the wastage of cell machinery and biomolecules. In other way it is the molecular mechanism of construction and destruction of gene product as per required of metabolism of living organism. In this way it is responsible for maintenance of cell homeostasis. Regulation is done in positive as well as negative way. In positive regulation activator promotes the production of enzymes while in negative regulation repressor checks the production of enzyme. In the prokaryotes regulation of gene expression can be understand by two operon model, I.e. Lac operon (subject to both negative and positive Regulation). Tryptophan operon (repressible or biosynthetic opron).