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1. GENE REGULATION IN PROKARYOTES

2. INTRODUCTION What is gene regulation? Regulation of gene expression, or gene regulation, includes a wide range of mechanisms that are used by cells to increase or decrease the production of specific gene products like proteins. It is the process of turning genes ‘on’ and ‘off ’. Gene regulation ensures that the appropriate genes are expressed at the proper times. Gene regulation in prokaryotes is most extensively observed at the initiation of transcription or occurs at the transcriptional level.

3. TRANSCRIPTIONAL REGULATION To understand how gene expression is regulated, we must first understand how a gene codes for a functional protein in a cell. Prokaryotic cell lack nucleus and their DNA therefore floats freely in the cell cytoplasm. So to synthesize a protein ,the processes of transcription and translation occur almost simultaneously. When the resulting protein is no longer needed, transcription stops, and when more protein is required, more transcription occurs.

4. CONSTITUTIVE, INDUCIBLE , AND REPRESSIVE GENE EXPRESSION Constitutive gene expression:- Genes are always active in this type. Gene products such as tRNA molecules.,ribosomal protein ,rRNA molecule that are referred as cellular housekeeping function are essential components of all living cells.So the Genes that specify this type of gene products which are continuously expressed in most cell are known as constitutive genes. Inducible gene expression:- The process of turning on the expression of Genes in response to a substance in the environment is called induction. Genes whose expression is regulated in this manner are called inducing genes;and their products ,enzymes ,are called inducible enzymes. Repressive gene expression:- When E.coli cells are present in an environment containing enough tryptophan to support optimal growth, the continued synthesis of the tryptophan biosynthetic enzymes would be waste.Thus a regulatory mechanism has evolved in E.coli that turns off the synthesis of tryptophan. So a gene whose expression has been turned off in this way is said to be “repressed” and the process is called Repression.

5. PRINCIPLES OF TRANSCRIPTIONAL REGULATION Gene regulation is controlled by regulatory proteins which are of two types: Positive regulator or activators Negative regulator or repressor TYPES OF GENE REGULATION Positive regulation:- In this case ,the genes are expressed only when an active regulator protein e.g. an activator, is present, if not operon will turn off. Negative regulation:- in this case ,the genes in the operon are expressed unless they are switched off by a repressor protein.Thus the operon will turn on when the repressor is inactivated.

6. In prokaryotes, operon model is used explain gene expression. In 1961,Francois Jacob and Jacques Monod proposed the operon model of gene regulation in bacteria. The model was based on their study of the genes in E.coli that code for enzymes that affect the breakdown of lactose. Lac operon An operon is a regulatory unit consisting of a few structural genes under the control of one promoter. It encodes polycistronic mRNA that contains the coding sequences for two or more structural genes.

7. Lac operon consist of two types of genes Structural genes : - lac Z:- encodes the enzyme beta- galactosidase which hydrolyzed lactose to form glucose and galactose and also converts lactose into allolactose. lacY :- It encodes the enzyme beta- galactoside permease which facilitate the entry of lactose into the cell lacA :- encodes the enzyme beta- galactoside transacetylase . It transfers an acetyl grp from acetyl CoA to toxic beta- galatosidase Regulatory genes include LacI :- It encodes repressor protein that binds to the operator. Promoter :- It serves as the binding site for RNA polymerase. Operator : - It serves as the binding site for repressor protein Regulatory genes regulate the transcription of structural genes.

8. Lac operon occurs only when two conditions are favoured:- Availability of lactose. (detected by lac repressor act as lactose sensor) Glucose not available.( detected by CAP acts as glucose sensor) In presence of lactose:- When lactose is available, some molecules will converted into allolactose (an isomer of lactose) which binds to the lac repressor and change its conformation to inhibit the binding to DNA. The lac repressor is the protein that inhibits the transcription and loses its ability to bind DNA and floats off the operator in presence of lactose. Allolactose is an inducer that turns on the lac operon which is usually off, so it is also known as inducible operon.

9. RNA POLYMERASE

10. In absence of lactose:- When lactose is not present , the lac repressor binds tightly to the operator and prevents transcription of RNA polymerase (sigma 70 factor)and lacZ, lacY and lacA genes.

11. Presence of high glucose: - When glucose is present, then the rate of transcription initiation is very low, resulting in synthesis of only low levels of lac mRNA and the protein. Glucose inhibits the synthesis of the cAMP , and CAP cannot bind DNA without cAMP and remains inative.(cyclic adenosine monophosphate). Absence of glucose:- Once the glucose depleted from the media and the intracellular glucose conc.falls .cells respond by synthesizing cyclic cAMP. As the concentration of cAMP increases ,it binds to the CAP protein , causing a conformational change that allows the protein to bind to the CAP site in the lac transcriptional control region. The bound CAP- cAMP complex interacts with the polymerase bound to the promoter,stimulating the rate of transcription initiation.This activation leads to synthesis of high levels if lac mRNA and subsequently of the protein encoded by lac operon.

12. ROLE OF TRYPTOPHAN REPRESSOR IN GENE REGULATION Ecoli can synthesize tryptophan using enzymes that are encoded by five structural genes located next to each other in trp operon. When environmental tryptophan is low,the operon is turned on, this means that transcription is initiated ,the genes are expressed. The repressor itself does not bind to the operator and tryptophan is synthesized. However, when tryptophan accumulates in the cell,two molecule bind to the trp repressor molecule, which changes its shape,allowing it to bind to the operator and blocks RNA polymerase from transcribing the structural genes, stopping expression of the operon.

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