Operon

Lac operon

1. WELCOME TOPIC: GENE REGULATION IN PROKARYOTES (LAC OPERON) BY, SIJI SKARIAH PG 1 BOTANY

2. Different genes in an organism are meant for the synthesis of different proteins. All these proteins are not needed at one time. The coded information present in DNA is expressed by the synthesis of specific proteins’ However,some genes are constantly transcribed and expressed in all cells at all time,because their product are constantly needed in all cells.

3. Such genes are called constitutive genes or house keeping genes. But some genes expressed only in some type of cells or on special occasion. Such genes are called non constitutive or luxury genes. The contol of gene expression or protein synthesis is called gene regulation or it is the process of turning genes on and off

4. In prokaryotes, protein synthesis or gene expression is controlled at transcription level The regulation may be negative or positive i ) Negative regulation : In – ve regulation an inhibitor or repressor is present in the cell.The binding of inhibitor prevents or supresses transcription of relevant gene.

5. ii) Positive regulation : In positive regulation,an inducer molecule activates the promoter,which promotes transcription of mRNA. A negatively regulated system may be either inducible or repressible. In inducible gene regulation the structural gene or genes are induced or ‘switch on’ to produce mRNA.

6. The substance which induces the gene for protein synthesis is known as inducer . The gene complex responsible for the synthesis of inducible enzymes is called inducible system. Eg:lac operon An eg . of repressible system- Trp operon

7. REGULATORY PROTEINS 1 . ACTIVATORS : A transcriptional  activator  is a protein that increases transcription of a gene or set of genes. Most activators are DNA binding proteins that bind to  promoter. The DNA site bound by the activator is referred to as an "activator site.

8. Eg:The   catabolite activator protein (CAP; also known as  cAMP repressor protein, CRP) activates transcription at the  lac operon  of the bacterium  E.coli . 2. REPRESSORS : A  repressor  is a DNA or RNA-binding protein that inhibits the expression of one or more genes by binding to the operator.

9. A DNA-binding repressor blocks the attachment of RNA polymerase to the promoter. Thus preventing transcription of the genes into mRNA. An RNA-binding repressor binds to the mRNA and prevents translation of the mRNA into protein. This blocking of expression is called  repression. Eg:lac operon

10. A   corepressor  is a molecule that can bind to repressor and make it bind to the operator tightly, which decreases transcription. A repressor that binds with a corepressor is termed an  aporepressor   or  inactive repressor . One type of aporepressor is the  trp repressor , an important metabolic protein in bacteria.

11. OPERON Francis Jacob and Jacques Monad (1961),explained that gene regulation is by operon model.

12. The operon model for lactose catabolism is called lac operon . An operon is a group of closely linked structure genes and associated control genes. The operon consist of regulatory gene,structural gene, operator and promoter site.

13. STRUCTURE OF LAC OPERON 1.Regulatory gene: The regulatory gene is the i gene that code for the repressor protein This i gene is expressed in all the time hence it is also known as a constitutive gene. 2.Structural genes: Three structural genes lac z,lac y,lac a involved in the synthesis of enzymes for the lactose catabolism.

14. The 3 genes as transcribed as a polycistronic mRNA. Lac z code for Betagalactosidase Lac y code for Lac permease Lac a code for Transacetylase 3.Promoter 4.Operator

15. Negative control of lac operon The lac repressor is synthesized through the activity of lac i gene called the regulator gene. This repressor is an allosteric protein(A protein whose shape is changed when it binds a particular molecule) i )it bind the operator site ii)it bind the lactose sugar(inducer)

16. In the absence of inducer,DNA binding site of repressor is functional. The repressor protein binds to the DNA at the operator site of the lac locus and blocks the transcription of lac genes by RNA polymerase. The lac enzyme synthesis is inhibited .

17. Positive control of lac operon It is an additional regulatory mechanism which allows the lac operon to sense the presence of glucose,an alternative and preferred energy source to lactose. If glucose and lactose are both present,cells will use up the glucose first. The presence of glucose in the cell switch off the lac operon by a mechanism called catabolite reperssion , which involves a protein called catabolite activator protein(CAP)

18. CAP only binds in the presence of derivative of ATP called cyclic Adenosine Monophosphate ( cAMP ) CAP binds to a DNA sequence upstream of the lac promoter and enhances binding of the RNA polymerase Transcription of the operon is enhanced

19. cAMP levels are influenced by glucose. The enzyme adenylate cyclase catalyze the formation of cAMP and is inhibited by glucose. When glucose is available in the cell,adenylate cyclase is inhibited and cAMP levles are low Under these condition CAP does not bind upstream of the promoter and the lac operon is transcribed at very low level.

20. When glucose is low, adenylate cyclase is not inhibited, cAMP is higher and CAP binds increasing the level of transcription. If lactose and glucose are present together the lac operon will only transcribed at a low level.