LAC OPERON.pptx

The control of gene expression or protein synthesis is called gene regulation or it is the process of turning genes on and off
Gene regulation in prokaryotes is most extensively observed at the initiation of transcription.
Most genes are controlled by extracellular signals- present in medium.
Repressor, a negative regulatory molecule, binds to the operator gene and interferes with the expression of genes. Activator, a positive regulatory molecule, enhances the expression of the genes.
Operon : a group or cluster of structural genes under a single promoter; bacterial operons are polycistronic transcripts that are able to produce multiple proteins from one mRNA
Francois Jacob and Jacques Monod in 1961
“Lac operon is an operon or a group of genes with a single promoter that encode genes for the transport and metabolism of lactose in E.coli and other bacteria.”
Lac operon concept is an example of prokaryotic gene regulation.

1. Gene regulation in prokaryotes LAC OPERON ELIZABETH PHILIP

2. Gene regulation Bacterial genomic DNA has many genes that undergoes gene expression and forms proteins; all these proteins are not needed all the time; therefore few genes are constantly expressed others are expressed occasionally. The control of gene expression or protein synthesis is called gene regulation or it is the process of turning genes on and off Gene regulation in prokaryotes is most extensively observed at the initiation of transcription. Most genes are controlled by extracellular signals- present in medium. Repressor , a negative regulatory molecule, binds to the  operator  gene and  interferes  with the expression of genes.  Activator , a positive regulatory molecule,  enhances  the expression of the genes.

3. The lac operon concept Operon : a group or cluster of structural genes under a single promoter; bacterial operons are polycistronic transcripts that are able to produce multiple proteins from one mRNA Francois Jacob and Jacques Monod in 1961 “Lac operon is an operon or a group of genes with a single promoter that encode genes for the transport and metabolism of lactose in E.coli and other bacteria.” Lac operon concept is an example of prokaryotic gene regulation.

4. STRUCTURE OF LAC OPERON Lac operon of E.coli consists of   structural  and the  regulatory  genes. a. Structural genes of lac operon: -A cluster of 3 structural genes encoding proteins- involved in lactose metabolism Lac Z – encodes beta galactosidase - breaks down lactose into glucose and galactose Lac Y – encodes permease - transport lactose into cell Lac A - encodes transacetylase - addition of acetyl group to galactose b.Regulatory genes: includes  promoter gene, operator gene, lac I , and  catabolite activator protein  (CAP) binding site. i )The  promoter  is the binding site for  RNA polymerase , the enzyme that performs transcription. ii)The  operator  is a  negative regulatory site  where the lac repressor protein binds. It is located between the promoter and the structural genes.

5. iii ) Lac I (repressor) gene  codes for the lac operon  repressor . This gene is located adjacent to the promoter of the lac operon, with its  own promoter  and  terminator  and is  always  transcribed, hence the repressor is always synthesized. Repressor binds to the operator to repress (turn off) the operon iv) Catabolite Activator Protein (CAP) binding site  is a  positive  regulatory site located just upstream of the lac operon promoter, where the  catabolite activator protein  ( CAP ) binds. The CAP has binding sites for  cAMP  and DNA. When cAMP binds CAP, its  affinity  for the DNA increases. When bound to DNA, CAP  promotes   transcription  by aiding  RNA polymerase  bind to the  promoter  more efficiently.

6. Mechanism When lactose is absent: Control of the lac genes depends on the availability of lactose to the bacterium. The lac repressor is synthesized always through the activity of lacI gene. In the absence of lactose, the repressor protein binds to the DNA at the operator site and interferes with the binding and transcription by the  RNA polymerase  Hence, RNA polymerase  cannot transcribe  the structural genes of the lac operon and the lactose- metabolising enzymes are not produced. Lac operon – TURNED OFF

7. When lactose is present: When lactose is  present  ,the  structural genes  should be  transcribed  to produce lactose- metabolising enzymes. The repressor is prevented from binding to the operator by the action of the molecule called  inducer -   allolactose . The molecule of allolactose is obtained from the  lactose  (which is now present in the environment), using the enzyme  β-galactosidase  (low level of which is always present in the cell).  Allolactose binds to the repressor protein and causes  conformational change  in the repressor due to which, the repressor protein loses its ability to bind to the operator. As the repressor do not bind to the operator, the  RNA polymerase  can bind to the  promoter  site and  transcribes  the genes to further synthesize the lactose-metabolizing proteins.

8. Regulation of lac operon in the presence of glucose If glucose is present in the medium bacteria will take up glucose first. The presence of glucose in the cell can switch off the lac operon by a mechanism called catabolite repression , which involves a protein called catabolite activator protein(CAP) CAP (Catabolite Activator Protein) site is present near the promoter region CAP binds to a derivative of ATP called cyclic Adenosine Monophosphate (cAMP) and is available only in the  absence of glucose

9. Glucose (-) lactose (+) CAP  is functional only when  cAMP  is bound to it, and cAMP in the cell is available only in the  absence of glucose . Hence CAP allows the transcription of lac operon genes and subsequent metabolism of lactose, only in absence of glucose. When no glucose present- cAMP levels increase . cAMP binds to CAP – forms complex – go and attach to CAP binding site on lac promoter. H elps RNA polymerase anchor onto the promoter and the efficiency of transcription is highly enhanced 

10. When glucose is available in the cell, cAMP levles are low. W ithout cAMP, CAP cannot bind DNA, which in turn causes weak binding of  RNA polymerase  to the lac operon  promoter.   As a result only  few transcripts  are produced and lactose utilizing enzymes are not efficiently produced. If lactose and glucose are present together the lac operon will only transcribed at a low level. Glucose(+) lactose (+)

11. REFERENCE: https://thebiotechnotes.com/2020/03/20/the-lac-operon/ https://en.wikipedia.org/wiki/Lac_operon https://youtu.be/CaCq4gglw0g