Few genetic terms
- Operon -> A unit of genetic expression consisting of one or more related genes and the operator, repressor and promoter sequences that regulate their transcription.
- For example -> Lac operon.
- Inducer -> A molecule that when bound to a regulatory protein , produces an increase in the expression of a given gene. Lactose is the inducer of Lac operon, IPTG also.
FEW GENETIC TERMS
- Constitutive gene – The gene that is expressed in a constant rate and not known to be subject of regulation. They are often called house keeping genes.
- Gratuitous inducer – The inducer that is not a substrate of the enzyme.
- For example – Isopropyl Thiogalactosidase (IPTG)
- Structural genes for 3 enzymes of lactose metabolism in Ecoli (LacZ, LacY, LacA) along with Lac promoter, Lac operator and Lac repressor constitute the Lac operon.
- This genetic arrangement allows coordinate expression of 3 enzymes of lactose metabolism in bacteria.
- All the genes are transcribed into a single mRNA
When Ecoli is presented with lactose or IPTG
- The expression of the activities of β galactosidase, β galactoside permease, and thiogalactoside transacetylase are increased 10 to 100 fold. CAP-cAMP complex required.
- When there is no lactose or IPTG -> The genes of the above enzymes will not be expressed.
E.coli is exposed to both lactose and glucose
- The organism first metabolizes glucose, then temporarily stop growing until the genes of Lac operon become induced.
- When glucose is exhausted then only lactose will induce the genes of lactose metabolism.
- When no inducer is present -> I gene is a constitutive gene i.e. it is not regulated and is constantly produced.
- I gene products are the repressor molecules which bind with the operator and prevent binding of RNAP to the promoter and thus prevent subsequent transcription of Lac Z, Lac Y and Lac A.
When lactose or IPTG present
- The repressor molecules have high affinity for the inducer ( lactose ). So the repressor binds to the inducer and cannot bind to the operator. As a result there is induction of Lac Z, Lac Y and Lac A.
- For binding of RNAP to the promoter site CAP- cAMP complex is required and the repressor protein must not block the operator locus.
When both lactose and glucose are present
- Glucose inhibits cAMP so CAP- cAMP will be lacking. Without CAP – cAMP complex RNAP cannot initiate transcription of Lac Z, Lac Y and Lac A.
- When glucose is exhausted CAP – cAMP complex will reappear and transcription will start.
- The operon that deals with the anabolic ( synthetic) enzymes is called an anabolic operon.
- In prokaryotes another operon is there which deals with the enzymes of histidine synthesis. This is called His operon.
- Lac operon is a catabolic operon because it deals with the enzymes of lactose catabolism.
- Lactose and IPTG are inducers and +ve regulators.
- CAP- cAMP complex is +ve regulator.
- Glucose and repressor molecules are –ve regulators.
- Regulation of gene expression in prokaryotes is well explained by Lac operon.
- Operon is a unit of gene expression that consists of structural genes & regulatory elements.
- Lac operon is studied in E.coli where lactose is the inducer
Other Biochemistry Notes
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