2. Identify the function of the proteins encoded by the lac operon. 3. Identify

Question

2. Identify the function of the proteins encoded by the lac operon.

3. Identify the lac operon as an inducible and/or repressible operon and identify its corresponding inducer/co-repressor.

4. Explain what happens when the lac operon is under negative regulation.

5. Explain what happens when the lac operon is under positive regulation.

6. Identify what conditions must exist for maximum expression of the lac operon and explain why these conditions are needed.

7. Identify the protein that is encoded by the lacI gene AND explain what constitutive expression means. Why is it important that the lacI gene is constitutively expressed?

Explanation / Answer

2. function of proteins encoded by lac operon:

a. lacZ gene encodes protein beta galactosidase which cleaves the lactose into glucose and galactose which can then enter glycolysis.

Transglycosylation of lactose into allolactose is also done by this enzyme, this allolactose binds to the repressor and give positive feedback to enzyme.

b. lacY: This protein encodes lactose permease enzyme which is a transmembrane symporter and help in pumping beta galactosidase inside the cell using proton gradient.

c. lac A encodes galactoside O-acetyltransferase:  transfers an acetyl groupfrom acetyl-CoA to -galactosides. and helps in cell detoxification

3. 3. lac operon is an inducible operon.allolactose is the indicer in lac operon. Only in the presence of this molecule which forms as a catalysis molecule of lactose lac operon become active.

4. Under negative control the lac operon genes constitutively expressed unless a repressor is there. So if repreesor binds to the operator then the way of promoter gets blocked and there will be no expression of any of gene.

5. under positive control: the genes are expressed only when an active regulator protein,called activator ex. catabolite gene activating protein is present. Thus the operon will be turned off when the positive regulatory protein is absent or inactivated.

6. when  High levels of glucose, low levels of lactose will be present there will be maximum expression of lac operon.

normally in lac operon there is utilsation f lactose using beta galactosidase producing glucose and lactose. But in the low lactose there will be insefficient production of enzyme. On the other hand if there is presence of high glucose then cell will utilise this source more readily. In such conditions The lac operon uses a two-part control mechanism to ensure that the cell expends energy producing -galactosidase, -galactoside permease and thiogalactoside transacetylase if necessary.

These processes will be acheived by lac repressor which repress the production in absence of lactose and CAP which will support production in absence of glucose. It is a kind of diauxic growth in microbes, Which always lead to the highest expression.

7. lac I: gene encoding for the lac repressor protein.

Constitutive expression: The expression of a gene always at a constant level is called constitutive expression.

lacI has its own promoterand it expresses always, This is the main part of regulation of lac operon. In the absence of inducer, Lac repressor is bound to lacO, preventing expression of the operon In the presence of inducer, inducer binds to Lac repressor and prevents repressor from binding to lacO, leading to transcription of the operon

so lacI is always necessary.

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