The gene machine program shows you what happens when lactose is present in E col

Question

The gene machine program shows you what happens when lactose is present in E coli, and how the laco peron is under negative control. However, the lac operon is also under positive control from a protein called CRP. cAMP Receptor Protein. The absence of the lac repressor is essential but not sufficient for effective transcription of the lac operon. RNA polymerase also depends on the presence of CRP. Like the lac repressor, which can bind to the DNA and lactose. CRP also has two types of binding sites: a site that binds to the DNA near the promoter, and a site that binds to cAMP. cAMP (cyclic AMP) is a signal molecule whose concentration in the cell is inversely proportional to that of glucose. cAMP is created from ATP by the enzyme adenylyl cyclase, and is used as an intercellular signaling molecule to regulate sugar and lipid metabolism. When glucose is present, E. coli will always prefer to metabolize glucose, but when glucose levels are low cAMP levels will begin to increase and the CRP-cAMP complex levels will rise. The CRP-cAMP complex will bind to the CRP binding site on the DNA upstream of the lac operon where it will increase the expression of the lac operon genes by enhancing the binding of RNA polymerase to the promoter sequence. Using the figure above, explain why RNA polymerase is not bound to the promoter when both lactose and glucose is present. Explain why the lac operon status is OFF when neither glucose or lactose is present, even though RNA polymerase is bound to the promoter. The lac operon is under both negative (the repressor) and positive (CRP) control. Why do you think this is? Why would this be beneficial?

Explanation / Answer

Lac operon is an inducible operon that utilises lactose as energy source and is activated when glucose is low and lactose is present.Lac operon encodes for genes required to utilize lactose as an energy source.

Lac operon contains an

operator

promoter

structural genes (transcribed together)

These structural genes are under the control of catabolic activator protein (CAP) or repressor.

1. E.coli uses othyer sugars as energy source when glucose level is low. To do so, cAMP-CAP complex serves as a positive regulator to induce transcription to metabolise the lactose as energy source.

CAP binds to the operator sequence upstream of promoter that initiates the transcription of lac operon. cAMP-CAP complex stimulates RNA polymerase activity and increases RNA synthesis. In the presence of cAMP CAP complex, RNA synthesis is blocked when repressor is bound to the operator. In the figure given, RNA polymerase activity is ceased in the presence of both glucose and lactose. This occurs because though both are present the concentration of glucose might be higher than lactose.

In E.coli lac operon, when a repressor binds to the operator, RNA polymerase cannot attach to it and mRNA is not transcribed and no lactose catabolysing enzymes are produced.

This operon has an inducer caled allolactose. When glucose is present , some lactose isomerises into allolactose . This allolactose binds to the allosteric site of represor causing it to release from the operator . Now RNA polymerase can attach and the 3 enzymes required for lactose catabolisation are transcribed.

2.Lac operon is turned off when neither glucose nor lactose are present even though RNA polymersae binding occurs as lac operon has two part control to ensure the cell expends energy producing the enzymes required for metabolism. For activation of lac operon the level of glucose must be low or none and lactose must be present. When glucose concentration is less cAMP concentration is high required for the activation of lac operon.Even when there is shortage of glusoce catabolites and CAP-cAMP comple forms, the enzyme necessary for lactose metabolism are produced only when lactose is present.

3.this level of control of positive and negative is done because lac operon inducers must bind to the repressor to remove it from operator and prevent transcription of lac operon. Thus cell conserves energy and resources by producing lactose metabolizing enzyme only when required and useful. In this way it is beneficial.

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.