In the research lab, you are growing E. coli bacteria which encode the lac opero
ID: 134616 • Letter: I
Question
In the research lab, you are growing E. coli bacteria which encode the lac operon. You can either grow the bacteria in a solution of glucose or lactose. a.) When E. coli are grown in the glucose solution without any lactose, do you expect the lac genes to be expressed or not? Explain in your answer what is going on with the repressor protein, operator, promoter, and RNA polymerase. b.) When E. coli are grown in the lactose solution without any glucose, do you expect the lac genes to be expressed or not? Explain in your answer what is going on with the repressor protein, operator, promoter, and RNA polymerase.Explanation / Answer
Ans.
#1. Regulation of lac operon in Presence of Glucose: The amount of glucose in E. coli (or any other cell) is inversely proportional to the amount of cyclic adenosine monophosphate (cAMP). When there is plenty of glucose in the cell, [cAMP] is low. The lower [cAMP] does not facilitate the formation of cAMP-CAP complex at significant level. Absence of significant amount of cAMP-CAP further results NO recruiting of the CAP protein to the promotor of operon. The promotor without CAP bound to it does not facilitate the recruitment of RNA pol to the site. Therefore, neither RNA pol binds to the promotor region, nor the operon is transcribed in presence of glucose in the cell.
#2. Lac operon Expression in the Presence of Lactose ( simultaneously with Absence of Glucose: A prokaryotic cell with depleted level of glucose and having lactose in the surrounding expresses lac operon. Since, the operon is induced ‘switched on’ by the presence of intracellular lactose, it is also called an inducible operon.
The amount of glucose in E. coli is inversely proportional to the amount of cyclic adenosine monophosphate (cAMP). A cell with diminished or depleted glucose level has high level of cAMP. In such condition, cAMP receptor protein (CRP), also known as catabolite gene activator protein (CAP), binds to cAMP (cAMP acts as allosteric effector of CAP). The cAMP-CRP complex binds to the promotor of lac operon brings localized conformational changes in the DNA. It further makes the efficient binding of RNA polymerase to the promotor more feasible.
The cell generally has very few molecules of permease even in the abundance of glucose in its cytoplasm. So, it can import lactose from the surrounding in times of glucose deficiency without the expression of lac operon. When glucose is depleted in the cytoplasm, few lactose molecules imported in the cytoplasm bind to the diffusible repressor protein. The lactose bound repressor proteins are unable to bind to the operator, thus also can’t inhibit the translation of structural genes. Thus, the lac operon is expression is triggered by both cAMP-CRP binding to the promotor and lactose binding to the repressor protein.