Question

13. The rate of B-galactosidase synthesis in E. coli is influenced by the bacteria's genotype and its growth conditions. Indicate whether you would expect the rate of β-galactosidase synthesis to increase, decrease, or remain the same after each of the following changes to the growth medium. Explain your answer. a. E. coli cells are cultured in minimal medium containing glycerol as the sole b. E. coli cells are cultured in minimal medium containing glucose as the sole c. E. coli cells are cultured in minimal medium containing both glucose and lactose d. An E. coli lacl mutant is cultured in minimal medium that contains glycerol carbon source. Then lactose is added to the medium. carbon source. Then lactose is added to the medium as carbon sources. Then cAMP is added to the medium as the sole carbon source. Then lactose is added to the medium

Answer 1

Catabolite repression is the positive control on the lac operon as a regulator protein increases transcription of this operon. cAMP activates a catabolite repressor protein that will bind to it. This cAMP-CAP complex will bind the promoter and stimulates RNA polymerase to bind the promoter and induce transcription. Glucose will induce catabolite repression as it decreases cAMP levels.

a. Glycerol shows no catabolite repression of lac operon. Glycerol is a poor source of energy. It does not deplete cAMP levels significantly. Lac operon is not operational when glycerol is present. Hence, when glycerol is replaced by lactose, allolactose will bind to repressor protein. cAMP is available and it will bind to CAP. cAMP-CAP will stimulate RNA polymerase binding to promoter. As repressor is not bound to operator, RNA polymerase will induce transcription of beta galactosidase. Hence, rate of beta galactosidase synthesis will increase.

b. Presence of glucose will reduce levels of cAMP. Hence, CAP will not bind to its site on promoter. When lactose is added, allolactose will bind the repressor, thereby inactivating it. RNA polymerase requires CAP binding for activity. Hence, despite no repressor bound to operator, the lac operon is transcribed at low level. The rate of transcription of beta galactosidase will remain same as long as glucose is present. Lac operon is not transcribed till glucose is exhausted. It cannot be said that there is a decline/decrease, as minimal beta galactosidase synthesis will occur.

c. As glucose and lactose are present, the transcription of beta galactosidase is minimal. However, when cAMP is added, CAP can bind to cAMP and activate RNA polymerase binding to lac promoter. Also, allolactose is bound to repressor protein. Thus, RNA polymerase will initiate transcription and the rate of beta galactosidase synthesis will increase.

D. This mutant lacks lacI gene product, which is the repressor protein. Lac I- mutants show constitutive expression of lac operon. Hence, the repressor is not bound to operator. When this mutant is grown in glycerol, the cAMP will not decrease. Similarly, there is no repressor bound to the operator, showing constitutive expression. When lactose is added, there will still be constitutive synthesis of beta galactosidase as allolactose is not required to bind the repressor. Hence, rate of beta galactosidase synthesis will remain same.