Homework Sourse5 Is the CAPcAMP effect on the transcription of the lac oper
5.) Is the CAP/cAMP effect on the transcription of the lac operon an example of positive or negative regulation? Why?
6.) Explain what happens during catabolite repression of the lac operon when both Glucose and Lactose are in high amounts. What is the biological relevance of this control?
7.) Why are Oc mutations said to be “dominant” over Is mutants?
8.) The rate of transcription of the trp operon in E. coli is controlled by what two regulatory processes? These two processes can regulate the trp operon up to about _____-fold due the combined effects of these processes. By what mechanisms do these two regulatory processes modulate trp operon transcript levels?
9.) Explain how high levels of tryptophan in the environment in which an E. coli cell is growing results in premature termination of transcription of the trp operon.
Solution
5) CAP-cAMP system is example of positive control. In this the lac operon expression occurs in the presence of an activating signal which is the CAP-cAMP complex. In the Presence of cAMP, the CAP binds to DNA at specific sites and promote the binding of RNAP. CAP thus can promote promote pathways like sugar and amino acids metabolism , transport processes, protein folding, toxin production and pilus synthesis.
6)If both lactose and glucose are present in high amounts, the -galactosidase synthesis is not induced till all the glucose is utilized. When glucose concentration is high, cAMP is low. Lac operon is activated in the presence of high concentration of cAMP. The low concentration of cAMP is not sufficient to induce lac operon. Hence in high glucose concentration, low cAMP is present and lac operon is not activated and hence no B galactosidase is produced.
7)Oc mutants are operator constitutive mutants that are due to changes in the DNA sequence of the operator. The changes in the operator impairs the binding of the lac repressor. Hence the repressor cannot repress the lac operon . So the lac operon wil Oc operator cannot be turned off. This is because the operator can control the genes on the same DNA strand.
