Need help solving part a and b. Please! Suppose you isolated a mutation in the C
ID: 56751 • Letter: N
Question
Need help solving part a and b. Please! Suppose you isolated a mutation in the CRP gene that results in the production of a CRP protein that cannot bind cAMP. What effect would such a mutation have on expression of the lac operon in the presence versus the absence of lactose (assume that glucose is absent in both cases)? Suppose you isolated a different mutation in the CRP gene that results in the production of a CRP protein that binds to the CRP? binding site in the lac promoter even when there is no cAMP available. What effect would this mutation have on expression of the lac operon in the presence versus the absence of lactose, if glucose were also available in both cases?Explanation / Answer
In Lac operon, transcription of Lac genes by RNA polymerase occurs by two mechanisms
Negative regulation by lac repressor (in absence of glucose)
Positive regulation (in response to glucose)
In negative regulation, repressor binds to the operator region in abcence of lactose and vice-vera.
In negative regulation, cAMP / CRP complex determines rate of transcription with respect to repressor state. Binding of cAMP / CRP protein to CRP binding sites leads change in conformation of DNA in promoter region which enhances binding of RNA polymerase and thus transcription or vice-versa. However, further transcription is governed by repressor i.e. if bound to operator can still inhibit transcription.
a)
In this case, we have following conditions
no glucose
mutant CAP (not able to bind cAMP)
This condition shows negative regulation, as lactose is the limiting factor
In presence of lactose, repressor falls off the operator allowing RNA polymerase to bind to promoter and transcribe genes weakly.
In absence of lactose, repressor binds to operator preventing RNA polymerase binding and no transcription of genes.
b)
In glucose mediated transcription of lac operon, the role of cAMP is to enhance binding of CRP to CRP binding site. In this second case the mutation has done that job.
So,
In presence of lactose, glucose and mutation; repressor will fall from operator, glucose will have no effect on mutant CRP binding, thus lac operon genes will be transcribed at large amounts.
In absence of lactose, presence of glucose and mutation; repressor is bound to operator and there will be no transcription of lac genes even if CRP is bound to CRP binding site.