In: Biology
The expression of genes is regulated in S. pneumoniae positive and negative regulators that respond to signals from the environment.
a. How can the presence or absence of glucose change the expression of the lac operon?
b. How can the presence or absence of lactose change the expression of the lac operon?
c. How could the presence of absence of tryptophan change expression of the trp operon?
d. How could the presence of absence of chorismate change expression of the trp operon?
e. If a medium contained both chorismate and tryptophan, how would the expression of the trp operon change, if at all?
a. the presence of glucose reduces the concentration of cAMP and thereby, inactivates CRP/CAP. RNA polymerase cannot bind to the promoter and therefore, transcription is repressed.
when glucose is present in the medium, there is no need of lac operon. the lac operon is transcribed only in the presence of lactose and absence of glucose.
b. presence of lactose inactivates the repressor and turns ON the transcription from lac operon.
the absence of lactose turns OFF the transcription from the lac operon.
c. tryptophan acts as a co-repressor for tryptophan operon. when tryptophan is present, it activates the repressor and thereby, inactivates the trp operon. in its absence, Trp operon is transcribed.
d. chorismate acts as the precursor of tryptophan. since this is the main substrate, the presence of chorismate requires the tryptophan operon to be expressed.
e. chorismate is the substrate for tryptophan biosynthesis. the enzymes required for the synthesis of tryptophan from chorismate are encoded by tryptophan operon. when tryptophan is present in the medium, it would not allow the transcription to proceed. it represses the transcription.