Question

1. The blob operon produces enzymes that convert compound A into compound B. The operon is controlled by a regulatory gene S. Normally, the enzymes are synthesized only in the absence of compound B. If gene S is mutated, the enzymes are synthesized in the presence and in the absence of compound B. Does gene S produce a regulatory protein that exhibits positive or negative control? Is this operon inducible or repressible?

2.A mutation prevents the catabolite activator protein (CAP) from binding to the promoter in the lac operon. What will the effect of this mutation be on transcription of the operon?

3.Compare the regulation of the lac operon and the effect on the production of ß-galactosidae when E. coli is grown under the following conditions:

a) In the presence of high levels of lactose and absence of glucose

b) In the presence of high levels of lactose and high levels of glucose

Answer 1

Answer: 1) An operon is a group of genes that code for proteins that work for the same regulation. They have a single promoter also. There are inducible and repressible operons.
The blobe operon here produces enzymes that convert A to B. Thus B is the required product and enzymes are needed for its synthesis. Thus the operon has to be activated only when B is absent. So, to control this, a regulatory protein is present, here the gene code for this protein is S.
The regulatory protein, in the normal case, gets translated from the mRNA which got transcribed from the operon. This protein, get bound with B when it is present, will bind to the operator region and thus prevents the transcription of structural genes. That is why the operon gets inactivated in the absence of B.
This is seen in repressible operons like trp operon. It is also given that the mutation of the regulatory S gene causes the production of enzymes in the presence and also in the absence of B. It suggests that the regulatory protein exhibits a negative control.

2) The lac operon produces enzymes which disintegrate lactose to produce energy in the absence of glucose in the bacterial medium. The absence of glucose will activate cAMP which binds to the Catabolite Activator Protein(CAP) which is a dimeric protein. CAP has a ligand-binding site and a DNA binding site. They will bind to the DNA which is upstream to the DNA binding are of the RNA polymerase. Thus CAP helps in the activation of transcription by activating RNA polymerase. The protein-protein interaction of CAP with the alpha subunit of RNA polymerase forms a strong binding of the polymerase in the bacterial lac operon.

Here the mutated CAP cannot bind to the RNA polymerase, thus polymerase cannot attach strongly to the DNA and thus transcription rate will decrease significantly.

3) Lac operon works to convert lactose to by-products yielding energy for bacteria in case glucose is not available. Yet, glucose is the primary priority. When glucose is absent, the lactose gets inside the bacterial cell through the membrane, it will convert to allolactose which binds to the repressor causing the decreased interaction of repressor protein with the operator. Thus RNA polymerase can sit on the promoter and can proceed transcription. The absence of glucose causes the production of cAMP which will bind to CAP and activate them. The cAMP-CAP complex helps in the strong interaction of RNA polymerase and promoter. Thus transcription proceeds. Here, in the case of betta-galactosidase production, it is an enzyme for the catabolism of lactose. It is produced from the structural gene Z of the lac operon. Thus the production of betta galactosidase means activated lac operon. Let's look at the conditions;
1. Lactose is high, glucose is absent- LAC OPERON GET ACTIVATED AND TRANSCRIPTION WILL PROCEED TO YIELD ENERGY FROM LACTOSE.

2. Lactose is high, glucose is also high- BACTERIA HAS GLUCOSE AS THEIR FIRST PRIORITY, SO IN THE PRESENCE OF GLUCOSE, THEY TAKE GLUCOSE AS THEIR ENERGY SOURCE. AS NO LACTOSE IS TAKEN, TRANSCRIPTION WILL BE INACTIVATED.
Repressor will remain at the operator, cAMP will be low and RNA polymerase cannot attach to the promoter.