5. You have isolated two mutants (reg1 and reg2) that cause constitutive express

Question

5. You have isolated two mutants (reg1 and reg2) that cause constitutive expression of the OSU operon which contains genes bev1 and bev2. One reg mutant has a defect in the DNA binding site, the other has a defect in the genes encoding a protein that binds to that site (loss of function). A. Is the DNA-binding protein a positive or negative regulator of gene expression? How do you know? B. To determine which mutant has a defect in the site and which one has a mutation in the binding protein, you decide to do a merodiploid analysis. Assuming that you can assay levels of Bev1 and Bev2 proteins, what results do you predict for for Bev1 and Bec2 expression given the two strains below (i and ii) if reg2 encodes the regulatory protein and regl is the regulatory site? i. F reg1- reg2+ bev1-bev2+/ reg1+ reg2+ bev1+ bev2- ii. Freg1+ reg2- bev1-bev2+/ regl+ reg2+ bev1+ bev2- Bevi:

Explanation / Answer

A. constitutive expression of Bev1 and Bev2
B. constitutive expression of bev1 only
C. constitutive expression of bev2 only
D. constitutive repression of Bev2 and constitutive expression of bev1

C. Constitutive expression of Bev2 only

the given is an example of the merodiploid strain that is one copy of operon on the chromosome and one on the F plasmid.

In order to make Bev1Bev2 inducible both the DNA binding region(reg1) and DNA binding protein(reg2) must be in their wild-type form that is must not have mutations. So, that the intact protein can bind to this DNA binding site, in the absence of inducer blocking the transcription. this is the reason why both reg1 and reg2 mutants are causing constitutive expression of the Bev1Bev2 operon.

Now in this strain reg1+reg2-bev1+bev2- is the situation present on the chromosome which indicates a mutation in DNA binding protein reg2. but this protein can be taken from the F plasmid having reg1-reg2+bev1-bev2+(active reg2+) so that it binds to the reg1 operator on the chromosome and the expression of bev1 become inducible. the reg2 protein derived from the plasmid cannot bind to reg1 of the plasmid as the later is mutant so that the expression of bev2 remain constitutive. so we can say that DNA binding protein (reg2) are trans acting meaning they are capable of acting on the other chromosome whereas DNA binding site is cis-acting(active on the same chromosome).

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