You have isolated two different mutants ( reg1 and reg2 ) causing constitutive e

Question

You have isolated two different mutants (reg1 and reg2) causing constitutive expression of the emu operon (emu1 emu2). One mutant contains a defect in a DNA binding site, and the other has a loss-of-function defect in the gene encoding a protein that binds to the site.

To determine which mutant has a defect in the site and which one has a mutation in the gene encoding the binding protein, you decide to do an analysis using F' plasmids. Assuming you can assay levels of the Emu1 and Emu2 proteins, what results would you predict for the strain below if reg2 encodes the regulatory protein and reg1 encodes the regulatory site?

F' reg1- reg2+ emu1- emu2+ /reg1+ reg2- emu1+ emu2-

A. constitutive expression of Emu1 and Emu2

B. constitutive expression of Emu1 only

C. constitutive expression of Emu2 only

D. constitutive repression of Emu2 and constitutive expression of Emu1

A. constitutive expression of Emu1 and Emu2

B. constitutive expression of Emu1 only

C. constitutive expression of Emu2 only

D. constitutive repression of Emu2 and constitutive expression of Emu1

Explanation / Answer

C. contitutive expression of emu2 only.

The given is an example of merodiploid strain, i,e. one copy of the operon on the chromosome and the other is on F plasmid.

In order to make the emu1emu2 operon inducible, both the DNA binding region (reg 1) and DNA binding protein (reg 2) must be in their wild type form (should not have mutation), so that the intact protein can bind to this DNA binding site,in the absence of inducer and blocking the transcription. This is the reason why, both Reg1 and Reg 2 mutants are causing contitutive expression of emu1emu2 operon.

Now in this strain, reg1+reg2-emu1+emu2- is the situation present on the chromosome, which indicates the mutation in reg2 (DNA binding protein). But this protein can be taken from the F plasmid having reg1-reg2+emu1-emu2+ (active reg2+), so that it bind to the reg1 operator on the chromosome and the expression of emu1 become inducible. Note that this reg2 protein derived from plasmid cannot bind to reg1 of the plasmid as the later is mutant, so that the expression of emu2 remain contitutive.Therefore we can say that the DNA binding proteins (reg2 in this case) are trans acting means they are capable on acting on other chromosome, whereas DNA binding sites are cis acting (active only on the same chromosome)

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