Plasmid pBR322 has PstI restriction enzyme site within gene amp^R that confers ampicillin resistance. If this enzyme is used for inserting a gene for β-galactoside production and the recombinant plasmid is inserted in an E.coli strain

The correct answer is: it will not be able to confer ampicillin resistance to the host cell.

Here is the step-by-step educational explanation:

1. Understanding the Structure of Plasmid pBR322:
Plasmid pBR322 is a widely used cloning vector in genetic engineering. It contains two antibiotic resistance marker genes:
- amp^R, which confers resistance to the antibiotic ampicillin.
- tet^R, which confers resistance to the antibiotic tetracycline.

2. Target Site for the Restriction Enzyme:
The question states that the restriction enzyme PstI has its recognition and cleavage site located specifically within the amp^R gene sequence.

3. Mechanism of Insertional Inactivation:
When the PstI restriction enzyme is used to cut the plasmid, it cleaves the DNA double strand right inside the functional sequence of the amp^R gene.
When a foreign gene (in this case, the gene for β-galactoside production) is inserted at this cleavage site and ligated, it disrupts the continuity and reading frame of the amp^R gene. This disruption makes the gene non-functional, meaning it can no longer produce the active protein responsible for ampicillin resistance. This phenomenon is known as insertional inactivation.

4. Consequence on the Transformed Host Cell:
When this recombinant plasmid is introduced into an E. coli host strain, the host cell will express the newly inserted gene (β-galactoside production) but will fail to express a functional ampicillin resistance protein. Consequently, the host cell will lose its resistance to ampicillin and will not be able to survive on a medium containing ampicillin.