Question

Why is the coding sequence of an enzyme β-galactosidase a preferred selectable marker in comparison to the ones named above?

Answer 1

The gene for the enzyme β-galactosidase is an alternative selectable marker. When the foreign gene is inserted within the β-galactosidase gene, the enzyme β- galactosidase gets inactivated. Then the bacteria are grown on a chromogenic substrate. Non-recombinants will produce blue-coloured colonies, while recombinants will produce colourless colonies.

Answer 2

The coding sequence of the enzyme β-galactosidase is a preferred selectable marker compared to antibiotic resistance markers like those in pBR322 because it simplifies the identification of recombinants. Antibiotic-based selection is cumbersome as it requires plating on multiple antibiotic-containing media. In contrast, insertional inactivation of β-galactosidase allows differentiation based on colour. When recombinant DNA is inserted within the β-galactosidase gene, it inactivates the enzyme, resulting in colourless colonies on media with a chromogenic substrate, while non-recombinants produce blue colonies. This visual distinction makes β-galactosidase an efficient and convenient selectable marker for identifying transformants.