Question

Which of the following are considered as useful selectable markers for Escherichia coli?

a. Ampicillin resistance gene
b. Chloramphenicol resistance gene
c. Tetracycline resistance gene
d. Kanamycin resistance gene

• A. a, b and c
• B. a only
• C. a and b
• D. a, b, c and d

Answer 1

The correct option is D a, b, c and d

Selectable marker is a gene in a vector that helps in the selection of transformants (host cells which have successfully taken up the foreign DNA) and non-transformants (cells which have not taken up the vector DNA).

For example, antibiotic resistance genes in the vector act as selectable markers because they confer antibiotic resistance to the transformants while the non-transformants remain susceptible to antibiotics.

Mechanism of action of selectable markers is as follows:

• The vector containing the selectable markers would carry the gene of interest into the host cell where multiple copies of the gene can be made.
• The host cells are cultivated in the medium containing the same antibiotics.
• If the host cell gains resistance to the antibiotic due to the presence of rDNA, then it indicates that transformation of the host cell with the recombinant DNA is successful.
• Thus the presence of the selectable marker helps in identifying and eliminating non-transformants and selectively permitting the growth of transformants.

Criteria to choose selectable markers is as follows:

• The host cell must not carry the same selectable marker or antibiotic resistance gene in its nuclear material.
• In this case the host cell should not be naturally resistant to the same antibiotic.
• Escherichia coli is not naturally resistant to the antibiotics ampicillin, chloramphenicol, tetracycline or kanamycin.
• Therefore, these selectable markers can be used for E.coli.

Hence the correct option is
d. a, b, c and d