A complete retroposon was cloned under the entire galactose-inducible promoter. The construct, as shown below, was inserted in the yeast genome to study the transposition event.

Some of the predicted outcomes are listed below:

A. Cells grown in the presence of glucose or galactose lead to an increase in the copy number of the transposon.
B. The transposed copies will be the same as the construct inserted in the genome.
C. The transposed copies cannot transpose further.
D. The transposed copies will not respond to either glucose or galactose in the media.

Assuming that the hypothesis is correct, choose the option that has all likely outcomes.

Correct option is D

• A (Cells grown in glucose or galactose lead to increased transposon copy number)

  • Incorrect:
    • The GAL promoter is inducible by galactose but is repressed by glucose in yeast.
    • If the transposon integrates into a new location without the GAL promoter, it will no longer be inducible by galactose.
    • This means that after the initial transposition, further increases in copy number may not happen.

• B (Transposed copies will be the same as the construct inserted in the genome)

  • Incorrect:
    • The original construct includes the GAL promoter.
    • However, new transposed copies may not include the GAL promoter, depending on how transposition occurs.
    • If the transposed copies do not carry the promoter, they will not behave exactly the same as the original construct.

• C (Transposed copies cannot transpose further)

  • Incorrect:
    • If a transposed copy lands in a region with an active promoter, it could still transpose.
    • The ability to transpose further depends on whether the regulatory elements (like the GAL promoter) are still functional in the new location.

• D (Transposed copies will not respond to glucose or galactose in the media)

  • Correct:
    • Once the retroposon has moved to a new location, it is no longer under the control of the GAL promoter unless it happens to integrate into a site near another GAL-regulated sequence.
    • This means that new copies will not respond to glucose or galactose, as their activity will be dependent on the regulatory elements at their new location.