The transcription of a gene in-vitro is activated by protein A. This activation is repressed when protein B is present which is coded for on a different chromosome. Before protein B is added, protein A can be isolated from the reaction mixture, but when protein B is also present, only a much larger protein is able to be isolated. What is the explanation for this observation?
A) The cis-acting protein B is a repressor that binds to protein A, preventing it from binding to the activation sequence.
B) The cis-acting protein B is binding to the activation sequence, preventing protein A from being able to access and bind to the target sequence.
C) Chromatin remodelling is preventing protein A and/or B from binding to the activation sequence.
D) The trans-acting protein B is a repressor that binds to protein A, preventing it from binding to the activation sequence.
E) The trans-acting protein B is binding to the activation sequence, preventing protein A from being able to access and bind to the target sequence.

Question

Quizplus · Accepted Answer

The observation that a much larger protein can be isolated when both proteins A and B are present suggests that protein B is binding to protein A, forming a larger complex. Since protein B is coded for on a different chromosome, it acts in trans, not in cis, to exert its effect on the transcription of the gene activated by protein A. Therefore, the correct explanation is that the trans-acting protein B is a repressor that binds to protein A, preventing it from binding to the activation sequence.

The Transcription of a Gene In-Vitro Is Activated by Protein