Suppose there is a protein "A" which binds to a DNA sequence and increases transcription of the gene CBC23. In the presence of molecule "C", protein "A" changes structure and can no longer bind to the DNA sequence, resulting in reduced transcription of CBC23. What terms describe "A" and "C" respectively?

A = Activator

B = Effector

Explanation:

An activator is a protein which typically binds to a short (50–1500 bp) region of DNA which might be located either upstream (mainly) or downstream of a gene so as to cause increased transcription. This particular region of DNA is known as enhancer and activator is also  known as transcription factor. Activator is a trans-acting factor which binds to the cis-acting factor which is enhancer so as to enhance transcriptional expression.

But another protein named as effector may restrict activator from binding to the enhancer leading to a decrease in transcriptional expression by binding to the activator allosterically. Allosteric binding of effector to the activator causes conformational change in activator so it can no longer bind the enhancer.