Question

question 15
to turn the lac operon off, a regulatory protein called a(n) ______ binds to an operator to block access to the promoter.
o activator
o signal molecule
o translator
o repressor

Explanation:

In the lac operon, the repressor protein binds to the operator region. When it binds, it blocks RNA polymerase from accessing the promoter, thus turning off the operon. The activator helps in enhancing transcription (turning on when needed, like in the presence of low glucose). A signal molecule (like allolactose) can bind to the repressor and change its conformation so it detaches (related to turning on). There is no "translator" in this regulatory context.

Answer:

repressor