How can mutations in promoter regions affect gene expression?

• A. They do not affect transcription factor binding.
• B. They can alter transcription factor binding, changing transcription rates and mRNA output.
• C. They only affect translation.
• D. They always increase transcription rate.

Answer: (B)

Mutations in promoter regions affect gene expression by altering transcription factor binding. The promoter is the DNA sequence where RNA polymerase II and transcription factors assemble to start transcription. If a mutation changes a binding site, it can change how strongly transcription factors bind, which in turn changes how often transcription is initiated. This can lead to more or less mRNA being produced, depending on whether the mutation strengthens or weakens activator binding or creates/loses repressor interactions. Translation is downstream and depends on mRNA, so promoter mutations mostly influence transcription, not translation. Therefore, expression can go up or down, not just up, and promoter mutations don’t simply “do nothing” to binding.