What is a transcriptional repressor?

• A. A protein that enhances transcription by recruiting RNA polymerase
• B. A protein that binds to DNA to increase transcription
• C. A protein that binds DNA to block transcription initiation or progression
• D. A protein that degrades mRNA after transcription

Answer: (C)

Transcriptional repressors are proteins that regulate gene expression by binding to specific DNA sequences and blocking transcription from starting or progressing. When a repressor sits at a promoter or operator region, it can physically prevent RNA polymerase from binding or moving along the DNA, which reduces or silences transcription. In bacteria, a classic example is a repressor that binds to the operator to stop transcription of an operon when the effector molecule isn’t present. In eukaryotes, repressors can recruit co-repressors and chromatin remodelers to keep chromatin in a closed state, making it harder for the transcription machinery to access the gene. This is distinct from activators that help recruit RNA polymerase and boost transcription, and from post-transcriptional controls that degrade or modify mRNA after it’s made.