What is the function of a transcriptional repressor?

• A. A transcriptional repressor binds RNA to degrade transcripts.
• B. A transcriptional repressor enhances transcription.
• C. A transcriptional repressor binds DNA to block transcription, often by blocking RNA polymerase or recruiting co-repressors and chromatin modifiers.
• D. A transcriptional repressor unwinds DNA.

Answer: (C)

A transcriptional repressor works by lowering gene expression, mostly by preventing RNA polymerase from transcribing the gene. They usually bind to specific DNA sequences near the promoter or regulatory regions. By sitting on the DNA, they can block RNA polymerase from binding or from moving along the template, stopping transcription. In many cases, they also recruit other proteins that compact chromatin or modify histones, making the DNA less accessible and further reducing transcription. This dual mode—direct interference with the transcription machinery and recruitment of repressive chromatin modifiers—explains how repressors keep genes turned off in different contexts. For example, bacterial repressors like the lac repressor block transcription by occupying the operator, while in eukaryotes, repressors recruit co-repressors and chromatin-modifying enzymes to create a repressive environment.

Options that talk about RNA binding to degrade transcripts describe RNA decay rather than transcriptional control, while those claiming enhancement of transcription describe activators. Unwinding DNA is a helicase-like activity, not the typical function of a transcriptional repressor.