What is a common mechanism by which a transcriptional repressor inhibits transcription?

• A. Blocking RNA polymerase binding or recruitment of co-repressors.
• B. Recruiting RNA polymerase to promoter.
• C. Guiding mRNA splicing.
• D. Degrading mRNA after transcription.

Answer: (A)

Transcriptional repression usually works by preventing the start of transcription, either by blocking RNA polymerase from binding the promoter or by drawing in corepressor proteins that modify chromatin to a closed, less accessible state. When RNA polymerase can’t bind or can’t be recruited efficiently, the transcription initiation complex can’t form, so transcription doesn’t proceed. Corepressors recruited by repressors often bring enzymes like histone deacetylases that tighten chromatin structure, further hindering access to the promoter and keeping transcription off.

The other options don’t fit this scenario: bringing RNA polymerase to the promoter would boost transcription, not repress it; guiding mRNA splicing and degrading mRNA after transcription occur after transcription has started and affect RNA processing or stability rather than initiation.