Question

Under certain circumstances, an elongating RNA polymerase can become arrested and cease transcribing. To deal with this situation, the cell has machinery that removes the arrested polymerase and at the same time recruits repair enzymes. Name the enzyme(s) which is part of the machinery.

• A. Endonuclease $\text{Uvr(A)(B)(C)}$
• B. Exonuclease
• C. DNA polymerase
• D. Reverse transcriptase

Hint:

Transcription-coupled repair (TCR) is significantly faster than global genomic repair (GGR) because the stalled RNA polymerase acts as an immediate distress signal that directly recruits the UvrABC endonuclease machinery.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Transcription-coupled repair (TCR) is a highly specialized sub-pathway of Nucleotide Excision Repair (NER).
TCR preferentially and rapidly repairs DNA damage on the actively transcribed template strand of genes.
When RNA polymerase encounters a DNA lesion (such as a UV-induced thymine dimer), it becomes arrested and physically blocks access to the damaged site.
The cell must utilize specific coupling proteins to remove or displace the stalled RNA polymerase and recruit the repair machinery to the lesion site.

Step 2: Detailed Explanation:
In Escherichia coli, when RNA polymerase stalls at a lesion, a coupling protein called Mfd (mutation frequency decline protein) binds to both the DNA and the stalled polymerase.
Mfd uses its ATPase activity to displace the arrested RNA polymerase from the DNA template.
At the same time, Mfd directly recruits the UvrA2B damage recognition complex to the damaged site.
This recruitment initiates the standard Nucleotide Excision Repair pathway.
The core catalytic assembly responsible for the subsequent incision of the damaged DNA strand is the UvrABC endonuclease complex.
The UvrA protein acts as a molecular matchmaker to detect the lesion and leaves the complex.
Then, UvrB and UvrC form an active endonuclease assembly that cleaves the phosphodiester backbone on both sides of the damaged nucleotide.
The excised oligonucleotide is subsequently removed, and the gap is filled by DNA polymerase I and sealed by DNA ligase.
Thus, the UvrABC endonuclease complex is recruited as a direct consequence of this transcription-coupled repair machinery.

Step 3: Final Answer:
The enzyme complex that is recruited to resolve the lesion is the Endonuclease $\text{Uvr(A)(B)(C)}$.