Q.12 In eukaryotes, 5S rRNA is synthesized by
1. RNA polymerase I
2. RNA polymerase II
3. RNA polymerase III
4. RNA polymerase (sigma) subunit
Eukaryotes 5S rRNA: Synthesized by RNA Polymerase III
Ribosome biogenesis in eukaryotes requires precise transcription of four rRNAs by specialized polymerases. The correct answer is 3. RNA polymerase III, which uniquely transcribes the 5S rRNA gene.
Unlike other rRNAs processed from a massive 45S precursor, 5S rRNA is independently transcribed and assembles separately before joining the large ribosomal subunit.
Why RNA Polymerase III Handles 5S rRNA
RNA polymerase III (Pol III) transcribes small non-coding RNAs like tRNAs, 5S rRNA, and U6 snRNA using internal promoters (A-box, intermediate element, C-box). In eukaryotes, 5S genes reside outside the nucleolus in euchromatin, unlike Pol I’s nucleolar 45S transcription.
Pol III requires transcription factor TFIIIA (with zinc fingers binding the internal control region), plus TFIIIB and TFIIIC for initiation. This yields ~120 nt 5S transcripts that bind ribosomal protein L5, forming the 5S RNP for later 60S subunit integration.
Breakdown of All Polymerase Options
Each polymerase has distinct roles—Pol III specializes in short RNAs.
| Option | Enzyme | Primary Targets | Why Not 5S rRNA? |
|---|---|---|---|
| 1 | RNA polymerase I | 45S pre-rRNA (18S, 5.8S, 25S/28S) | Nucleolar, large ribosomal transcripts only |
| 2 | RNA polymerase II | mRNAs, snRNAs, miRNAs | Protein-coding genes; 7-methylguanosine capped |
| 3 | RNA polymerase III | 5S rRNA, tRNAs, 5.8S-like | Correct: Short RNAs with type 1 promoters |
| 4 | RNA polymerase (sigma) subunit | Bacterial sigma factors for promoter recognition | Prokaryotic only—no eukaryotic equivalent |
Ribosomal RNA Transcription Summary
-
Pol I: ~90% transcription; rDNA tandem arrays.
-
Pol II: Protein components.
-
Pol III: 5S + small RNAs; excess 5S stored as 7SL particles.
This division ensures stoichiometric ribosome assembly. Mutations disrupting Pol III affect growth dramatically.
Correct Answer: 3. RNA polymerase III
