Q.64 tRNA is synthesized by
1. RNA polymerase I
2. RNA polymerase II
3. Ribosomes
4. RNA polymerase III
tRNA is synthesized by RNA polymerase III, which transcribes small non-coding RNAs including tRNA genes. This eukaryotic polymerase specializes in compact promoters like the A and B boxes within tRNA intragenic sequences.
Eukaryotic RNA Polymerases Overview
Eukaryotes use three nuclear RNA polymerases: Pol I (rRNA, nucleolus), Pol II (mRNA, snRNA), Pol III (tRNA, 5S rRNA, small ncRNAs).
Pol III recognizes internal promoters (A/B boxes for tRNA) via TFIIIC/TFIIIB factors; produces abundant stable RNAs for translation.
Ribosomes translate, not transcribe.
Correct Answer: 4. RNA polymerase III
Pol III transcribes all cellular tRNAs from ~500 genes, plus 5S rRNA and others; regulated by MAF1, p53 for growth control.
Initiation involves TFIIIB bending DNA upstream of TSS; terminates at oligo(U) tracts.
Option Breakdown
| Option | Transcribes What? | tRNA Role? |
|---|---|---|
| 1. RNA polymerase I | rRNA (28S, 18S, 5.8S); nucleolar. | No |
| 2. RNA polymerase II | mRNA, snRNA, miRNA; Pol II CTD regulation. | No |
| 3. Ribosomes | Protein synthesis machinery. | No |
| 4. RNA polymerase III | tRNA, 5S rRNA, U6 snRNA. | Yes |
Pol III Specifics
tRNA promoters: Box A (~TSS), Box B (downstream); TFIIIC binds both, recruits TFIIIB/Pol III.
High output matches tRNA abundance; cancer links via MYC/Pol III deregulation.
