Transcription machinery – eukaryotic RNA polymerases: In eukaryotic nuclei, which RNA polymerase is primarily responsible for transcribing tRNA genes (and also 5S rRNA)?

Introduction / Context:
Eukaryotes partition transcriptional duties among multiple nuclear RNA polymerases. Knowing which polymerase handles tRNA genes is critical for interpreting promoters, transcription factors, and drug/toxin sensitivities in cell biology and genetics.

Given Data / Assumptions:

• Pol I → large rRNA (28S/18S/5.8S in nucleolus).
• Pol II → mRNA and most snRNA/snoRNA precursors.
• Pol III → tRNA and 5S rRNA (and some other small RNAs).

Concept / Approach:
tRNA genes typically use internal promoter elements (A and B boxes) recognized by TFIIIC/TFIIIB complexes to recruit RNA polymerase III. Pol III transcription produces short, structured RNAs required for translation and RNA processing. This functional division ensures efficient regulation and specialized transcription factor usage across RNA classes.

Step-by-Step Solution:
Match RNA classes to polymerases. Identify tRNA → Pol III. Select “RNA polymerase III.”

Verification / Alternative check:
Inhibitors like alpha-amanitin strongly affect Pol II at low doses, while Pol III is inhibited only at higher concentrations; Pol I is resistant—functional differences that support assignments.

Why Other Options Are Wrong:
Pol I does rRNA; Pol II does mRNA; “Any of these” is false; mitochondrial RNA polymerase transcribes mitochondrial genes, not the nuclear tRNA genes asked here.

Common Pitfalls:
Confusing 5S rRNA (Pol III) with the larger rRNAs (Pol I).

Final Answer:
RNA polymerase III.