What is the role of RNA polymerase III in the process of transcription in eukaryotes?

RNA polymerase III (Pol III) is one of the three main RNA polymerases found in the nuclei of eukaryotic cells.

Its specific role is to synthesize the small, abundant RNA molecules that are essential for protein synthesis and RNA processing. These Pol III‑transcribed genes include:

• Transfer RNAs (tRNA) – the adaptor molecules that deliver amino acids to the ribosome during translation.
• 5S ribosomal RNA (5S rRNA) – a component of the large ribosomal subunit that helps stabilize rRNA structure.
• Small nuclear RNAs (snRNA) – most notably the U6 snRNA, which is a core part of the spliceosome that removes introns from pre‑mRNA.

Pol III recognizes internal promoter elements located within the coding region of tRNA and 5S rRNA genes (such as the A‑box and B‑box for tRNA) and external promoter sequences for certain snRNA genes. Once bound, Pol III initiates transcription, elongates the RNA chain, and terminates at a stretch of thymidine residues (poly‑T) in the DNA template, producing a short, uncapped, non‑polyadenylated RNA transcript.

In contrast, RNA polymerase I synthesizes the large ribosomal RNAs (28S, 18S, 5.8S) and RNA polymerase II synthesizes messenger RNAs (pre‑mRNA) and most snRNAs. Therefore, the distinctive function of Pol III is limited to the transcription of tRNA, 5S rRNA, and snRNA.