Core component of RNA polymerase III (Pol III) which synthesizes small non-coding RNAs using the four ribonucleoside triphosphates as substrates. Can mediate Pol I proofreading of the nascent RNA transcript. Anchors into the Pol III active site to constantly monitor transcription fidelity, cleaves mis-incorporated 5'-ribonucleotides and restarts the transcription process. Once Pol III reaches the poly(dT) termination signal, can induce Pol III clamp opening and transcription termination. Pol III plays an important role in sensing and limiting infection by intracellular bacteria and DNA viruses. Acts as a nuclear and cytosolic DNA sensor involved in innate immune response. Can sense non-self dsDNA that serves as template for transcription into dsRNA. The non-self RNA polymerase III transcripts, such as Epstein-Barr virus-encoded RNAs (EBERs) induce type I interferon and NF-kappa-B through the RIG-I pathway.