Cryo-EM structures of human RNA polymerase III in its unbound and transcribing states

Abstract

Cryo-EM structures of human Pol III in both apo- and elongating states reveal metazoan-specific differences in the regulation of transcription termination and identify mutations relevant to human disease. RNA polymerase III (Pol III) synthesizes transfer RNAs and other short, essential RNAs. Human Pol III misregulation is linked to tumor transformation, neurodegenerative and developmental disorders, and increased sensitivity to viral infections. Here, we present cryo-electron microscopy structures at 2.8 to 3.3 angstrom resolution of transcribing and unbound human Pol III. We observe insertion of the TFIIS-like subunit RPC10 into the polymerase funnel, providing insights into how RPC10 triggers transcription termination. Our structures resolve elements absent from Saccharomyces cerevisiae Pol III such as the winged-helix domains of RPC5 and an iron-sulfur cluster, which tethers the heterotrimer subcomplex to the core. The cancer-associated RPC7 alpha isoform binds the polymerase clamp, potentially interfering with Pol III inhibition by tumor suppressor MAF1, which may explain why overexpressed RPC7 alpha enhances tumor transformation. Finally, the human Pol III structure allows mapping of disease-related mutations and may contribute to the development of inhibitors that selectively target Pol III for therapeutic interventions.