Architecture of TFIIIC and its role in RNA polymerase III pre-initiation complex assembly

In eukaryotes, RNA Polymerase III (Pol III) is specifically responsible for transcribing genes encoding tRNAs and other short non-coding RNAs. The recruitment of Pol III to tRNA-encoding genes requires the transcription factors (TF) IIIB and IIIC. TFIIIC has been described as a conserved, multi-subunit protein complex composed of two subcomplexes, called τA and τB. How these two subcomplexes are linked and how their interaction affects the formation of the Pol III pre-initiation complex (PIC) is poorly understood. Here we use chemical crosslinking mass spectrometry and determine the molecular architecture of TFIIIC. We further report the crystal structure of the essential TPR array from τA subunit τ131 and characterize its interaction with a central region of τB subunit τ138. The identified τ131–τ138 interacting region is essential in vivo and overlaps with TFIIIB-binding sites, revealing a crucial interaction platform for the regulation of tRNA transcription initiation. TFIIIC is a RNA polymerase III-specific general transcription factor complex essential for tRNA synthesis. Here the authors combine chemical crosslinking/mass spectrometry and X-ray crystallography to define the architecture of TFIIIC and suggest a model for the assembly of pre-initiation complexes at tRNA genes.