Required for RNA polymerase III-mediated transcription. Component of TFIIIC that initiates transcription complex assembly on tRNA and is required for transcription of 5S rRNA and other stable nuclear and cytoplasmic RNAs. Binds to the box B promoter element (By similarity). Part of the TFIIIC subcomplex TFIIIC2, consisting of six subunits, GTF3C1, GTF3C2, GTF3C3, GTF3C4, GTF3C5 and GTF3C6. Interacts with IGHMBP2. Interacts with MAF1. Belongs to the TFIIIC subunit 1 family.