Human transcription factors IIIC2, IIIC1 and a novel component IIIC0 fulfil different aspects of DNA binding to various pol III genes

Human transcription factor IIIC2 interacts with the TFIIIA-5S DNA complex and forms a ternary TFIIIA/IIIC2-5S DNA complex. Formation of this complex does not preclude simultaneous binding of TFIIIC2 to the B-box sequence of a second template. This suggests that the domain(s) or subunit(s) required for indirect recognition of the 5S promoter by TFIIIC2 are different from those necessary for direct binding of TFIIIC2 to B-box-containing pol III promoters. Whereas TFIIIC2 is only required for transcription of the 'classical' pol III genes, TFIIIC1 is generally required for transcription of all pol III genes, including that of the U6 gene. The activity of TFIIIC1 strongly enhances specific binding of basal pol III factors TFIIIA, TFIIIC2 and the PSE binding protein (PBP) to their cognate promoter elements and it acts independently of the corresponding termination regions. Moreover, we characterize an activity, TFIIIC0, purified from phosphocellulose fraction C, which shows strong DNase I protection of the termination region of several pol III genes and which is functionally and chromatographically distinct from TFIIIC1 and TFIIIC2.