Human transcription factor IIIC binds to its cognate promoter sequences in a metal coordinated fashion

Transcription factor IIIC from human cells (hTFIIIC) contains a 55 kDa polypeptide which specifically binds to the promoter of the VAI and 5S gene. This interaction can be abolished by depleting divalent metal cations from the free protein through chelation with EDTA. Prior association of the protein with its DNA-binding sequence renders the complex refractory to chelation by EDTA. Specific binding of hTFIIIC to its cognate promoter sequences--shown by electrophoretic mobility shift and DNase I protection assays--can be restored by the addition of zinc ions. In contrast to the binding of hTFIIIA to the 5S gene, which was monitored in parallel and which exclusively requires Zn2+, the binding of hTFIIIC to the VAI and 5S gene can also be reconstituted--albeit with a lower efficiency--by the transition metals Co2+, Fe2+ and Mn2+ but not by Ni2+ or Cu2+. These results show that hTFIIIC binds to its promoter sequences in a metal coordinated fashion which differs from that observed for the binding of hTFIIIA to the 5S gene.