DNA-BINDING DOMAIN AND SUBUNIT INTERACTIONS OF TRANSCRIPTION FACTOR-IIIC REVEALED BY DISSECTION WITH POLIOVIRUS 3C PROTEASE

Transcription factor IIIC (TFIIIC) is a general RNA polymerase III tra nscription factor that binds the B-box internal promotor element of tR NA genes and the complex of TFIIIA with a 5S rRNA gene. TFIIIC then di rects the binding of TFIIIB to DNA upstream of the transcription start site. TFIIIB in turn directs RNA polymerase III binding and initiatio n. Human TFIIIC contains five different subunits. The 243-kDa alpha su bunit can be specifically cross-linked to B-box DNA, but its sequence does not reveal a known DNA binding domain. During poliovirus infectio n, TFIIIC is cleaved and inactivated by the poliovirus-encoded 3C prot ease (3Cpro). Here we analyzed the cleavage of TFIIIC subunits by 3Cpr o in vitro and during poliovirus infection of HeLa cells. Analyses of the DNA binding activities of the resulting subcomplexes indicated tha t an N-terminal 83-kDa domain of the alpha subunit associates with the beta subunit to generate the TFIIIC DNA binding domain. Cleavage with 3Cpro also generated an similar to 125-kDa C-terminal fragment of the alpha subunit which remained associated with the gamma and epsilon su bunits.