Identification of a 150-kilodalton polypeptide that copurifies with yeast TFIIIC and binds specifically to tRNA genes.

The transcription in vitro of eucaryotic tRNA genes by RNA polymerase III requires two transcription factors, designated TFIIIB and TFIIIC. One of the critical functions of TFIIIC in the transcription of tRNA genes is that it interacts directly and specifically with the two internal promoter elements of these genes. We have partially purified Saccharomyces cerevisiae TFIIIC by chromatography on Bio-Rex 70, DEAE-cellulose, and phosphocellulose resins. A 150-kilodalton (kDa) DNA-binding polypeptide copurified with TFIIIC activity. This 150-kDa protein coeluted with the DNA-binding activity of TFIIIC after rechromatography of TFIIIC on phosphocellulose and its elution with a linear salt gradient. The stable and high-affinity interaction of this protein with tRNA genes was demonstrated by the maintenance of a protein-DNA complex under conditions of high ionic strength. Finally, we showed by two criteria that the interaction of this protein with tRNA genes was specific. First, the protein-DNA complex was competed with only by DNA-containing tRNA genes; second, the protein preferentially bound to DNA fragments containing a tRNA gene. These results strongly suggest that the DNA-binding domain of the yeast TFIIIC is contained within this 150-kDa polypeptide.