The Saccharomyces cerevisiae RNA polymerase Ill transcription factor (TF) IIIB has been assembled from three components. An assembly pathway of these polypeptides, which specifies their interactions, has been determined. TheTATA-binding protein, TBP, and the TFIIB-related BRF7 gene product BRF, together reconstitute the transcription factor activity and TFlllC-dependent DNA-binding activity of the B’component of TFIIIB. BRF alone weakly binds to a TFIIIC-tRNA gene complex; TBP greatly stabilizes this interaction. B” transcription factor actitivity is recovered with its previously identified 90 kd polypeptide from SDS-polyacrylamide gels. Incorporation of the 90 kd 8” protein into the transcription complex requires TBP. The heparin-resistant TFIIIB-DNA complex retains all three of its constituent proteins, TBP, BRF, and B”.