A suppressor of TBP mutations encodes an RNA polymerase III transcription factor with homology to TFIIB.

The TDS4 gene of S. cerevisiae was isolated as an allele-specific high copy suppressor of mutations within the basic region of the TATA-binding protein (TBP). The gene is essential for viability and encodes a 596 aa protein. The first 300 aa of the TDS4 protein exhibit significant sequence similarity to the RNA polymerase II transcription factor TFIIB. However, TDS4 is required for RNA polymerase III transcription in vivo and in vitro. Antibodies specific for TDS4 or TBP react with the TFIIIB complex, indicating that both proteins are components of the RNA polymerase III initiation complex. These findings suggest that the RNA polymerase II and III initiation mechanisms are extremely similar, and they explain how the TATA-binding protein can function in both systems.