A stable complex of a novel transcription factor IIB-related factor, humanTFIIIB50, and associated proteins mediate selective transcription by RNA polymerase III of genes with upstream promoter elements

Transcription factor IIIB (TFIIIB) is directly involved in transcription in itiation by RNA polymerase III in eukaryotes. Yeast contain a single TFIIIB activity that is comprised of the TATA-binding protein (TBP), TFIIB-relate d factor 1 (BRF1), and TFIIIB", whereas two distinct TFIIIB activities, TFI IIB-alpha and TFIIIB-beta, have been described in human cells. Human TFIIIB -beta is required for transcription of genes with internal promoter element s, and contains TBP, a TFIIIB" homologue (TFIIIB150), and a BRF1 homologue (TFIIIB90). whereas TFIIIB-alpha is required for transcription of genes wit h promoter elements upstream of the initiation site. Here we describe the i dentification, cloning, and characterization of TFIIIB50, a novel homologue of TFIIB and TFIIIB90. TFIIIB50 and tightly associated factors, along with TBP and TFIIIB150, reconstitute human TFIIIB-alpha activity. Thus, higher eukaryotes, in contrast to the yeast Saccharomyces cerevisiae, have evolved two distinct TFIIB-related factors that mediate promoter selectivity by RN A polymerase III.