TATA box-mediated in vitro transcription by RNA polymerase III. Evidence for TATA-binding protein in a polymerase III type complex.

We have shown previously that under specific conditions, a TATA box will mediate efficient in vitro transcription by RNA polymerase (pol) III in the absence of a PSE or other promoter elements. The reaction requires a HeLa cell phosphocellulose protein fraction, fraction B, which must be preincubated with the template DNA. Fraction B does not contain any detectable pol II type transcription factor IID (TFIID) activity. In this report, the relationship between fraction B and TFIID was further examined. Purified human TATA-binding protein (TBP) can substitute for fraction B to mediate TATA-dependent pol III transcription. Both TBP and fraction B prefer a reverse TATA box for pol III transcription, yet TBP bound to a reverse TATA box functions poorly for pol II transcription. Like TFIID, fraction B forms a template-committed complex with TATA-containing promoters. TBP, however, will not template commit for pol III transcription unless premixed with phosphocellulose fraction C. TBP-mediated pol III transcription is also more sensitive to the detergent Sarkosyl (N-lauroylsarcosine, Sigma) than is the fraction B reaction unless it is premixed with fraction C. Together, the data suggest that TBP can complex with a component of fraction C, and this complex is then functionally equivalent to fraction B. We propose that fraction B contains TBP in a complex with some other component(s) of the pol III transcription machinery and that this B complex TBP may be specific for pol III transcription.