hTFIIIB-beta stably binds to pol II promoters and recruits RNA polymerase III in a hTFIIIC1 dependent way.

It has been shown that under specific conditions, transcription of protein coding genes can be efficiently initiated by RNA polymerase (pol) III in vitro. We examined the formation and composition of such pol III transcription complexes on the duck histone H5 and alphaA-globin promoters and found that the essential step for the formation of pol III transcription complexes on these pol II promoters was the stable binding of transcription factor (TF) IIIB-beta. For this process, the intact TFIIIB-beta complex, consisting of TBP and associated factors (TAFs) was needed and the prior association of pol III assembly factors was not necessary. We demonstrate for the first time that hTFIIIB-beta alone is able to bind to pol II promoter DNA. This resulted in a very stable complex which was resistant to high concentrations of heparin. Although immunodepletion revealed that TBP is essentially required for complex formation, other components of hTFIIIB-beta must also be involved, since TBP itself is unable to form heparin-resistant complexes and does not mediate pol III commitment per se. pol III is recruited to these pol II promoters in a strictly TFIIIC1 dependent way. After binding of TFIIIB-beta, the addition of TFIIIC1 and pol III were sufficient to yield productive pol III transcription complexes, which utilized the correct pol II initiation site. From these findings, we postulate that TFIIIC1 is involved in the recruitment of pol III and may thus form a bridge between TFIIIB-beta and the enzyme. This finding provides the first evidence for functional contacts between TFIIIC1 and pol III, which could be of general importance for the assembly of pol III transcription complexes.