Formation of the transcription initiation complex on mammalian rDNA.

Steps for the formation of transcription initiation complex on the human rRNA gene (rDNA) in vitro were analyzed with partially purified transcription factors and RNA polymerase I. The reaction requires at least two factors besides RNA polymerase I for maximal efficiency. Preincubation and short-pulse analyses of the accurate transcripts revealed the following steps. First, the species-dependent factor, designated TFID, bound to the rDNA template, forming a preinitiation complex (PIC-1) which was resistant to a moderate concentration (0.015 to 0.02%) of Sarkosyl. Other factors, designated TFIA and RNA polymerase I, were then added to convert it to the final preinitiation complex PIC-3. This complex incorporated the first two nucleoside triphosphates of the starting site to complete the initiation complex (IC), which was resistant to a high concentration (0.2%) of Sarkosyl. Binding of TFID was rate limiting in the overall initiation reaction in vitro. Together with the kinetics of incorporation, the results are interpreted to mean that TFID, one bound, remains complexed with rDNA together with TFIA as the PIC-2 for many rounds of transcription by RNA polymerase I. Thus, the formation of PIC-2 may be a prerequisite for the stable opening of rDNA for transcription in vivo.