TBP-TAF complex SL1 directs RNA polymerase I pre-initiation complex formation and stabilizes upstream binding factor at the rDNA promoter

Knowledge of the role of components of the RNA polymerase I transcription machinery is paramount to understanding regulation of rDNA expression. We describe key findings for the roles of essential transcription factor SL1 and activator upstream binding factor (UBF). We demonstrate that human SL1 can direct accurate Pol I transcription in the absence of UBF and can interact with the rDNA promoter independently and stably, consistent with studies of rodent SL1 but contrary to previous reports of human SL1. UBF itself does not bind stably to rDNA but rapidly associates and dissociates. We show that SL1 significantly reduces the rate of dissociation of UBF from the rDNA promoter. Our findings challenge the idea that UBF activates transcription through recruitment of SL1 at the rDNA promoter and suggest that the rate of pre-initiation complex (PIC) formation is primarily determined by the rate of association of SL1, rather than UBF, with the promoter. Therefore, we propose that SL1 directs PIC formation, functioning in core promoter binding, RNA polymerase I recruitment, and UBF stabilization and that SL1-promoter complex formation is a necessary prerequisite to the assembly of functional and stable PICs that include the UBF activator in mammalian cells.