Formation of the Pol I preinitiation complex requires the binding of selective factor 1 (SL1 or TIF-IB) to the core element of the rDNA promoter. SL1 is a complex composed of TBP and at least three TBP-associated factors (TAFs). For basal levels of transcription, only SL1 and the initiation-competent form of Pol I (Pol Iβ), characterized by RRN3 binding, are required.
For activated transcription levels, UBTF (UBF) is also required. UBTF binds as a dimer to both the upstream control element (UCE) and core element of the rDNA promoter, bending the DNA to form an enhanceosome. SL1 has been found to stabilize the binding of UBTF to the rDNA promoter.
The subunits of the Pol I PIC differ between organisms.