Component of the transcription factor SL1/TIF-IB complex, which is involved in the assembly of the PIC (pre-initiation complex) during RNA polymerase I-dependent transcription. The rate of PIC formation probably is primarily dependent on the rate of association of SL1/TIF-IB with the rDNA promoter. SL1/TIF-IB is involved in stabilization of nucleolar transcription factor 1/UBTF on rDNA. Formation of SL1/TIF-IB excludes the association of TBP with TFIID subunits (By similarity). Component of the transcription factor SL1/TIF-IB complex, composed of TBP and at least TAF1A, TAF1B, TAF1C and TAF1D. In the complex interacts directly with TBP, TAF1A and TAF1B. Interaction of the SL1/TIF-IB subunits with TBP excludes interaction of TBP with the transcription factor IID (TFIID) subunits. Interacts with UBFT (By similarity). Interacts with CEBPA (isoform 1 and isoform 4) (PubMed:20075868).