Figure 1

An integrated prototype of the TBP regulatory network. Three parallel assembly pathways proceed temporally from left to right along line segments that represent promoter DNA. Two pathways, directed by the multisubunit TFIID complex ('D', in cyan) and the compositionally related SAGA complex ('S', in green) lead to productive pre-initiation complex (PIC) assembly, which goes on to produce RNA. The RNA ultimately is degraded leading to a steady-state balance between production and degradation. In this model, TBP ('T', in yellow) resides as a self-inhibited dimer when not bound to DNA. Dissociation into monomers is required for DNA binding. The TAND domain of TFIID's TAF1 subunit further inhibits TBP binding to DNA along the TFIID pathway. The third pathway loads TBP onto promoter DNA in a nonproductive manner, and interferes with PIC assembly unless dissociated by the combined action of NC2 and Mot1 ('N' and 'M' in red) [24]. NC2 and Mot1 also dissociate TBP loaded via the SAGA pathway. While Mot1 dissociates TBP in the absence of NC2 in vitro, it appears to be linked to NC2 function in vivo (Figure S1 in Additional data file 1). Numbers correspond to steps defined in Figure 2a. The presence of chromatin and general transcription regulators is implicit and not shown because their contributions to PIC assembly are not being tested here.