Assembly of an RNA Polymerase I Initiation Complex in Vertebrates

The identification of the cis-acting sequences and protein factors involved in ribosomal DNA transcription strongly suggested that the network of protein-protein and protein—DNA interactions among UBF, SL1, and the promoter elements plays a major role in the assembly of a stable preinitiation complex. A number of studies have established that UBF and SL1 play a key role in this process and are necessary to direct a high level of RNA Pol I transcription in vitro. The current stepwise model of factors assembly predicts that the binding of UBF dimer to the UCE and CORE elements is a prerequisite for the recruitment of the selectivity factor SL1 to the rDNA promoter (Fig. 4, step I). Biochemical studies have indicated that two subunits of SL1, TAFi-18 and TBP, interact directly with the carboxy-terminal acidic domain of UBF (Beckmann et al., 1995 Tuan et al., 1999 step II). This finding demonstrates that the function of the carboxyl-terminal activation domain of UBF is to recruit the essential 

Grummt, 1991). The fate of each component of the preinitiation complex once transcription is initiated has been recently investigated in vitro using immobilized DNA templates (Aprikian et al., 2001 Panov et al., 2001). In the human system, UBF and SL1 appear to remain bound to the promoter, ready to recruit a new RRN3/Pol I complex for a new round of transcription. Similar experiments in yeast have indicated that UAF is the only factor that remains bound to the promoter once the polymerase initiates transcription, whereas the core factor (CF), TBP, and RRN3 dissociate from the promoter-bound upstream activator factor (UAF) once elongation of transcription by RNA Pol I occurs. Whether the differences in the mechanisms of RNA Pol I postassembly reflect species-related differences remains to be further investigated. 

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