Factors Involved in Ribosomal RNA Transcription

The development of a cell-free transcription system, which faithfully transcribes a synthetic rDNA template, was first described about two decades ago (Grummt, 1981b Haglund and Rothblum, 1987 Learned and Tjian, 1982 Mishima et al., 1981 Wilkinson and Sollner-Webb, 1982). This accomplishment encouraged a large number of biochemical studies on the dissection of the RNA Pol I transcription system. In vitro transcription assays coupled to column fractionation analyses of cell extracts from a 

One of the fractions required to direct rDNA transcription contains the RNA polymerase I enzymatic activity. Purification of mammalian RNA polymerase I indicates that the core enzyme is a multisubunit complex with a molecular mass of 500 kDa. (Hannan et aL, 1998c Song et al., 

Biochemical fractionation studies indicated that purified RNA polymerase I activity can be dissociated into two fractions, both of which possess RNA polymerization activity, but only one of which has promoter-specific activity. The analysis of these different forms of RNA polymerases led to the identification of a set of polypeptides that are loosely associated with 

The factor TIF-IC has been identified as a 65-kDa protein loosely associated with RNA Pol I (Schnapp et al., 1994b). Biochemical characterization of this factor indicated that it is required for the assembly of the preinitiation complex and the formation of the first internucleotide bond. In addition, TIF-IC functions in chain elongation and suppresses RNA polymerase pausing during transcription elongation (Schnapp et al., 1994b). To date, TIF-IC has only been identified in the murine system. Because this factor has not been purified to homogeneity and cloned, its molecular identity remains unknown. 

DNA structures in a conformation-specific rather than sequence-specific manner (Copenhaver etal., 1994). For this reason, it has been proposed that UBF plays an architectural role at the ribosomal DNA promoter. In agreement with this model, electron microscopic imaging analysis indicated that DNA is wrapped around a UBF dimer in a structure reminiscent of the nucleosome (Bazettjones et al., 1994). Therefore, UBF may function as a scaffold protein, which, by establishing the proper DNA-protein structure, facilitates the formation of the transcription initiation complex at the ribosomal DNA promoter. This model also predicts that the UCE and core elements will cooperate with each other in the recruitment of UBF. Indeed, changes in the spacing between the UCE and core have profound consequences on the ability of UBF to stimulate RNA Pol I transcription. 

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