TBP-associated factors interact with DNA and govern species specificity of RNA polymerase I transcription.

Abstract

Unlike genes transcribed by RNA polymerases II and HI, transcription by RNA polymerase I is highly speciesspecific. Ribosomal promoter selectivity is brought about by a multisubunit transcription factor (SL1/TIF-IB) which consists of the TATA-binding protein (TBP) and three TBP-associated factors (TAFs). To determine the basis for the inability of SL1/TIF-IB to recognize heterologous rDNA, the transcriptional properties and the subunit composition of the murine and the human factor, as well as a chimeric complex containing epitopetagged human TBP and murine TAFs, have been compared. We show that TBP can be exchanged between the human and mouse factor indicating that the variable N-terminal domain of TBP does not play a significant role in rDNA promoter selectivity. Instead, DNA binding is brought about by the TAFs. UV crosslinking experiments demonstrate that binding to the ribosomal gene promoter is mediated by two TAFs (TAF148 and TAF168) which have the same electrophoretic mobility in the human and mouse factor. The largest TAF is different in both species and is suggested to play a role in the species-specific assembly of productive preinitiation complexes. Thus, evolutionary changes of rDNA promoter sequences have been accompanied by changes in specific TAFs.

Cloning of murine RNA polymerase I-specific TAF factors: Conserved interactions between the subunits of the species-specific transcription initiation factor TIF-IB/SL1

“…3A, each of the mouse and human TAF I s is able to bind to GST-mTBP but not to GST alone, indicating that the interactions between TBP and TAF I s are conserved. This finding is in accord with previous data showing that differences in the N termini of human and mouse TBP do not contribute to the promoter selectivity of TIF-IB͞SL1 (9).…”

“…Nevertheless, they exhibit different template specificities in that they require the homologous template to promote initiation complex formation, indicating that subtle structural differences account for these functional differences. TBP has been shown to be exchangeable between the human and mouse factor, and therefore differences in the variable N-terminal domains in human and mouse TBP do not appear to play a significant role in rDNA promoter selectivity (9,18). Consequently, the molecular basis for species-specific promoter recognition should reside in differences between the human and mouse TAF I s.…”

“…Most of the factors, i.e., UBF, pol I, TIF-IA, and TIF-IC, are interchangeable between human and mouse (3,(6)(7)(8)(9) whereas TIF-IB͞SL1 has been found to be the speciesspecific component in the preinitiation complex (3,9). A significant advance toward a functional characterization of this selectivity factor was the discovery that TIF-IB͞SL1 is a multiprotein complex consisting of TATA box binding protein (TBP) and three TBP-associated factors (TAFs) (10,11).…”

Cell cycle-dependent regulation of RNA polymerase I transcription: The nucleolar transcription factor UBF is inactive in mitosis and early G 1

“…Recent work has established that silencing of cellular pre-rRNA synthesis during mitosis is caused by inactivation of the pol I-specific transcription initiation factor TIF-IB͞SL1 by cdc2͞cyclin B-mediated phosphorylation (10). TIF-IB͞SL1 is a multiprotein complex containing TBP and three pol I-specific TAFs (TAF I s) (11)(12)(13). Binding of TIF-IB͞SL1 to the core element of the ribosomal gene promoter is enhanced by the upstream binding factor (UBF), a member of the family of high mobility group (HMG) box proteins (14).…”

DNA-dependent protein kinase: a potent inhibitor of transcription by RNA polymerase I.

“…Transcription complex formation is nucleated by binding of the promoter selectivity factor, termed TIF-IB in rodents and SL1 in humans. TIF-IB/SL1 is a multiprotein complex containing the TATAbinding protein (TBP) and three TBP-associated factors (polypeptides) (TAFs) (Comai et al 1992;Eberhard et al 1993;Rudloff et al 1994). TIF-IB/SL1 then recruits Pol I and two additional essential factors, TIF-tA and TIF-IC, to the promoter and transcriptional initiation proceeds (Schnapp and Grummt 1991;Schnapp et al 1993Schnapp et al , 1994.…”