xUBF and Rib 1 are both required for formation of a stable polymerase I promoter complex in X. laevis.

Abstract

We show that three protein fractions are required for accurate transcription initiation at a Xenopus laevis ribosomal gene promoter in vitro: RNA polymerase I, Rib1 and xUBF. The Rib1 and xUBF fractions are both necessary and sufficient for formation of a stable initiation complex. The xUBF fraction can be completely replaced by recombinant xUBF. We also report the sequence of a cDNA clone for xUBF. xUBF is 701 amino acids in length, contains domain which are related to a domain found in chromosomal proteins HMG 1 and 2, and has an acidic carboxy terminus of 87 amino acids. xUBF is closely similar in amino acid sequence to its previously reported human homolog, hUBF, except that xUBF has only three of the HMG‐related domains while hUBF has four and therefore is 63 amino acids longer than xUBF.

Phosphorylation of the N‐terminal domain of Xenopus TATA‐box binding protein by DNA‐dependent protein kinase depends on the C‐terminal core domain

“…TIF-IB has been shown to consist of TBP and 3 pol Ispecific TAFs [39]. Ribl appears to be its Xenopus equivalent but has not been molecularly characterized [40]. These observations raised the possibility that DNA-PK phosphorylates and inactivates a polypeptide in the promoter-bound TBP-TAF complex.…”

The RNA polymerase I transcription factor UBF is a sequence-tolerant HMG-box protein that can recognize structured nucleic acids

“…In this regard, UBF is a likely candidate because we have shown that it is clearly a sequence-tolerant protein previously shown to produce discrete footprints on the promoters of all vertebrate species examined (7,(12)(13)(14)33). In addition, UBF can functionally substitute across species boundaries in some cases (13,14), and it is apparently involved in recruiting or stabilizing the TBP-containing activity in transcription complexes on the promoter (10,13,18,33,34,53).…”

“…UBF was originally purified by two independent means, first as a human cell fraction necessary for RNA polymerase I transcription in vitro and as an rRNA gene enhancer binding protein (10)(11)(12). UBF has now been purified and cloned from human, frog, rat and mouse (7,(13)(14)(15)(16)(17)(18)(19)(20). The UBF protein consists of an amino terminal dimerization domain (19, 21,22) followed by five repeated 80 amino acid domains (17) termed HMG-boxes due to their similarity to the DNA binding domains of High Mobility Group (HMG) proteins 1 and 2 (16), and a highly acidic carboxyl terminal tail.…”

“…In human extracts the pol I transcription activity named SL1 [Selectivity Factor 1; (26)] contains TBP and at least three TBPAssociated Factors (TAFs) (27). In other species the equivalent activity has been named TFID, Factor D, Rib-I or TIF-1B (15,18,(28)(29)(30)(31)(32). Human SLI requires interaction with UBF to form a promoter complex which can be visualized as a DNase I footprint that is more extensive than the footprint produced by UBF (33).…”

“…In mice, however, UBF is not required for SLI to bind the promoter (34,35) but both factors still interact to form a footprint more extensive than either activity alone (13). Furthermore, several studies have revealed that mouse rRNA transcription is greatly *To whom correspondence should be addressed Q--n) 1994 Oxford University Press stimulated by UBF apparently by acting early in the assembly of stable pre-initiation complexes (10,15,18,34). It has been proposed that UBF acts as both an activator, by stabilizing the interaction of the TBP-containing activity and/or RNA polymerase I with the promoter and as an anti-repressor by preventing histone (or other DNA binding factors) mediated repression (35,36).…”