The mouse ribosomal gene terminator consists of three functionally separable sequence elements.

Abstract

The structural requirements for 3′ end formation of mouse pre‐rRNA have been studied. Three sequence elements are shown to be required for accurate and efficient transcription termination by RNA polymerase I (pol I) assayed both in a cell‐free transcription system and in vivo after transfection of rDNA minigene constructs into 3T6 cells. The essential termination signal is the previously identified 18‐bp conserved element (AGGTCGACCAGATTANTCCG) that contains a SalI restriction site. This sequence motif (the ‘Sal box’) interacts with a specific nuclear protein that directs transcription termination. Here we demonstrate that the ‘Sal box’ sequence motif is sufficient for termination of pol I transcripts and the release of the nascent RNA chains from the template. However, in addition to this termination signal, pyrimidine‐rich sequences flanking the box at the 5′ and 3′ side play a role in the efficient and correct formation of authentic pre‐rRNA termini. Downstream sequences contribute to the efficiency of the termination reaction, whereas the position of 3′ end formation (i.e. 21 bp upstream of the ‘Sal box’) is affected by 5′ flanking regions. These flanking regions are recognized by at least two different nuclear factors which specifically bind to DNA sequences located upstream and downstream of the ‘Sal box’.

Binding of the Termination Factor Nsi1 to Its Cognate DNA Site Is Sufficient To Terminate RNA Polymerase I Transcription In Vitro and To Induce Termination In Vivo

“…Nsi1 terminated RNA polymerase I transcription, whereas neither Pol II nor Pol III transcription termination was comparably affected in the employed in vitro assay. These results resemble those obtained in experiments investigating mammalian RNA polymerase I transcription termination by the transcription termination factor TTF1 (12,38,46). Hence, at least in such an artificial in vitro system, a specific molecular interplay between Pol I and Nsi1 is required to promote transcription termination.…”

“…In mouse, binding of the transcription termination factor TTF1 to a conserved 18-bp DNA element was shown to be sufficient to stop Pol I transcription in vitro (10)(11)(12) and to release the transcripts with the help of Pol I transcript release factor (PTRF) (13,14). The same DNA cis element was able to terminate transcription in vivo (12). Adjacent DNA elements could modulate termination efficiency.…”

Transcription termination by nuclear RNA polymerases

“…Ninety percent of yeast RNAPI terminates at the first terminator (T1), situated upstream of a pause-inducing factor-binding site that is recognized in vitro by Reb1 (Ju et al 1990;Lang and Reeder 1995). In mammals, the major terminator, called the Sal box (Grummt et al 1985;Kuhn et al 1988), is recognized by TTF-I (transcription termination factor for polI) , and like the Reb1-binding site, it must be correctly oriented to cause termination (Grummt et al 1985;Lang and Reeder 1993). The mouse Sal box is an 18-bp element repeated 10 times in the IGS, while humans contain a shorter Sal box (11 bp) that is recognized by the human TTF-I (Bartsch et al 1987;Pfleiderer et al 1990;.…”

Transcription termination at the chicken beta H-globin gene.

“…Eucaryotic RNA polymerase I termination (27), although factor dependent, bears little resemblance to p-dependent termination in E. coli but instead resembles aspects of the protein blockage mechanism described by Sellitti et al (50). For polymerase I terminators, it is the DNA to which the factor binds (27), and the signal follows, rather than precedes, the site of termination (27,28).RNA polymerase II appears to incorporate elements of all the above termination mechanisms in its repertoire of strategies. Transcription of the human gastrin gene appears to terminate upstream of the termination signal as for RNA polymerase I (3, 49).…”

“…Eucaryotic RNA polymerase I termination (27), although factor dependent, bears little resemblance to p-dependent termination in E. coli but instead resembles aspects of the protein blockage mechanism described by Sellitti et al (50). For polymerase I terminators, it is the DNA to which the factor binds (27), and the signal follows, rather than precedes, the site of termination (27,28).…”