Termination of Mammalian rDNA Replication: Polar Arrest of Replication Fork Movement by Transcription Termination Factor TTF-I

Abstract

A replication fork barrier (RFB) at the 3' end of eukaryotic ribosomal RNA genes blocks bidirectional fork progression and limits DNA replication to the same direction as transcription. We have reproduced the RFB in vitro in HeLa cell extracts using 3' terminal murine rDNA fused to an SV40 origin-based vector. The RFB is polar and modularly organized, requiring both the Sal box transcription terminator and specific flanking sequences. Mutations within the terminator element, depletion of the RNA polymerase I-specific transcription termination factor TTF-I, or deletion of the termination domain of TTF-I abolishes RFB activity. Thus, the same factor that blocks elongating RNA polymerase I prevents head-on collision between the DNA replication apparatus and the transcription machinery.

Transcription termination by nuclear RNA polymerases

“…3; Takeuchi et al 2003;Huang et al 2006). TTFI also acts as an RFB when it is bound to the second Sal box and flanking regions of the IGS (Gerber et al 1997). Unexpectedly, the RFB was found to act as an additional RNAPI transcription terminator in a xrn1D strain depleted of Rat1 (El Hage et al 2008).…”

Functional architecture of the Reb1-Ter complex of Schizosaccharomyces pombe

“…This interaction mediates at least four different functions namely termination of DNA replication (29-31), activation of transcription (8, 32), termination of transcription, and a long-range chromosome-to-chromosome interaction called "chromosome kissing" (31). The mammalian ortholog TTF-I also performs similar functions (26,27,33,34).Replication termination is of critical importance, especially during the S phase of the cell cycle, when chromosomal DNA is often transcribed and replicated at the same time. Ter sites are needed to prevent collisions between the replication and transcription machineries approaching each other from opposite directions, thereby preventing R-loop formation and potential genome instability (35,36).…”

“…This interaction mediates at least four different functions namely termination of DNA replication (29)(30)(31), activation of transcription (8,32), termination of transcription, and a long-range chromosome-to-chromosome interaction called "chromosome kissing" (31). The mammalian ortholog TTF-I also performs similar functions (26,27,33,34).…”

Rad22Rad52-dependent Repair of Ribosomal DNA Repeats Cleaved by Slx1-Slx4 Endonuclease

“…These sites are often called replication fork barriers (RFBs). It is somewhat paradoxical that in some cases these "programmed" RFBs are thought to play important roles in maintaining genome integrity.The best-characterized RFBs occur in the ribosomal DNA genes (rDNA), which are present in long tandem repeats at one or a few chromosomal loci in most eukaryotic species, including yeasts and humans (Brewer and Fangman, 1988;Linskens and Huberman, 1988;Little et al, 1993;Gerber et al, 1997;Sanchez et al, 1998). The budding yeast Saccharomyces cerevisiae carries ϳ150 copies of the rDNA genes.…”