of nucleosomes. Pol I does seem to transcribe through nucleosomes, either bypassing or disrupting them, perhaps assisted by chromatin-remodeling activities. In addition, UBF might also act as positive feedback, enhancing Pol I elongation through an anti-repressor function. An additional factor, TIF-IC, can also stimulate the overall rate of transcription and suppress pausing of Pol I. As Pol I proceeds along the rDNA, supercoils form both ahead of and behind the complex. These are unwound by topoisomerase I or II at regular intervals, similar to what is seen in Pol II-mediated transcription. Elongation is likely to be interrupted at sites of DNA damage. Transcription-coupled repair occurs similarly to Pol II-transcribed genes and requires the presence of several DNA repair proteins, such as TFIIH, CSB, and XPG. Termination In higher eukaryotes, TTF-I binds and bends the termination site at the 3' end of the transcribed region. This will force Pol I to pause. TTF-I, with the help of transcript-release factor PTRF and a T-rich region, will induce Pol I into terminating transcription and dissociating from the DNA and the new transcript. Evidence suggests that termination might be rate-limiting in cases of high rRNA production. TTF-I and PTRF will then indirectly stimulate the reinitiation of transcription by Pol I at the same rDNA gene. In organisms such as budding yeast the process seems to be much more complicated and is still not completely elucidated. Recombination hotspot Recombination hotspots are DNA sequences that increase local recombination. The HOT1 sequence in yeast is one of the most well studied mitotic recombination hotspots. The HOT1 sequence includes an RNA polymerase I transcription promoter. In a yeast mutant strain defective in RNA polymerase I the HOT1 activity in promoting recombination is abolished. The level of RNA polymerase I transcription activity that is dependent on the promoter in the HOT1 sequence appears to determine the level of nearby mitotic recombination. See also RNA polymerase RNA polymerase II RNA polymerase III Selective factor 1 References EC 2.7.7 Gene expression Proteins