Eukaryotic transcription part 2: Polymerase I mediated transcription

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In the process of transcription (by any polymerase), there are three main stages:

Initiation: the construction of the RNA polymerase complex on the gene's promoter with the help of transcription factors
Elongation: the actual transcription of the majority of the gene into a corresponding RNA sequence
Termination: the cessation of RNA transcription and the disassembly of the RNA polymerase complex.

Initiation

Initiation: the construction of the polymerase complex on the promoter. Pol I requires no TATA box in the promoter, instead relying on a UCS (Upstream Control Sequence).

UBF (Upstream Binding Factor) binds the UCS.
UCS recruits and binds a protein complex incorporating TBP (TATA Binding Protein) and three TAFs (TBP Associated Factors) called SL1 or TIF-IB. The TBP is forced to bind non-sequence specifically.
Rrn3/TIF-IA gets phosphorylated and binds Pol I
Pol I binds to the UBF/SL1 complex via Rrn3/TIF-IA, and transcription starts.

Elongation

As Pol I escapes and clears the promoter, UBF and SL1 remain-promoter bound, ready to recruit another Pol I. Indeed, each active rDNA gene can be transcribed multiple times simultaneously, as opposed to Pol II-transcribed genes, which associate with only one complex at a time. While elongation proceeds unimpeded in vitro, it is unclear at this point whether this process happens in a cell, given the presence 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 and behind the complex. These are unwound by topoisomerase I or II at regular interval, 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 require 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. Source of the article published in description is Wikipedia. I am sharing their material. Copyright by original content developers of Wikipedia.
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