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. Author manuscript; available in PMC: 2026 Feb 4.
Published in final edited form as: Cell Rep. 2025 Feb 24;44(3):115325. doi: 10.1016/j.celrep.2025.115325

Figure 7. Models for RNAPI transcription.

Figure 7

(A) Shallow backtracking can be released by Rpa12-mediated cleavage of the nascent transcript. We propose that release from deep backtracking is initiated with oligo-adenylation of the nascent transcript by the TRAMP complex and promoted by (1) Rpa12-driven release and/or (2) a reverse torpedo mechanism.

(B) Left: inactive rDNA repeats are packaged in nucleosomes but can be opened by the cooperative activities of multiple polymerases, each linked by torsional entrainment. Right: a single stalled or deeply backtracked RNAPI will break the convoy of polymerases. Continued transcription by downstream polymerases can be facilitated by Top2 recruitment. The stalled polymerase may be released by combined torpedo (Rat1) and reverse torpedo (TRAMP, exosome) activities. Relative numbers and sizes of nucleosomes and RNAPI complexes are approximately correct and consistent with the length of the rDNA. Nascent pre-ribosomes would be very much larger in size but are omitted for clarity.

(C) Model components are summarized to scale, plus the forward and reverse torpedoes.