Parameters used for modeled curves shown in Figure 2.

V: Velocity of transcription in nucleotides (nt) incorporated sec−1.P: Probability that pre-rRNA will undergo nascent transcript cleavage (NTC) rather than released transcript cleavage (RTC).NTC window: The distance traveled (in nt) by the transcribing polymerase downstream of site A2 prior to the NTC event.Equilibration time: Time required for [3H] tritium uptake and equilibration of the internal nucleotide pool, prior to linear incorporation of label into newly synthesized RNA.Lifetimes are in seconds.