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.V：转录速率，单位为每秒掺入的核苷酸（nucleotides, nt）数。 P：预核糖体RNA（pre-rRNA）发生新生转录本切割（nascent transcript cleavage, NTC）而非释放后转录本切割（released transcript cleavage, RTC）的概率。 NTC窗口：转录聚合酶在发生NTC事件前，于A2位点下游移动的核苷酸距离。 平衡时间：在标记物线性掺入新合成RNA之前，[3H]氚（tritium）摄取以及内部核苷酸池达到平衡所需的时间。 寿命单位为秒。