The nucleotide addition phase and the pausing phase are colored in green and blue, respectively. At the beginning of a nucleotide addition cycle, the transcription elongation complex (TEC) with a transcript length of n thermally fluctuates between the pre-translocated state (TECn,0) and the post-translocated state (TECn,1) with a forward rate constant k1 and a reverse rate constant k−1. After translocation, the incoming NTP binds to the active site with a binding rate constant k2 and a dissociation rate constant k−2. NTP binding is followed by NTP sequestration, bond formation, and PPi release, which are collectively described by a single catalysis rate constant k3 in our study. Upon the release of the PPi, TEC is reset to the pre-translocated state (TECn+1,0) and ready for the next nucleotide addition cycle. From the pre-translocated state, the polymerase can also enter the off-pathway pausing phase by backtracking. The pausing kinetics are determined by the backward stepping rate constants kbn and forward stepping rate constants kfn. The inset shows cartoon configurations of the TEC in a pre-translocated and a post-translocated state.