||p.5 3rd paragraph:"As shown in the example trajectories (Figure 2B,C), transcription elongation is punctuated by pauses of various durations. Pause density, ?pause, is defined as the average number of pauses per bp of template transcribed. As the concentration of NTP goes up, the pause-free velocity increases and
the apparent ?pause, which counts pauses lasting longer than 1 s, decreases (Figure 2E). The same trend
was also observed for the mutant Pol II (Figure 2F). The inverse relationship between v and ?pause
indicates that elongation and pausing are in kinetic competition and that pausing occurs prior to NTP binding (Artsimovitch and Landick, 2000 Davenport et al., 2000 Forde et al., 2002 Landick, 2006 Mejia et al., 2008). Note that pausing has also been observed to occur after NTP binding at
certain sequences for Escherichia coli RNAP however, yeast Pol II does not seem to employ such mechanism (Kireeva and Kashlev, 2009). The pause-free velocities and apparent pause densities at various NTP concentrations are summarized in Table 1."