Figure 7

Mechanistic model for the role of Spt5 and Pol II pausing in the regulation of initiation, and for the role of Spt5 during elongation

In the WT situation, promoter‐proximal paused Pol II inhibits new initiating Pol II from being recruited probably via steric hindrance if paused Pol II is located too close to the PIC or if Pol II was paused further downstream of the TSS resulting in a pile‐up of Pol II behind it. Once paused Pol II is released into productive elongation, a new initiating Pol II can be recruited into the PIC. As a consequence, if the duration of paused Pol is decreased, such as upon Spt5 depletion, the frequency of new Pol II recruitment into the PIC would increase resulting in more rounds of initiation and more Pol II release into the gene body. Furthermore, as shown in this study, Spt5 depletion also decreases the number of Pol II complexes that elongate further than 15–20 kb from the TSS. Kinetic analysis of elongating Pol II in mammalian cells has shown that Pol II accelerates into gene bodies and transitions to its maximum speed and processivity ~15 kb from the TSS (Danko et al, 2013; Jonkers et al, 2014). We propose that Spt5 facilitates, but is not essential for, this transition to maximal processivity. In effect, the presence of Spt5 maximizes the number of Pol II molecules that can successfully transition to full processivity. In its absence, fewer Pol II molecules are able to successfully navigate past this zone and reach the end of genes. Importantly, the data show that Pol II complexes that do progress past this transition stage are not dependent on Spt5 for further elongation and processivity. As a result, reduced Spt5 levels would be expected to cause gene expression changes, especially of long genes, over time.