Figure 1.

The P-TEFb kinase facilitates Pol II pause release. After transcription of a short (> 18 nt-long) RNA chain (red line), the negative elongation factors DSIF and NELF promote pausing by interactions with Pol II and nascent RNA. At this point, the Pol II CTD is in its hypo-phosphorylated form (S5P) by the action of the TFIIH kinase (not shown for simplicity). In response to stimulation, the P-TEFb kinase stimulates the hyper-phosphorylation of the Pol II CTD (S5P and S2P), NELF and DSIF. These phosphorylation events cause the release of NELF from the paused, early elongation complex and transforms DSIF into a positive elongation factor, stimulating the release of Pol II into productive elongation. The arrow denotes the position of the TSS, and the open circle indicates the elongation bubble.