P-TEFb

[1] Immediately following initiation Pol II becomes trapped in promoter proximal paused positions on the majority of human genes (Figure 1).

[2][3] P-TEFb is a cyclin dependent kinase that can phosphorylate the DRB sensitivity inducing factor (DSIF)[4] and negative elongation factor (NELF),[5] as well as the carboxyl terminal domain of the large subunit of Pol II[6] and this causes the transition into productive elongation leading to the synthesis of mRNAs.

[20] The structure of HIV Tat bound to P-TEFb demonstrated that the viral protein forms extensive contacts with the cyclin T1 subunit (Figure 2).

[7] As shown in Figure 3 P-TEFb is held in the 7SK snRNP by the double stranded RNA binding protein HEXIM (HEXIM1 or HEXIM2 in humans).

[24][25] When P-TEFb is extracted from the 7SK snRNP, 7SK RNA undergoes a conformation change, HEXIM is ejected and hnRNPs take the place of the factors removed.

Figure 1. RNA polymerase II elongation control. Pol II comes under the control of negative elongation factors (DSIF and NELF) shortly after initiation. P-TEFb mediates a transition into productive elongation by phosphorylating the two negative factors and the polymerase and is regulated by association with the 7SK snRNP.
Figure 2. Structure of P-TEFb bound by HIV Tat. PDB ID: 3MIA Cdk9 (blue), cyclin T1 (cyan), Tat (orange), ATP (magenta), magnesium (purple), zinc atoms (yellow).
Figure 3. Reversible association of P-TEFb with the 7SK snRNP. P-TEFb is released from the 7SK snRNP by Brd4 or HIV Tat. HEXIM is ejected and the two proteins are replaced by hrRNPs. The reverse of this process requires other unknown factors.