Hyperphosphorylation (Ser2) of RNA Pol II CTD by the P-TEFb(Cyclin T1:Cdk9) complex

The association between Tat, TAR and P-TEFb is believed to bring the catalytic subunit of P-TEFb(Cyclin T1:Cdk9) in close proximity to Pol II where it hyperphosphorylates the CTD of Pol II (Herrmann et al., 1995; Zhou et al. 2000). In the presence of Tat, P-TEFb(Cyclin T1:CDK9) has been shown to phosphorylate serine 5 in addition to serine 2 suggesting that modification of the substrate specificity of CDK9 may play a role in the ability of Tat to promote transcriptional elongation (Zhou et al. 2000).

Tat、TAR与P-TEFb之间的关联被认为能将P-TEFb的催化亚基（Cyclin T1:Cdk9）置于与Pol II紧密相邻的位置，从而在Pol II的CTD上过度磷酸化（Herrmann等人，1995年；Zhou等人，2000年）。在Tat的存在下，P-TEFb（Cyclin T1:CDK9）已被证实除在丝氨酸2上磷酸化外，还可在丝氨酸5上磷酸化，这暗示了底物特异性对CDK9的修饰可能在其促进转录延伸的能力中扮演着一定角色（Zhou等人，2000年）。