TFIIH inhibits CDK9 phosphorylation during human immunodeficiency virus type 1 transcription

Tat stimulates human immunodeficiency virus, type 1 (HIV-1), transcription elongation by recruitment of the human transcription elongation factor P-TE Fb, consisting of CDK9 and cyclin T1, to the TAR RNA structure. It has been demonstrated further that CDK9 phosphorylation is required for high affini ty binding of Tat/P-TEFb to the TAR RNA structure and that the state of P-T EFb phosphorylation may regulate Tat transactivation. We now demonstrate th at CDK9 phosphorylation is uniquely regulated in the HIV-1 preinitiation an d elongation complexes. The presence of TFIIH in the HIV-1 preinitiation co mplex inhibits CDK9 phosphorylation. As TFIIH is released from the elongati on complex between +14 and +36, CDK9 phosphorylation is observed. In contra st to the activity in the "soluble" complex, phosphorylation of CDK9 is inc reased by the presence of Tat in the transcription complexes. Consistent wi th these observations, we have demonstrated that purified TFIIH directly in hibits CDK9 autophosphorylation. By using recombinant TFIIH subcomplexes, o ur results suggest that the XPB subunit of TFIIH is responsible for this in hibition of CDK9 phosphorylation. Interestingly, our results further sugges t that the phosphorylated form of CDK9 is the active kinase for RNA polymer ase II carboxyl-terminal domain phosphorylation.