(a) Nonpermissive state for RNA elongation
(b) State of transition
(c) Permissive state for RNA elongation
Figure 4: This figure describes the role of the P-TEFb complex in stabilizing RNA elongation. Panel (a) illustrates a nonpermissive state for RNA elongation. In this configuration, RNA pol II is associated with N-TEF, and the RNA pol II CTD is not phosphorylated. In this case, RNA transcription can start but stops prematurely. Panel (b) shows a state of transition, in which P-TEFb is about to substitute N-TEF in the transcription complex. P-TEFb contains the complex Cdk9/cyclin partner. Panel (c) exhibits a permissive state for RNA elongation. The presence of P-TEFb in the transcription complex phosphorylates the RNA pol II CTD, which, in turn, allows for the stabilization of the RNA transcript elongation. Abbreviations: RNA pol II: RNA polymerase II; N-TEF: negative transcription factor; CTD: carboxyl terminal domain of RNA pol II; P-TEFb: positive transcription factor b.