Pinning Down Transcription: Regulation of RNA Polymerase II Activity During the Cell Cycle

Pin1 is a peptidyl-prolyl cis/trans isomerase that was initially discovered as an important mitotic regulator. In keeping with this, it targets specifically phosphorylated ser or thr–pro dipeptides, which are found in numerous mitotic proteins as a result of cyclin-dependent kinase activity. But Pin1 also modulates the activity of numerous other proteins, and a strong candidate for such regulation has been RNA polymerase II (RNAP II). The unique C-terminal domain of the RNAP II largest subunit contains as many as 100 ser-pro dipeptides laid out in an essentially linear array, and which are subject to different patterns of phosphorylation. Here we summarize findings indicating that Pin1 can indeed directly modulate the structure and function of RNAP II, and propose that this activity plays a role both in mitotic gene silencing and during the transcription cycle.