Essential PPIase specific for phosphoserine and phosphothreonine N-terminal to the proline residue. Required for efficient pre-mRNA 3'-end processing and transcription termination, probably by inducing conformational changes by proline-directed isomerization in the C-terminal domain (CTD) of RPB1, thereby altering cofactor binding with the RNA polymerase II transcription complex. Also targets the SIN3-RPD3 histone deacetylase complex (HDAC). [protein]-peptidylproline (omega=180) = [protein]-peptidylproline (omega=0) Inhibited by 5-hydroxy-1,4-naphthoquinone (juglone), but not by FK506 or cyclosporin A. Interacts with the RNA polymerase II largest subunit (RPB1) and with the SIN1-RDP3 HDAC subunit SIN3. The WW domain binds to the phosphorylated tandem 7 residues repeats of RPB1. Present with 4401 molecules/cell in log phase SD medium. Belongs to the PpiC/parvulin rotamase family.