Preferentially catalyzes the dephosphorylation of 'Ser-5' within the tandem 7 residue repeats in the C-terminal domain (CTD) of the largest RNA polymerase II subunit POLR2A. Negatively regulates RNA polymerase II transcription, possibly by controlling the transition from initiation/capping to processive transcript elongation. Recruited by REST to neuronal genes that contain RE-1 elements, leading to neuronal gene silencing in non-neuronal cells. H2O + O-phospho-L-seryl-[protein] = L-seryl-[protein] + phosphate H2O + O-phospho-L-threonyl-[protein] = L-threonyl-[protein] + phosphate Binds 1 Mg(2+) ion per monomer. Stimulated by GTF2F1. Inhibited by beryllofluoride anions. Optimum pH is 5. Monomer (By similarity). Interacts with GTF2F1. Interacts with REST. Colocalizes with RNA polymerase II. Expression is restricted to non-neuronal tissues. Highest expression in skeletal muscle, spleen, lung and placenta.